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Eric D'Asaro

Senior Principal Oceanographer

Professor, Oceanography

Email

dasaro@apl.washington.edu

Phone

206-685-2982

Research Interests

Physical oceanography, internal waves, air-sea interaction, upper ocean dynamics, Arctic oceanography, ocean instrumentation

Biosketch

Dr. D'Asaro's research spans a wide number of environments from upper ocean mixed layers to nearshore coastal fronts to fjords to deep convection. It retains studies of turbulence and internal waves, but has increasingly moved toward understanding the role of these ocean mixing processes in controlling biochemical processes in the ocean, especially gas exchange and biological productivity. By measuring big signals, like hurricanes or major blooms, it is easier to unravel the underlying processes because the signal to noise is high. For the past 20 years, D'Asaro has focused on exploiting the unique capabilities of "Lagrangian Floats", a class of instruments that try to accurately follow the three dimensional motion of water parcels particularly in regions of strong mixing. This turns out to be a novel but effective way to measure turbulence in regions of strong mixing. Lagrangian techniques have not been used very much in measuring mixing and turbulence. Accordingly one of the more exciting aspects of this work is learning how to use Lagrangian floats in the ocean. This understanding draws both upon basic ideas in fluid mechanics and upon understanding of mixing in the ocean. It strongly influences float design, use, and the oceanographic problems studied. The work thus spans a wide range of topics, from fluid mechanics to oceanography to engineering. That makes it particularly fun and interesting. Chemical species in the ocean and many microbial plants and animals drift with the ocean currents. Floats mimic this behavior, making them excellent platforms for studying aspects of ocean chemistry and biology. There is an ongoing revolution in these fields as electronic sensors become capable of making measurements formerly possible only in the laboratory. Floats equipped with such sensors are potentially very powerful tools. Dr. D'Asaro works to realize this potential, which is especially challenging and interesting as he collaborates with ocean biologists and chemists to design and operate multidisciplinary floats.

Department Affiliation

Ocean Physics

Education

B.A. Physics, Harvard University, 1976

M.S. Applied Physics, Harvard University, 1976

Ph.D. Oceanography, MIT/WHOI, 1980

Projects

Autonomous Lagrangian Floats for Oxygen Minimum Zone Biogeochemistry

Researchers are developing a new, in situ, autonomous tool for studying N loss in oxygen minimum zones (OMZs). It will allow observation of variability over a range in temporal and spatial scales that are critical for understanding controlling processes and better estimating the magnitude of N loss. The sustained deployments possible with autonomous platforms will be critical for detecting any response of OMZs to climate change.

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31 May 2012

Intense oxygen minimum zones of the world's oceans, though constituting a small fraction of total oceanic volume, host critical biogeochemical processes and are central to understanding the ocean's N cycle and its biogeochemical and isotopic signatures. OMZs are sites for a large portion of marine combined N loss to N2 (25 to 50%) and dominate the ocean N isotope budget through cogeneration of 15N and 18O enriched NO3.

Hurricane Lagrangian Floats

Lagrangian floats, designed to follow the water parcel that surrounds them, are deployed by aircraft ahead of hurricanes. As the hurricane passes they sample the evolving surface mixed layer and then surface to telemeter their data by satellite to scientists.

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Hurricanes draw their energy from warm ocean waters, but cool the warm ocean beneath them by mixing up cold water from below. Forecasters can accurately predict the track that hurricanes will take, but not their intensity. This is partially because they do not properly model the cooling of the upper ocean during hurricane passage. The purpose of our float deployments is to study this cooling.

Lateral Mixing

Small scale eddies and internal waves in the ocean mix water masses laterally, as well as vertically. This multi-investigator project aims to study the physics of this mixing by combining dye dispersion studies with detailed measurements of the velocity, temperature and salinity field during field experiments in 2011 and 2012.

 

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Publications

2000-present and while at APL-UW

Mixing to monsoons: Air-sea interactions in the Bay of Bengal

Lucas, A.J., et al. including E.A. D'Asaro, "Mixing to monsoons: Air-sea interactions in the Bay of Bengal," EOS, Trans. AGU, 95, 269-270, 2014.

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29 Jul 2014

More than 1 billion people depend on rainfall from the South Asian monsoon for their livelihoods. Summertime monsoonal precipitation is highly variable on intraseasonal time scales, with alternating "active" and "break" periods. These intraseasonal oscillations in large-scale atmospheric convection and winds are closely tied to 1°C—2°C variations of sea surface temperature in the Bay of Bengal.

A calibration equation for oxygen optodes based on physical properties of the sensing foil

McNeil, C.L. and E.A. D'Asaro, "A calibration equation for oxygen optodes based on physical properties of the sensing foil," Limnol. Oceanogr. Methods, 12:139-154, doi:10.4319/lom.2014.12.139, 2014.

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1 Mar 2014

We present a new physically based calibration equation for Aanderaa Inc. oxygen sensing optodes. We use the two site fluorescence quenching model of Demas et al. (1995) to describe the nonlinear Stern-Volmer response of the optode foil to oxygen partial pressure. Seven (minimally six) coefficients quantify foil response to oxygen and temperature; another quantifies response to hydrostatic pressure. These eight coefficients are related, theoretically, to basic physical properties of the foil. The equation provides a framework to study causes of variability and drift in optodes and to develop better quality control and handling procedures. We tested the equation using factory calibrations of 24 optode foils. When accurate multi-point calibration data are unavailable, two additional coefficients empirically correct the usually large differences observed between factory foil calibrations and post-factory laboratory/field calibrations; we cannot eliminate this major cause of uncertainty in optode calibrations.

Excluding two potentially anomalous foils, the calibration equation fits 13 similarly calibrated foils, totaling 455 calibration points over 3 – 40°C to –0.57 ± 1.48 mbar. Analysis of the resulting best fit coefficients reveals an underlying variability in optodes associated with variability in site 2 and site 1 Stern-Volmer coefficients of 32% and 20%, respectively. The fraction of unquenched fluorophores responding with the more accessible site 1 quenching characteristics varies by only 3%. The equation fits multi-point data for two optodes within manufacturer's specifications, the greater of ± 2.5 µmol kg-1 and ± 1.5%. Detailed measurements of calibration changes over time will be required to understand the causes of optode drift.

Turbulence in the upper-ocean mixed layer

D'Asaro, E.A., "Turbulence in the upper-ocean mixed layer," Ann. Rev. Mar. Sci., 6, 101-115, doi:10.1146/annurev-marine-010213-135138, 2014.

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3 Jan 2014

Nearly all operational models of upper-ocean mixing assume that the turbulence responsible for this mixing is driven by the atmospheric fluxes of momentum, heat, and moisture and the shear imposed by the ocean circulation. This idealization is supported by historical measurements of dissipation rate within the boundary layer. Detailed measurements made recently by many investigators and supported by theoretical and numerical results have found significant deviations from this classical view attributable to the influence of surface waves. Although a review of these measurements finds strong support for the influence of waves — and, in particular, for the predictions of large-eddy simulations, including the Craik-Leibovich vortex force — there are insufficient data to give definitive support to a new paradigm.

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Quantifying upper ocean turbulence driven by surface waves

D'Asaro, E.A., J. Thomson, A.Y. Shcherbina, R.R. Harcourt, M.F. Cronin, M.A. Hemer, and B. Fox-Kemper, "Quantifying upper ocean turbulence driven by surface waves," Geophys. Res. Lett, 41, 102-107, doi:10.1002/1013GL058193, 2013.

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1 Jan 2014

Nearly all operational ocean models use air-sea fluxes and the ocean shear and stratification to estimate upper ocean boundary layer mixing rates. This approach implicitly parameterizes surface wave effects in terms of these inputs. Here, we test this assumption using parallel experiments in a lake with small waves and in the open ocean with much bigger waves. Under the same wind stress and adjusting for buoyancy flux, we find the mixed layer average turbulent vertical kinetic energy in the open ocean typically twice that in the lake. The increase is consistent with models of Langmuir turbulence, in which the wave Stokes drift, and not wave breaking, is the dominant mechanism by which waves energize turbulence in the mixed layer. Applying these same theories globally, we find enhanced mixing and deeper mixed layers resulting from the inclusion of Langmuir turbulence in the boundary layer parameterization, especially in the Southern Ocean.

Waves and the equilibrium range at Ocean Weather Station P

Thomson, J., E.A. D'Asaro, M.F. Cronin, W.E. Rogers, R.R. Harcourt, and A. Shcherbina, "Waves and the equilibrium range at Ocean Weather Station P," J. Geophys. Res., 118, 5951-5962, doi:10.1002/2013JC008837, 2013.

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1 Nov 2013

Wave and wind measurements at Ocean Weather Station P (OWS-P, 50°N 145°W) are used to evaluate the equilibrium range of surface wave energy spectra. Observations are consistent with a local balance between wind input and breaking dissipation, as described by Philips (1985). The measurements include direct covariance wind stress estimates and wave breaking dissipation rate estimates during a 3 week research cruise to OWS-P. The analysis is extended to a wider range of conditions using observations of wave energy spectra and wind speed during a 2 year mooring deployment at OWS-P. At moderate wind speeds (5–15 m/s), mooring wave spectra are in agreement, within 5% uncertainty, with the forcing implied by standard drag laws and mooring wind measurements. At high wind speeds (>15 m/s), mooring wave spectra are biased low, by 13%, relative to the forcing implied by standard drag laws and mooring wind measurements. Deviations from equilibrium are associated with directionality and variations at the swell frequencies. A spectral wave hindcast accurately reproduces the mooring observations, and is used to examine the wind input.

Statistics of vertical vorticity, divergence, and strain in a developed submesoscale turbulence field

Shcherbina, A.Y., E.A. D'Asaro, C.M. Lee, J.M. Klymak, M.J. Molemaker, and J.C. McWilliams, "Statistics of vertical vorticity, divergence, and strain in a developed submesoscale turbulence field," Geophys. Res. Lett., 40, 4706-4711, doi:10.1002/grl.50919, 2013.

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16 Sep 2013

A detailed view of upper ocean vorticity, divergence, and strain statistics was obtained by a two-vessel survey in the North Atlantic Mode Water region in winter 2012. Synchronous Acoustic Doppler Current Profiler sampling provided the first in situ estimates of the full velocity gradient tensor at O(1 km) scale without the usual mix of spatial and temporal aliasing. The observed vorticity distribution in the mixed layer was markedly asymmetric (skewness 2.5), with sparse strands of strong cyclonic vorticity embedded in a weak, predominantly anticyclonic background. Skewness of the vorticity distribution decreased linearly with depth, disappearing completely in the pycnocline. Statistics of divergence and strain rate generally followed the normal and χ distributions, respectively. These observations confirm a high-resolution numerical model prediction for the structure of the active submesoscale turbulence field in this area.

An ocean coupling potential intensity index for tropical cyclones

Lin, I.-I., P. Black, J.F. Price, C.-Y. Yang, S.S. Chen, C.-C. Lien, P. Harr, N.-H. Chi, C.-C. Wu, and E.A. D'Asaro, "An ocean coupling potential intensity index for tropical cyclones," Geophys. Res. Lett., 40, 1878-1882, doi:10.1002/grl.50091, 2013.

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16 May 2013

Timely and accurate forecasts of tropical cyclones (TCs, i.e., hurricanes and typhoons) are of great importance for risk mitigation. Although in the past two decades there has been steady improvement in track prediction, improvement on intensity prediction is still highly challenging. Cooling of the upper ocean by TC-induced mixing is an important process that impacts TC intensity. Based on detail in situ air-deployed ocean and atmospheric measurement pairs collected during the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign, we modify the widely used Sea Surface Temperature Potential Intensity (SST_PI) index by including information from the subsurface ocean temperature profile to form a new Ocean coupling Potential Intensity (OC_PI) index. Using OC_PI as a TC maximum intensity predictor and applied to a 14 year (1998–2011) western North Pacific TC archive, OC_PI reduces SST_PI-based overestimation of archived maximum intensity by more than 50% and increases the correlation of maximum intensity estimation from r2=0.08 to 0.31. For slow-moving TCs that cause the greatest cooling, r2 increases to 0.56 and the root-mean square error in maximum intensity is 11 m s1. As OC_PI can more realistically characterize the ocean contribution to TC intensity, it thus serves as an effective new index to improve estimation and prediction of TC maximum intensity.

Calibration and stability of oxygen sensors on autonomous floats

D'Asaro, E.A., and C. McNeil, "Calibration and stability of oxygen sensors on autonomous floats," J. Atmos. Ocean. Technol., 30, 1896-1906, doi:10.1175/JTECH-D-12-00222.1, 2013.

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16 Apr 2013

The calibration accuracy and stability of three Aanderaa 3835 optodes and three Seabird SBE-43 oxygen sensors were evaluated over four years using in situ and laboratory calibrations. The sensors were mostly in storage, being in the ocean for typically only a few weeks per year and operated for only a few days per year. Both sensors measure partial pressure of oxygen, or equivalently saturation at standard pressure; results are expressed in this variable. It is assumed that sensor drift occurs in the oxygen sensitivity of the sensors, not the temperature compensation; this is well justified for the SBE-43 based on multiple calibrations.

Neither sensor had significant long-term drift in output when sampling anoxic water. Sensor output at 100% saturation differed from the factory calibrations by up to ±6% (averaging –2.3%±3%) for the SBE-43 and up to –12.6% for the optodes. The optode output at 100% saturation is well described by a single decaying exponential with a decay constant of ~2 yr and an amplitude of 28%. The mechanism of this drift is unknown, but is not primarily due to sensor operation. It may be different from that experienced by sensors continuously deployed in the ocean. Thus, although the optodes in this study did not have a stable calibration, their drift was stable and, once calibrated, allowed optode and SBE-43 pairs mounted on the same autonomous floats to be calibrated to an accuracy of ±0.4% over a 4-yr period.

Observations of the cold wake of Typhoon Fanapi (2010)

Mrvaljevic, R.K., P.G. Black, L.R. Centurioni, Y.-T. Chang, E.A. D'Asaro, S.R. Jayne, C.M. Lee, R.-C. Lien, I.-I. Lin, J. Morzel, P.P. Niiler, L, Rainville, and T.B. Sanford, "Observations of the cold wake of Typhoon Fanapi (2010)," Geophys. Res. Lett., 40, 316-321, doi:10.1002/grl.50096, 2013.

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28 Jan 2013

Several tens of thousands of temperature profiles are used to investigate the thermal evolution of the cold wake of Typhoon Fanapi, 2010. Typhoon Fanapi formed a cold wake in the Western North Pacific Ocean on 18 September characterized by a mixed layer that was >2.5°C cooler than surrounding water, and extending to >80 m, twice as deep as the pre-existing mixed layer. The initial cold wake became capped after 4 days as a warm, thin surface layer formed. The thickness of the capped wake, defined as the 26°C to 27°C layer, decreased, approaching the background thickness of this layer with an e-folding time of 23 days, almost twice the e-folding lifetime of the Sea Surface Temperature (SST) cold wake (12 days). The wake was advected several hundreds of kilometers from the storm track by a pre-existing mesoscale eddy. The observations reveal new intricacies of cold wake evolution and demonstrate the challenges of describing the thermal structure of the upper ocean using sea surface information alone.

Eddy-driven stratification initiates North Atlantic spring phytoplankton blooms

Mahadevan, A., E. D'Asaro, C. Lee, and M.J. Perry, "Eddy-driven stratification initiates North Atlantic spring phytoplankton blooms," Science, 337, 54-58, doi:10.1126/science.1218740, 2012.

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6 Jul 2012

Springtime phytoplankton blooms photosynthetically fix carbon and export it from the surface ocean at globally important rates. These blooms are triggered by increased light exposure of the phytoplankton due to both seasonal light increase and the development of a near-surface vertical density gradient (stratification) that inhibits vertical mixing of the phytoplankton. Classically and in current climate models, that stratification is ascribed to a springtime warming of the sea surface. Here, using observations from the subpolar North Atlantic and a three-dimensional biophysical model, we show that the initial stratification and resulting bloom are instead caused by eddy-driven slumping of the basin-scale north-south density gradient, resulting in a patchy bloom beginning 20 to 30 days earlier than would occur by warming.

Particulate organic carbon and inherent optical properties during 2008 North Atlantic Bloom Experiment

Cetinić, I., M.J. Perry, N.T. Briggs, E. Kallin, E.A. D'Asaro, and C.M. Lee, "Particulate organic carbon and inherent optical properties during 2008 North Atlantic Bloom Experiment," J. Geophys. Res., 117, doi:10.1029/2011JC007771, 2012.

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30 Jun 2012

The co-variability of particulate backscattering (bbp) and attenuation (cp) coefficients and particulate organic carbon (POC) provides a basis for estimating POC on spatial and temporal scales that are impossible to obtain with traditional sampling and chemical analysis methods. However, the use of optical proxies for POC in the open ocean is complicated by variable relationships reported in the literature between POC and cp or bbp. During the 2008 North Atlantic Bloom experiment, we accrued a large data set consisting of >300 POC samples and simultaneously measured cp and bbp. Attention to sampling detail, use of multiple types of POC blanks, cross-calibration of optical instruments, and parallel measurements of other biogeochemical parameters facilitated distinction between natural and methodological-based variability. The POC versus cp slope varied with plankton community composition but not depth; slopes were 11% lower for the diatom versus the recycling community. Analysis of literature POC versus cp slopes indicates that plankton composition is responsible for a large component of that variability. The POC versus bbp slope decreased below the pycnocline by 20%, likely due to changing particle composition associated with remineralization and fewer organic rich particles. The higher bbp/cp ratios below the mixed layer are also indicative of particles of lower organic density. We also observed a peculiar platform effect that resulted in ~27% higher values for downcast versus upcast bbp measurements. Reduction in uncertainties and improvement of accuracies of POC retrieved from optical measurements is important for autonomous sampling, and requires community consensus for standard protocols for optics and POC.

Estimates of net community production and export using high-resolution, Lagrangian measurements of O2, NO3, and POC through the evolution of a spring diatom bloom in the North Atlantic

Alkire, M.B., E. D'Asaro, C. Lee, M.J. Perry, A. Gray, I. Cetinic, N. Briggs, E. Rehm, E. Kallin, J. Kaiser, and A. Gonzalez-Posada, "Estimates of net community production and export using high-resolution, Lagrangian measurements of O2, NO3, and POC through the evolution of a spring diatom bloom in the North Atlantic," Deep Sea Res. I, 64, 157-174, doi:10.1016/j.dsr.2012.01.012, 2012.

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1 Jun 2012

Budgets of nitrate, dissolved oxygen, and particulate organic carbon (POC) were constructed from data collected on-board a Lagrangian, profiling float deployed between April 4 and May 25, 2008, as part of the North Atlantic Bloom Experiment. These measurements were used to estimate net community production (NCP) and apparent export of POC along the float trajectory. A storm resulting in deep mixing and temporary suspension of net production separated the bloom into early (April 23–27) and main (May 6–13) periods over which ~264 and ~805 mmol C m-2 were produced, respectively. Subtraction of the total POC production from the NCP yielded maximum estimates of apparent POC export amounting to ~92 and 574 mmol C m-2 during the early and main blooms, respectively. The bloom terminated the following day and ~282 mmol C m-2 were lost due to net respiration (70%) and apparent export (30%). Thus, the majority of the apparent export of POC occurred continuously during the main bloom and a large respiration event occurred during bloom Termination. A comparison of the POC flux during the main bloom period with independent estimates at greater depth suggest a rapid rate of remineralization between 60 and 100 m. We suggest the high rates of remineralization in the upper layers could explain the apparent lack of carbon overconsumption (C:N>6.6) in the North Atlantic during the spring bloom.

Trapped core formation within a shoaling nonlinear internal wave

Lien, R.-C., E.A. D'Asaro, F. Henyey, M.-H. Chang, T.-T. Tang, and Y.-J. Yang, "Trapped core formation within a shoaling nonlinear internal wave," J. Phys. Oceanogr., 42, 511-525, doi:10.1175/2011JPO4578.1, 2012.

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1 Apr 2012

Large-amplitude (100–200 m) nonlinear internal waves (NLIWs) were observed on the continental slope in the northern South China Sea nearly diurnally during the spring tide. The evolution of one NLIW as it propagated up the continental slope is described. The NLIW arrived at the slope as a nearly steady-state solitary depression wave. As it propagated up the slope, the wave propagation speed C decreased dramatically from 2 to 1.3 m s-1, while the maximum along-wave current speed Umax remained constant at 2 m s-1. As Umax exceeded C, the NLIW reached its breaking limit and formed a subsurface trapped core with closed streamlines in the coordinate frame of the propagating wave. The trapped core consisted of two counter-rotating vortices feeding a jet within the core. It was highly turbulent with 10–50-m density overturnings caused by the vortices acting on the background stratification, with an estimated turbulent kinetic energy dissipation rate of O(10-4) W kg-1 and an eddy diffusivity of O(10-1) m2 s-1. The core mixed continually with the surrounding water and created a wake of mixed water, observed as an isopycnal salinity anomaly. As the trapped core formed, the NLIW became unsteady and dissipative and broke into a large primary wave and a smaller wave. Although shoaling alone can lead to wave fission, the authors hypothesize that the wave breaking and the trapped core evolution may further trigger the fission process. These processes of wave fission and dissipation continued so that the NLIW evolved from a single deep-water solitary wave as it approached the continental slope into a train of smaller waves on the Dongsha Plateau. Observed properties, including wave width, amplitude, and propagation speed, are reasonably predicted by a fully nonlinear steady-state internal wave model, with better agreement in the deeper water. The agreement of observed and modeled propagation speed is improved when a reasonable vertical profile of background current is included in the model.

Autonomous data describe North Atlantic spring bloom

Fennel, L., I. Cetinic, E. D'Asaro, C. Lee, and M.J. Perry, "Autonomous data describe North Atlantic spring bloom," Eos, Trans. AGU, 92, 465, doi:10.1029/2011EO500002, 2011.

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13 Dec 2011

Each spring, increasing sunlight and associated changes in the ocean structure trigger rapid growth of phytoplankton across most of the North Atlantic Ocean north of 30°N. The bloom, one of the largest in the world, is a major sink for atmospheric carbon dioxide and a prototype for similar blooms around the world. Models of the ocean carbon cycle, a necessary component of climate models, need to accurately reproduce the biological, chemical, and physical processes occurring during these blooms. However, a paucity of detailed observations severely limits efforts to evaluate such models.

Typhoon-ocean interaction in the western North Pacific: Part 1

D'Asaro, E., P. Black, L. Centurioni, P. Harr, S. Jayne, I.-I Lin, C. Lee, J. Morzel, R. Mrvaljevic, P.P. Niiler, L. Rainville, T. Sanford, and T.Y. Tang, "Typhoon-ocean interaction in the western North Pacific: Part 1," Oceanography, 24, 24-31, doi:10.5670/oceanog.2011.91, 2011

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5 Dec 2011

The application of new technologies has allowed oceanographers and meteorologists to study the ocean beneath typhoons in detail. Recent studies in the western Pacific Ocean reveal new insights into the influence of the ocean on typhoon intensity.

High-resolution observations of aggregate flux during a sub-polar North Atlantic spring bloom

Briggs, N., M.J. Perry, I. Cetinic, C. Lee, E. D'Asaro, A.M. Gray, and E. Rehm, "High-resolution observations of aggregate flux during a sub-polar North Atlantic spring bloom," Deep-Sea Res. I, 58, 1031-1039, doi:10.1016/j.dsr.2011.07.007, 2011.

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1 Oct 2011

An aggregate flux event was observed by ship and by four underwater gliders during the 2008 sub-polar North Atlantic spring bloom experiment (NAB08). At the height of the diatom bloom, aggregates were observed as spikes in measurements of both particulate backscattering coefficient (bbp) and chlorophyll a fluorescence. Optical sensors on the ship and gliders were cross-calibrated through a series of simultaneous profiles, and bbp was converted to particulate organic carbon. The aggregates sank as a discrete pulse, with an average sinking rate of ~75 m^2 d^-1; 65% of aggregate backscattering and 90% of chlorophyll fluorescence content was lost between 100 m and 900 m. Mean aggregate organic carbon flux at 100 m in mid-May was estimated at 514 mg C m^2 d^-1, consistent with independent flux estimates. The use of optical spikes observed from gliders provides unprecedented coupled vertical and temporal resolution measurements of an aggregate flux event.

A perfect focus of the internal tide from the Mariana Arc

Zhao, Z., and E.A. D'Asaro, "A perfect focus of the internal tide from the Mariana Arc," Geophys. Res. Lett., 38, doi:10.1029/2011GL047909, 2011.

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30 Jul 2011

The Mariana Arc of ridges and islands forms an ~1300-km-long arc of a circle, ~630 km in radius centered at 17N, 139.6E. The hypothesis that the westward-propagating internal tides originating from the arc converge in a focal region is tested by examining the dominant M2 internal tides observed with air-launched expendable bathythermographs (AXBTs) and altimetric data from multiple satellites. The altimetric and AXBT observations agree well, though they measure different aspects of the internal tidal motion. M2 internal tides radiate both westward and eastward from the Mariana Arc, with isophase lines parallel to the arc and sharing the same center. The westward-propagating M2 internal tides converge in a focal region, and diverge beyond the focus. The focusing leads to energetic M2 internal tides in the focal region. The spatially smoothed energy flux is about 6.5 kW/m, about four times the mean value at the arc; the spatially un-smoothed energy flux may reach up to 17 kW/m. The size of the focus is close to the Rayleigh estimate; it is thus a perfect focus.

Optimizing models of the North Atlantic spring bloom using physical, chemical, and bio-optical observations from a Lagrangian float.

Bagniewski, W., K. Fennel, M.J. Perry, and E.A. D'Asaro, "Optimizing models of the North Atlantic spring bloom using physical, chemical, and bio-optical observations from a Lagrangian float." Biogeosciences, 8, 1291-1307, doi: 10.5194/bg-8-1291-2011, 2011.

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25 May 2011

The North Atlantic spring bloom is one of the main events that lead to carbon export to the deep ocean and drive oceanic uptake of CO2 from the atmosphere. Here we use a suite of physical, bio-optical and chemical measurements made during the 2008 spring bloom to optimize and compare three different models of biological carbon export. The observations are from a Lagrangian float that operated south of Iceland from early April to late June, and were calibrated with ship-based measurements. The simplest model is representative of typical NPZD models used for the North Atlantic, while the most complex model explicitly includes diatoms and the formation of fast sinking diatom aggregates and cysts under silicate limitation. We carried out a variational optimization and error analysis for the biological parameters of all three models, and compared their ability to replicate the observations. The observations were sufficient to constrain most phytoplankton-related model parameters to accuracies of better than 15%. However, the lack of zooplankton observations leads to large uncertainties in model parameters for grazing. The simulated vertical carbon flux at 100 m depth is similar between models and agrees well with available observations, but at 600 m the simulated flux is larger by a factor of 2.5 to 4.5 for the model with diatom aggregation. While none of the models can be formally rejected based on their misfit with the available observations, the model that includes export by diatom aggregation has a statistically significant better fit to the observations and more accurately represents the mechanisms and timing of carbon export based on observations not included in the optimization. Thus models that accurately simulate the upper 100 m do not necessarily accurately simulate export to deeper depths.

Enhanced turbulence and energy dissipation at ocean fronts

D'Asaro, E., C. Lee, L. Rainville, L. Thomas, and R. Harcourt, "Enhanced turbulence and energy dissipation at ocean fronts," Science, 332, 318-322, doi:0.1126/science.1201515, 2011.

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15 Apr 2011

The ocean surface boundary layer mediates air-sea exchange. In the classical paradigm and in current climate models, its turbulence is driven by atmospheric forcing. Observations at a 1-km-wide front within the Kuroshio found the rate of energy dissipation within the boundary layer to be enhanced by 10 to 20 times, suggesting that the front not the atmospheric forcing supplied the energy for the turbulence. The data quantitatively support the hypothesis that winds aligned with the frontal velocity catalyzed a release of energy from the front to the turbulence. The resulting boundary layer is stratified, in contrast to the classically well-mixed layer. These effects will be strongest at the intense fronts found in the Kuroshio, Gulf Stream, and Antarctic Circumpolar Current, key players in the climate system.

Export and mesopelagic particle flux during a North Atlantic spring diatom bloom

Martin, P., R.S. Lampitt, M.J. Perry, R. Sanders, C. Lee, and E. D'Asaro, "Export and mesopelagic particle flux during a North Atlantic spring diatom bloom," Deep Sea Res. I, 58, 338-349, doi: 10.1016/j.dsr.2011.01.006, 2011.

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1 Apr 2011

Spring diatom blooms are important for sequestering atmospheric CO2 below the permanent thermocline in the form of particulate organic carbon (POC). We measured downward POC flux during a sub-polar North Atlantic spring bloom at 100 m using thorium-234 (234Th) disequilibria, and below 100 m using neutrally buoyant drifting sediment traps. The cruise followed a Lagrangian float, and a pronounced diatom bloom occurred in a 600 km2 area around the float. Particle flux was low during the first three weeks of the bloom, between 10 and 30 mg POC m/d. Then, nearly 20 days after the bloom had started, export as diagnosed from 234Th rose to 360-620 mg POC m2/d, co-incident with silicate depletion in the surface mixed layer. Sediment traps at 600 and 750 m depth collected 160 and 150 mg POC m2/ d, with a settled volume of particles of 1000-1500 mL m2/ d. This implies that 25-43% of the 100 m POC export sank below 750 m. The sinking particles were ungrazed diatom aggregates that contained transparent exopolymer particles (TEP). We conclude that diatom blooms can lead to substantial particle export that is transferred efficiently through the mesopelagic. We also present an improved method of calibrating the Alcian Blue solution against Gum Xanthan for TEP measurements.

Observations of air–sea exchange of N2 and O2 during the passage of Hurricane Gustav in the Gulf of Mexico during 2008

McNeil, C.L., E.A. D'Asaro, and J.A. Nystuen, "Observations of air–sea exchange of N2 and O2 during the passage of Hurricane Gustav in the Gulf of Mexico during 2008," in Gas Transfer at Water Surfaces, edited by S. Komori, W. McGillis, and R. Kurose, 368-376 (Kyoto: Kyoto University, 2011, 594 pp.)

15 Jan 2011

Measurement of vertical kinetic energy and vertical velocity skewness in oceanic boundary layers by imperfectly Lagrangian floats

Harcourt, R.R., and E.A. D'Asaro, "Measurement of vertical kinetic energy and vertical velocity skewness in oceanic boundary layers by imperfectly Lagrangian floats," J. Atmos. Ocean. Technol., 27, 1918-1935, doi:10.1175/2010JTECHO731.1, 2010.

1 Nov 2010

Modulation of equatorial turbulence by tropical instability waves

Lien, R.-C., E.A. D'Asaro, and C. Menkes, "Modulation of equatorial turbulence by tropical instability waves," J. Geophys. Lett., 35, doi:10.1029/2008GL035860, 2008.

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24 Dec 2008

The sea surface temperature in the Pacific equatorial cold tongue is influenced strongly by the turbulent entrainment flux. A numerical model using a level-1.5 turbulence closure scheme suggests strong modulation of the entrainment flux by tropical instability waves (TIWs). Turbulence observations taken by a Lagrangian float encountering a TIW confirm the spatial pattern of turbulent flux variation predicted by the model.

The strongest observed turbulence mixing occurred at the leading edge of the TIW trough; turbulence diffusivity K – 10-2 m2 s-1 and turbulent heat flux Q – 1000 W m-2 at the base of surface mixed layer. The weakest observed turbulence occurred at –2° south of the TIW trough; K – 10-4 m2 s-1 and Q – 10 W m-2. The TIW caused nearly two decades of turbulence variation within an O(1000 km) zonal scale and O(100 km) meridional scale. Model results suggest that the increased entrainment heat flux at the leading edge of the TIW trough can be explained by the enhancement of shear at the surface mixed layer base modulated by the TIWs.

A diapycnal mixing budget on the Oregon shelf

D'Asaro, E.A., "A diapycnal mixing budget on the Oregon shelf," Limnol. Oceanogr., 53, 2137-2150, 2008.

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1 Dec 2008

Although isopycnal mixing is undoubtedly important at global and gyre scales, the relative importance of isopycnal and diapycnal mixing on much smaller scales is uncertain. This issue is investigated using 35 d of data from a Lagrangian float deployed on a mid-depth isopycnal on the Oregon shelf. Measurements of temperature, salinity, and pressure maintain the float on the isopycnal; its high-frequency diapycnal deviations are used to estimate the diapycnal diffusivity using an inertial subrange method; lower-frequency deviations, including intentional profiles to the surface, are used to estimate diapycnal derivatives near the target isopycnal. Downward irradiance at 490 nm is used to calibrate chlorophyll fluorescence measurements and compute solar heating rates. Corrections for the diapycnal deviations provide a nearly continuous isopycnal time series of spice (a temperature and salinity combination nearly orthogonal to potential density) and chlorophyll.

A new formulation of the diffusion equation in isopycnal coordinates is derived and used to test the accuracy of purely diapycnal mixing balances for spice and chlorophyll. On vertical scales of about 10 m and timescales of about 2 d, isopycnal spice variations are mostly controlled by diapycnal mixing, although other processes, presumably isopycnal mixing, are sometimes important. Processes other than diapycnal mixing control isopycnal chlorophyll variations on these scales. Likely candidates include isopycnal mixing with a nearby bloom, planktonic sinking out of this bloom, or possibly local phytoplankton growth. Thus both isopycnal and diapycnal mixing can be important at these small scales.

Large-eddy simulation of Langmuir turbulence in pure wind seas

Harcourt, R.R., and E.A. D'Asaro, "Large-eddy simulation of Langmuir turbulence in pure wind seas," J. Phys. Oceanogr., 38, 1542-1562, 2008.

1 Jul 2008

Air-sea gas exchange at extreme wind speeds measured by autonomous oceanographic floats

D'Asaro, E.A., and C. McNeil, "Air-sea gas exchange at extreme wind speeds measured by autonomous oceanographic floats," J. Mar. Syst., 74, 722-736, doi:10.1016/j.jmarsys.2008.02.006, 2008.

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30 Jun 2008

Measurements of the air–sea fluxes of N2 and O2 were made in winds of 15–57 m s-1 beneath Hurricane Frances using two types of air-deployed neutrally buoyant and profiling underwater floats. Two "Lagrangian floats" measured O2 and total gas tension (GT) in pre-storm and post-storm profiles and in the actively turbulent mixed layer during the storm. A single "EM-APEX float" profiled continuously from 30 to 200 m before, during and after the storm. All floats measured temperature and salinity. N2 concentrations were computed from GT and O2 after correcting for instrumental effects. Gas fluxes were computed by three methods. First, a one-dimensional mixed layer budget diagnosed the changes in mixed layer concentrations given the pre-storm profile and a time varying mixed layer depth. This model was calibrated using temperature and salinity data. The difference between the predicted mixed layer concentrations of O2 and N2 and those measured was attributed to air–sea gas fluxes FBO and FBN. Second, the covariance flux FCO(z) = < w>O2%u2032%u3009(z) was computed, where w is the vertical motion of the water-following Lagrangian floats, O2' is a high-pass filtered O2 concentration and <>(z) is an average over covariance pairs as a function of depth. The profile FCO(z) was extrapolated to the surface to yield the surface O2 flux FCO(0). Third, a deficit of O2 was found in the upper few meters of the ocean at the height of the storm. A flux FSO, moving O2 out of the ocean, was calculated by dividing this deficit by the residence time of the water in this layer, inferred from the Lagrangian floats. The three methods gave generally consistent results. At the highest winds, gas transfer is dominated by bubbles created by surface wave breaking, injected into the ocean by large-scale turbulent eddies and dissolving near 10-m depth. This conclusion is supported by observations of fluxes into the ocean despite its supersaturation; by the molar flux ratio FBO/FBN, which is closer to that of air rather than that appropriate for Schmidt number scaling; by O2 increases at about 10-m depth along the water trajectories accompanied by a reduction in void fraction as measured by conductivity; and from the profile of FCO(z), which peaks near 10 m instead of at the surface.

At the highest winds O2 and N2 are injected into the ocean by bubbles dissolving at depth. This, plus entrainment of gas-rich water from below, supersaturates the mixed layer causing gas to flux out of the near-surface ocean. A net influx of gas results from the balance of these two competing processes. At lower speeds, the total gas fluxes, FBO, FBN and FCO(0), are out of the ocean and downgradient.

Convection and the seeding of the North Atlantic bloom

D'Asaro, E.A., "Convection and the seeding of the North Atlantic bloom," J. Mar. Syst., 69, 233-237, doi:10.1016/j.jmarsys.2005.08.005, 2008.

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28 Feb 2008

Observations of vertical velocities in deep wintertime mixed layers using neutrally buoyant floats show that the convectively driven vertical velocities, roughly 1000 m per day, greatly exceed the sinking velocities of phytoplankton, 10 m or less per day. These velocities mix plankton effectively and uniformly across the convective layer and are therefore capable of returning those that have sunk to depth back into the euphotic zone. This mechanism cycles cells through the surface layer during the winter and provides a seed population for the spring bloom. A simple model of this mechanism applied to immortal phytoplankton in the subpolar Labrador Sea predicts that the seed population in early spring will be a few percent of the fall concentration if the plankton sink more slowly than the mean rate at which the surface well-mixed layer grows over the winter. Plankton that sink faster than this will mostly sink into the abyss with only a minute fraction remaining by spring. The shallower mixed layers of mid-latitudes are predicted to be much less effective at maintaining a seed population over the winter, limiting the ability of rapidly sinking cells to survive the winter.

Mixing it up with krill

Kunze, E., J.F. Dower, R. Dewey, and E.A. D'Asaro, "Mixing it up with krill," Science, 318, 1239, doi: 10.1126/science.318.5854.1239b, 2007.

23 Nov 2007

Cold wake of Hurricane Frances

D'Asaro, E.A., T.B. Sanford, P.P. Niiler, and E.J. Terrill, "Cold wake of Hurricane Frances," Geophys. Res. Lett., 34, doi:10.1029/2007GL029922, 2007.

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11 Aug 2007

An array of instruments air-deployed ahead of Hurricane Frances measured the three-dimensional, time dependent response of the ocean to this strong (60 m s-1) storm. Sea surface temperature cooled by up to 2.2°C with the greatest cooling occurring in a 50-km-wide band centered 60–85 km to the right of the track. The cooling was almost entirely due to vertical mixing, not air-sea heat fluxes. Currents of up to 1.6 m s-1 and thermocline displacements of up to 50 m dispersed as near-inertial internal waves. The heat in excess of 26°C, decreased behind the storm due primarily to horizontal advection of heat away from the storm track, with a small contribution from mixing across the 26°C isotherm. SST cooling under the storm core (0.4°C) produced a 16% decrease in air-sea heat flux implying an approximately 5 m s-1 reduction in peak winds

High-frequency internal waves on the Oregon continental shelf

D'Asaro, E.A., R.-C. Lien, and F. Henyey, "High-frequency internal waves on the Oregon continental shelf," J. Phys. Oceanogr., 37, 195601976, doi:10.1175/JPO3096.1, 2007.

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1 Jul 2007

Measurements of vertical velocity by isopycnal-following, neutrally buoyant floats deployed on the Oregon shelf during the summers of 2000 and 2001 were used to characterize internal gravity waves on the shelf using measurements of vertical velocity. The average spectrum of Wentzel–Kramers–Brillouin (WKB)-scaled vertical kinetic energy has the level predicted by the Garrett–Munk model (GM79), plus a narrow M2 tidal peak and a broad high-frequency peak extending from about 0.1N to N and rising a decade above GM79. The high-frequency peak varies in energy coherently with time across its entire bandwidth. Its energy is independent of the tidal energy. The energy in the "continuum" region between the peaks is weakly correlated with the level of the high-frequency peak energy and is independent of the tidal peak energy. The vertical velocity is not Gaussian but is highly intermittent, with a calculated kurtosis of 19. The vertical kinetic energy varies geographically. Low energy is found offshore and nearshore. The highest energy is found near a small seamount. High energy is found over the rough topography of Heceta Bank and near the shelf break. The highest energy occurs as packets of high-frequency waves, often occurring on the sharp downward phase of the M2 internal tide and called "tidal solibores."

A few isolated waves with high energy are also found. Of the 1-h periods with the highest vertical kinetic energy, 31% are tidal solibores, 8% are isolated waves, and the remainder of the periods appear unorganized. The two most energetic tidal solibores were examined in detail. As compared with the steady, propagating, two-dimensional, inviscid, internal-wave solutions to the equations of motion with no background shear [i.e., the Dubreil–Jacotin–Long (DJL) equation], all but the most energetic observed waveforms are too narrow for their height to be solitary waves. Despite the large near-N peak in vertical kinetic energy, the M2 internal tide contributes over 80% of the energy, ignoring near-inertial waves. The tidal solibores make a very small contribution, 0.5%, to the overall internal-wave energy.

Solar power for autonomous floats

D'Asaro, E.A., "Solar power for autonomous floats," J. Atmos. Ocean. Technol., 24, 1309-1314, doi:10.1175/JTECH2041.1, 2007.

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1 Jul 2007

Advances in low-power instrumentation and communications now often make energy storage the limiting factor for long-term autonomous oceanographic measurements. Recent advances in photovoltaic cells, with efficiencies now close to 30%, make solar power potentially viable even for vehicles such as floats that only surface intermittently. A simple application, the development of a solar-powered Argos recovery beacon, is described here to illustrate the technology. The 65-cm2 solar array, submersible to at least 750 dbar, powers an Argos beacon. Tests indicate that with minor improvements the beacon will run indefinitely at any latitude equatorward of about 50°. Scaling up this design to current operational profiling floats, each profile could easily be powered by a few hours of solar charging, a shorter time than is currently being used for Argos data communications.

Air-sea gas exchange at extreme wind speeds measured by autonomous oceanographic floats

D'Asaro, E.A., and C. McNeil, "Air-sea gas exchange at extreme wind speeds measured by autonomous oceanographic floats," J. Mar. Syst., 66, 92-109, doi:10.1016/j.jmarsys.2006.06.007, 2007.

1 Jun 2007

Measurement of scalar variance dissipation from Lagrangian floats

D'Asaro, E.A., and R.-C. Lien, "Measurement of scalar variance dissipation from Lagrangian floats," J. Atmos. Ocean. Technol., 24, 1066-1077, doi:10.1175/JTECH2031.1, 2007.

1 Jun 2007

Parameterization of air-sea gas fluxes at extreme wind speeds

McNeil, C., and E.A. D'Asaro, "Parameterization of air-sea gas fluxes at extreme wind speeds," J. Mar. Sys., 66, 110-121, doi:10.1016/j.jmarsys.2006.05.013, 2007.

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1 Jun 2007

Air–sea flux measurements of O2 and N2 obtained during Hurricane Frances in September 2004 using air-deployed neutrally buoyant floats reveal the first evidence of a new regime of air–sea gas transfer occurring at wind speeds in excess of 35 m s-1. In this regime, plumes of bubbles 1 mm and smaller in size are transported down from near the surface of the ocean to greater depths by vertical turbulent currents with speeds up to 20–30 cm s-1. These bubble plumes mostly dissolve before reaching a depth of approximately 20 m as a result of hydrostatic compression. Injection of air into the ocean by this mechanism results in the invasion of gases in proportion to their tropospheric molar gas ratios, and further supersaturation of less soluble gases. A new formulation for air–sea fluxes of weakly soluble gases as a function of wind speed is proposed to extend existing formulations to span the entire natural range of wind speeds over the open ocean, which includes hurricanes.

The new formulation has separate contributions to air–sea gas flux from: 1) non-supersaturating near-surface equilibration processes, which include direct transfer associated with the air–sea interface and ventilation associated with surface wave breaking; 2) partial dissolution of bubbles smaller than 1 mm that mix into the ocean via turbulence; and 3) complete dissolution of bubbles of up to 1 mm in size via subduction of bubble plumes. The model can be simplified by combining "surface equilibration" terms that allow exchange of gases into and out of the ocean, and "gas injection" terms that only allow gas to enter the ocean. The model was tested against the Hurricane Frances data set. Although all the model parameters cannot be determined uniquely, some features are clear. The fluxes due to the surface equilibration terms, estimated both from data and from model inversions, increase rapidly at high wind speed but are still far below those predicted using the cubic parameterization of Wanninkhof and McGillis at high wind speed. The fluxes due to gas injection terms increase with wind speed even more rapidly, causing bubble injection to dominate at the highest wind speeds.

Air-sea exchange in hurricanes: Synthesis of observations from the Couple Boundary Layer Air-Sea Transfer experiment

Black, P.G., E.A. D'Asaro, W.M. Drennan, J.R. French, P.P. Niiler, T.B. Sanford, E.J. Terrill, E.J. Walsh, and J.A. Zhang, "Air-sea exchange in hurricanes: Synthesis of observations from the Couple Boundary Layer Air-Sea Transfer experiment," Bull. Am. Meterol. Soc., 88, 357-374, doi:10.1175/BAMS-88-3-357, 2007.

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1 Mar 2007

The Coupled Boundary Layer Air–Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air–sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (CD) values derived from CBLAST momentum flux measurements show CD becoming invariant with wind speed near a 23 m s-1 threshold rather than a hurricane-force threshold near 33 m s-1. Values above 23 m s-1 are lower than previous open-ocean measurements.

The Dalton number estimates (CE) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s-1, which is in approximate agreement with previous measurements at lower winds. These observations imply a CE/CD ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be "roll vortex" or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9–1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery.

As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and sub-surface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2°C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s-1 were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.

A gas tension device with response times of minutes

McNeil, C., E.A. D'Asaro, B. Johnson, and M. Horn, "A gas tension device with response times of minutes," J. Atmos. Ocean. Technol., 23, 1539-1558, doi: 10.1175/JTECH1974.1, 2006.

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1 Nov 2006

The development and testing of a new, fast response, profiling gas tension device (GTD) that measures total dissolved air pressure is presented. The new GTD equilibrates a sample volume of air using a newly developed (patent pending) tubular silicone polydimethylsiloxane (PDMS) membrane interface. The membrane interface is long, flexible, tubular, and is contained within a seawater-flushed hose. The membrane interface communicates pressure to a precise pressure gauge using low dead-volume stainless steel tubing. The pressure sensor and associated electronics are located remotely from the membrane interface. The new GTD has an operating depth in seawater of 0–300 m. The sensor was integrated onto an upper-ocean mixed layer, neutrally buoyant float, and used in air–sea gas exchange studies. Results of laboratory and pressure tank tests are presented to show response characteristics of the device. A significant hydrostatic response of the instrument was observed over the depth range of 0–9 m, and explained in terms of expulsion (or absorption) of dissolved air from the membrane after it is compressed (or decompressed). This undesirable feature of the device is unavoidable since a large exposed surface area of membrane is required to provide a rapid response. The minimum isothermal response time varies from (2 ± 1) min near the sea surface to (8 ± 2) min at 60-m depth. Results of field tests, performed in Puget Sound, Washington, during the summer of 2004, are reported, and include preliminary comparisons with mass-spectrometric analysis of in situ water samples analyzed for dissolved N2 and Ar. These tests served as preparations for deployment of two floats by aircraft into the advancing path of Hurricane Frances during September 2004 in the northwest Atlantic. The sensors performed remarkably well in the field. A model of the dynamical response of the GTD to changing hydrostatic pressure that accounts for membrane compressibility effects is presented. The model is used to correct the transient response of the GTD to enable a more precise measurement of gas tension when the float was profiling in the upper-ocean mixed layer beneath the hurricane.

Measurement of turbulent kinetic energy dissipation rate with a Lagrangian float

Lien, R.-C., and E.A. D'Asaro, "Measurement of turbulent kinetic energy dissipation rate with a Lagrangian float," J. Atmos. Ocean. Technol., 23, 964-976, 2006.

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1 Jul 2006

This study tests the ability of a neutrally buoyant float to estimate the dissipation rate of turbulent kinetic energy ε from its vertical acceleration spectrum using an inertial subrange method. A Lagrangian float was equipped with a SonTek acoustic Doppler velocimeter (ADV), which measured the vector velocity 1 m below the float's center, and a pressure sensor, which measured the float's depth. Measurements were taken in flows where estimates of ε varied from 10-8 to 10-3 W kg-1. Previous observational and theoretical studies conclude that the Lagrangian acceleration spectrum is white within the inertial subrange with a level proportional to ε. The size of the Lagrangian float introduces a highly reproducible spectral attenuation at high frequencies. Estimates of the dissipation rate of turbulent kinetic energy using float measurements εfloat were obtained by fitting the observed spectra to a model spectrum that included the attenuation effect. The ADV velocity measurements were converted to a wavenumber spectrum using a variant of Taylor's hypothesis. The spectrum exhibited the expected –5/3 slope within an inertial subrange. The turbulent kinetic energy dissipation rate εADV was computed from the level of this spectrum. These two independent estimates, εADV and εfloat, were highly correlated. The ratio εfloatADV deviated from one by less than a factor of 2 over the five decades of ε measured. This analysis confirms that ε can be estimated reliably from Lagrangian float acceleration spectra in turbulent flows. For the meter-sized floats used here, the size of the float and the noise level of the pressure measurements sets a lower limit of εfloat > 10-8 W kg-1.

The hurricane mixing front

D'Asaro, E.A., R. Harcourt, E. Terrill, P.P. Niiler, and T.B. Sanford, "The hurricane mixing front," Bull. Am. Meteorol. Soc., 87, 1492, 2006.

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26 Apr 2006

The temperature of the sea surface beneath the hurricane inner core is a key factor controlling the flux of enthalpy from the ocean to the hurricane and thus an important influence on hurricane intensification. Mixing caused by the hurricane winds produces a rapid cooling of the sea surface as cooler water is mixed upward from below. This produces a front in sea surface temperature beneath the storm. The position of this front relative to the eye should thus be related to SST-induced storm intensification. Data from the CBLAST measurements in hurricanes is used to map several examples of this front. The sensitivity of its position to storm properties is explored using simple models of ocean mixing.

Energy flux of nonlinear internal waves in northern South China Sea

Chang, M.H., R.-C. Lien, T.Y. Tang, E.A. D'Asaro, and Y.J. Yang, "Energy flux of nonlinear internal waves in northern South China Sea," Geophys. Res. Lett., 33, doi:10.1029/2005GL025196, 2006.

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4 Feb 2006

We analyze three sets of ADCP measurements taken on the Dongsha plateau, on the shallow continental shelf, and on the steep continental slope in the northern South China Sea (SCS). The data show strong divergences of energy and energy flux of nonlinear internal waves (NLIW) along and across waves' prevailing westward propagation path. The NLIW energy flux is 8.5 kW m-1 on the plateau, only 0.25 kW m-1 on the continental shelf 220 km westward along the propagation path, and only 1 kW m-1 on the continental slope 120 km northward across the propagation path. Along the wave path on the plateau, the average energy flux divergence of NLIW is ~0.04 W m-2, which corresponds to a dissipation rate of O(10-7 – 10-6) W kg-1. Combining the present with previous observations and model results, a scenario of NLIW energy flux in the SCS emerges. NLIWs are generated east of the plateau, propagate predominantly westward across the plateau along a beam of ~100 km width that is centered at ~21°N, and dissipate nearly all their energy before reaching the continental shelf.

Mixing, 3D mapping, and Lagrangian evolution of a thermohaline intrusion

Alford, M.H., M.C. Gregg, and E.A. D'Asaro, "Mixing, 3D mapping, and Lagrangian evolution of a thermohaline intrusion," J. Phys. Oceanogr., 35, 1689-1711, 2005

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30 Sep 2005

Observations of the three-dimensional structure and evolution of a thermohaline intrusion in a wide, deep fjord are presented. In an intensive two-ship study centered on an acoustically tracked neutrally buoyant float, a cold, fresh, low-oxygen tongue of water moving southward at about 0.03 m s-1 out of Possession Sound, Washington, was observed. The feature lay across isopycnal surfaces in a 50–80-m depth range. The large-scale structures of temperature, salinity, velocity, dissolved oxygen, and chlorophyll were mapped with a towed, depth-cycling instrument from one ship while the other ship measured turbulence close to the float with loosely tethered microstructure profilers. Observations from both ships were expressed in a float-relative (Lagrangian) reference frame, minimizing advection effects. A float deployed at the tongue's leading edge warmed 0.2°C in 24 h, which the authors argue resulted from mixing. Diapycnal heat fluxes inferred from microstructure were 1–2 orders of magnitude too small to account for the observed warming. Instead, lateral stirring along isopycnals appears responsible, implying isopycnal diffusivities O(1 m2 s-1). These are consistent with estimates, using measured temperature microstructure, from an extension of the Osborn–Cox model that allows for lateral gradients. Horizontal structures with scales O(100 m) are seen in time series and spatial maps, supporting this interpretation.

Energy of nonlinear internal waves in the South China Sea

Lien, R.-C., T.Y. Tang, M.H. Chang, and E.A. D'Asaro, "Energy of nonlinear internal waves in the South China Sea," Geophys. Res. Lett., 32, 10.1029/2004GL022012, 2005.

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12 Mar 2005

Four sets of ADCP measurements were taken in the South China Sea (SCS); these results were combined with previous satellite observations and internal-tide numerical model results. Analysis suggests that strong internal tides are generated in Luzon Strait, propagate as a narrow tidal beam into the SCS, are amplified by the shoaling continental slope near TungSha Island, become nonlinear, and evolve into high-frequency nonlinear internal waves (NIW). Internal waves in the SCS have geographically distinct characteristics. (1) West of Luzon Strait the total internal wave energy (Eiw ) is 10 x that predicted by Garrett-Munk spectra (EGM) (Levine, 2002). There is no sign of NIW. (2) Near TungSha Island Eiw = 13 x EGM. Strong nonlinear and high-harmonic tides are present. Repetitive trains of large-amplitude NIW appear primarily at a semidiurnal periodicity with their amplitudes modulated at a fortnightly tidal cycle. The rms vertical velocity of NIW shows a clear spring-neap tidal cycle and is linearly proportional to the barotropic tidal height in Luzon Strait with a 1.85-day time lag, consistent with the travel time of internal tides from Luzon Strait to TungSha Island. (3) At the northern SCS shelfbreak Eiw = 4 x EGM. Single depression waves are found, but no multiple-waves packets are evident. (4) On the continental shelf Eiw = 2 x EGM . Both depression and elevation NIW exist.

Turbulent structure of high-density suspensions formed under waves

Lamb, M.P., E.A. D'Asaro, and J.D. Parsons, "Turbulent structure of high-density suspensions formed under waves," J. Geophys. Res., 109, 10.1029/2004JC002355, 2004.

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22 Dec 2004

We performed a series of laboratory experiments on the interactions between turbulent wave boundary layers and a predominantly silt-sized sediment bed. Under a wide range of wave conditions similar to those observed on storm-dominated midshelf environments we produced quasi-steady high-density benthic suspensions. These suspensions were turbulent, while containing large near-bed concentrations of suspended sediment (17–80 g/L), and were separated from the upper water column by a lutocline. Detailed measurements of the vertical structure of velocity, turbulence, and sediment concentration revealed that the wave boundary layer, while typically >1 cm thick in sediment-free conditions, was reduced substantially in size, often to <3 mm, with the addition of suspendible sediment. This likely resulted from sediment-induced stratification that limited vertical mixing of momentum. Despite boundary layer reduction the flows were able to support high-density suspensions as thick as 8 cm because turbulent energy was transported upward from this thin but highly energetic near-bed region. Standard formulations of the Richardson number for shear flows are not applicable to our experiments since the suspensions were supported from transported rather than locally produced turbulence.

Measurements of turbulent vertical kinetic energy in the ocean mixed layer from Lagrangian floats

Tseng, R.-S., and E.A. D'Asaro, "Measurements of turbulent vertical kinetic energy in the ocean mixed layer from Lagrangian floats," J. Phys. Oceanogr., 34, 1984-1990, doi: 10.1175/1520-0485(2004)034, 2004.

1 Sep 2004

Lagrangian trajectories on the Oregon shelf during upwelling

D'Asaro, E.A., "Lagrangian trajectories on the Oregon shelf during upwelling," Cont. Shelf Res., 24, 1421-1436, doi:10.1016/j.csr.2004.06.003, 2004.

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1 Aug 2004

Neutrally buoyant, isopycnal-following floats were deployed on the Oregon continental shelf during the upwelling seasons of 2000 and 2001 and were carried southward by the mean current. The floats made CTD profiles and obtained GPS fixes twice daily, thus providing a hydrographic section along a known track. The floats followed the water accurately while at depth, but were displaced from the trajectories of the deep water during semidiurnal surfacings. These effects were large for water depths shallower than 100 m, but small on the rest of the shelf. Float trajectories, corrected for advection while on the surface, showed significant error when near the shore, but little net effect offshore. Some floats moved onshore and upward along the sloping isopycnals as expected during upwelling. Although the position of the isopycnal could be predicted accurately from the wind, the motion along the isopycnal showed significant fluctuations unrelated to the wind. These may be due to barotropic shelf waves. Some floats moved southward, roughly following the isobaths around Heceta Bank to Cape Blanco. Here they underwent large vertical and cross-shelf excursions and eventually moved offshore. Two floats passed through this region 25 days apart following different trajectories, indicating an unsteady flow. Overall, these data show the expected mix of a classical upwelling circulation in the north, an offshore jet with eddies in the south, and a strong influence of topography on both the mean flow and its fluctuations.

Air-sea heat flux measurements from nearly neutrally buoyant floats

D'Asaro, E.A., "Air-sea heat flux measurements from nearly neutrally buoyant floats," J. Atmos. Ocean. Technol., 21, 1086-1094, doi:10.1175/1520-0426(2004)021, 2004.

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1 Jul 2004

The ability of neutrally buoyant, high-drag floats to measure the air–sea heat flux from within the turbulent oceanic boundary layer is investigated using float data from four different winter and fall float deployments. Two flux estimates can be made: Q0A measures the vertical advection of heat, and Q0D integrates the Lagrangian heating rate. Because the floats are only imperfectly Lagrangian, a key issue is diagnosing the ability of a given set of float data to make accurate flux measurements. A variety of diagnostics are explored and evaluated. Here Q0A and Q0D are compared to heat flux measurements computed using bulk formulas and shipboard measurements for one 2-week cruise. Quality controlled float-based fluxes agree with bulk fluxes to within 10 W m-2 for both positive and negative values. The differences are well within the expected statistical errors in the float measurements and the bias errors of the bulk measurements.

Lagrangian estimates of diapycnal mixing in a simulated K–H instability

D'Asaro, E.A., K.B. Winters, and R.-C. Lien, "Lagrangian estimates of diapycnal mixing in a simulated K–H instability," J. Atmos. Ocean. Technol., 21, 799-809, 2004.

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1 May 2004

The Lagrangian properties of a high-resolution, three-dimensional, direct numerical simulation of Kelvin–Helmholtz (K–H) instability are examined with the goal of assessing the ability of Lagrangian measurements to determine rates and properties of ocean mixing events. The size and rotation rates of the two-dimensional K–H vortices are easily determined even by individual trajectories. Changes in density along individual trajectories unambiguously show diapycnal mixing. These changes are highly structured during the early phases of the instability but become more random once the flow becomes turbulent. Only 36 particles were tracked, which is not enough to usefully estimate volume-averaged fluxes from the average rates of temperature change. Similarly, time- and volume-averaged vertical advective flux can be estimated to only 20% accuracy. Despite the relatively low Reynolds number of the flow, the dissipation rates of energy and density variance are correlated with the spectral levels of transverse velocity and density in an inertial subrange, as expected for high-Reynolds-number turbulence. The Kolmogorov constants are consistent with previous studies. This suggests that these inertial dissipation methods are the most promising techniques for making useful measurements of diapycnal mixing rates from practical Lagrangian floats because they converge rapidly and have a clear theoretical basis.

Lagrangian spectra and diapycnal mixing in stratified flow

Lien, R.-C., and E.A. D'Asaro, "Lagrangian spectra and diapycnal mixing in stratified flow," J. Phys. Oceanogr., 34, 978-984, 2004.

1 Apr 2004

Meso- and submesoscale structure in a convecting field

Steffen, E.L., and E.A. D'Asaro, "Meso- and submesoscale structure in a convecting field," J. Phys. Oceanogr., 34, 44-60, doi: 10.1175/1520-0485(2004)034, 2004.

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1 Jan 2004

Intensive data collection in the region of the Labrador Sea northwest of former Ocean Weather Station Bravo during the winter of 1998 allowed examination of the meso- and submesoscale structure during active convection. Data used include shipboard CTDs, shipboard underway data, isobaric CTD profiling floats, and high-drag floats whose trajectories were approximately Lagrangian in the horizontal and vertical directions. On the mesoscale, O(20 km), horizontal variability was nearly 1°C and 0.1 psu. An anticyclonic eddy of 40-km diameter was found. On a smaller scale, O(5 km), variability of 0.04 psu and 0.3°C was found. By utilizing data from fully Lagrangian floats, this smaller-scale field was found to be organized into eddies of 1–12-km radius. Both cyclonic and anticyclonic features were found, with the anticyclones being larger. This observation may explain the excess of anticyclones reported in previous studies having lower spatial resolution. These features were unsteady, with an anticyclone doubling in size in less than a week. There was communication between eddies, with four of five floats escaping an anticyclone. This exchange produced horizontal diffusivities (250–350 m2 s-1) on the order of basin-scale values, implying these small-scale features could produce the majority of the stirring. The influence of these structures on convection was explored: convection occurred throughout the region sampled despite the presence of eddies, the deepest mixed layers were found within an anticyclone, and convective trajectories within small cyclones were found to be significantly tilted so as to avoid the surface centers of the cyclones.

Data Processing for Winters 1997 and 1998 Central Labrador Sea

Steffen, E.L., and E.A. D'Asaro, "Data Processing for Winters 1997 and 1998 Central Labrador Sea," APL-UW TM 4-03, November 2003

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30 Nov 2003

This work describes several techniques to process data collected in the central Labrador Sea during the winters of 1997 and 1998 as part of the Labrador Sea Deep Convection Experiment. Ship-based observations (CTD and intake logs) and float data (profiling isobaric floats and fully Lagrangian floats) are intercalibrated and used to estimate trends in mixed layer properties during the winter of 1998. RAFOS records are used to calculate the horizontal position of fully Lagrangian floats utilizing two methods.

Observations of the Labrador Sea eddy field

Lilly, J.M., P.B. Rhines, F. Schott, K. Lavender, J. Lazier, U. Send, and E.A. D'Asaro, "Observations of the Labrador Sea eddy field," Prog. Oceanogr., 59, 75-176, doi:10.1016/j.pocean.2003.08.013, 2003.

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26 Nov 2003

This paper is an observational study of small-scale coherent eddies in the Labrador Sea, a region of dense water formation thought to be of considerable importance to the North Atlantic overturning circulation. Numerical studies of deep convection emphasize coherent eddies as a mechanism for the lateral transport of heat, yet their small size has hindered observational progress. A large part of this paper is therefore devoted to developing new methods for identifying and describing coherent eddies in two observational platforms, current meter moorings and satellite altimetry. Details of the current and water mass structure of individual eddy events, as they are swept past by an advecting flow, can then be extracted from the mooring data. A transition is seen during mid-1997, with long-lived boundary current eddies dominating the central Labrador Sea year-round after this time, and convectively formed eddies similar to those seen in deep convection modeling studies apparent prior to this time. The TOPEX / Poseidon altimeter covers the Labrador Sea with a loose "net" of observations, through which coherent eddies can seem to appear and disappear. By concentrating on locating and describing anomalous events in individual altimeter tracks, a portrait of the spatial and temporal variability of the underlying eddy field can be constructed. The altimeter results reveal an annual "pulsation" of energy and of coherent eddies originating during the late fall at a particular location in the boundary current, pinpointing the time and place of the boundary current-type eddy formation. The interannual variability seen at the mooring is reproduced, but the mooring site is found to be within a localized region of greatly enhanced eddy activity. Notably lacking in both the annual cycle and interannual variability is a clear relationship between the eddies or eddy energy and the intensity of wintertime cooling. These eddy observations, as well as hydrographic evidence, suggest an active role for boundary current dynamics in shaping the energetics and water mass properties of the interior region.

The Labrador Sea Deep Convection Experiment data collection

Krahmann, G., M. Visbeck, W. Smethie, E.A. D'Asaro, P.B. Rhines, R.A. Clarke, J. Lazier, R.E. Davis, P.P. Niiler, P.S. Guest, J. Meincke, G.W. Kent Moore, R.S. Pickart, W. Brechner Owens, M.D. Prater, I.A. Renfrew, and F.A. Schott, "The Labrador Sea Deep Convection Experiment data collection," Geochem. Geophys. Geosyst., 4, 10.1029/2003GC000536, 2003.

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31 Oct 2003

Between 1996 and 1998, a concerted effort was made to study the deep open ocean convection in the Labrador Sea. Both in situ observations and numerical models were employed with close collaboration between the researchers in the fields of physical oceanography, boundary layer meteorology, and climate. A multitude of different methods were used to observe the state of ocean and atmosphere and determine the exchange between them over the experiment's period. The Labrador Sea Deep Convection Experiment data collection aims to assemble the observational data sets in order to facilitate the exchange and collaboration between the various projects and new projects for an overall synthesis. A common file format and a browsable inventory have been used so as to simplify the access to the data.

Performance of autonomous Lagrangian floats

D'Asaro, E.A., "Performance of autonomous Lagrangian floats," J. Atmos. Ocean. Technol., 20, 896-911, DOI: 10.1175/1520-0426(2003)020<0896:POALF>2.0.CO;2, 2003.

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1 Jun 2003

A truly Lagrangian float would follow all three components of oceanic velocity on all timescales. Progress toward this goal is reviewed by analyzing the performance of nearly Lagrangian floats deployed in a variety of oceanic flows. Two new float types, described in this paper, are autonomous with durations of months, can alternate between Lagrangian and profiling modes, relay data via satellite, and can carry a variety of sensors. A novel hull design is light, strong, and has a compressibility close to that of seawater.

The key to making floats accurately Lagrangian is an improved understanding of the factors that control float buoyancy and motion. Several insights are presented here. Anodized aluminum gains weight in seawater due to reactions between its surface and seawater. At low pressure the buoyancy of floats with O-ring seals varies as if attached bubbles of air were being compressed. The volume of "air" decays exponentially with a decay scale of a few days from 10 to 30 cc at deployment to an asymptotic value that depends on pressure. The drag of floats moving slowly through a stratified ocean is dominated by internal wave generation and is thus linear, not quadratic. Internal wave drag acting on an isopycnal-seeking float will cause the float to be Lagrangian for frequencies greater than about N/30, where N is the buoyancy frequency.

These floats have proven most useful in measuring the turbulence in ocean boundary layers and other regions of strong turbulence where the ability of the floats to be Lagrangian on short timescales matches the short timescale of the processes and where the size of the turbulent eddies exceeds the size of the float. On longer timescales, the floats successfully operate as isopycnal followers. Because truly Lagrangian floats are highly sensitive to minor perturbations, extension of the frequency band over which the floats are Lagrangian will require careful control of float buoyancy and thus a detailed understanding of the float's equation of state.

The ocean boundary layer below Hurrican Dennis

D'Asaro, E.A., "The ocean boundary layer below Hurrican Dennis," J. Phys. Oceanogr., 33, 561-579, 2003.

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1 Mar 2003

Three neutrally buoyant floats were air deployed ahead of Hurricane Dennis on 28 August 1999. These floats were designed to accurately follow three-dimensional water trajectories and measure pressure (i.e., their own depth) and temperature. The hurricane eye passed between two of the floats; both measured the properties of the ocean boundary layer beneath sustained 30 m s-1 winds. The floats repeatedly moved through a mixed layer 30–70 m deep at average vertical speeds of 0.03–0.06 m s-1. The speed was roughly proportional to the friction velocity. Mixed layer temperature cooled about 2.8° and 0.75°C at the floats on the east and west sides of the northward-going storm, respectively. Much of the cooling occurred before the eye passage. The remaining terms in the horizontally averaged mixed layer heat budget, the vertical velocity—temperature covariance and the Lagrangian heating rate, were computed from the float data. Surface heat fluxes accounted for only a small part of the cooling. Most of the cooling was due to entrainment of colder water from below and, on the right-hand (east) side only, horizontal advection and mixing with colder water. The larger entrainment flux on this side of the hurricane was presumably due to the much larger inertial currents and shear. Although these floats can make detailed measurements of the heat transfer mechanisms in the ocean boundary layer under these severe conditions, accurate measurements of heat flux will require clusters of many floats to reduce the statistical error.

The Kolmogorov constant for the Lagrangian velocity spectrum and structure function

Lien, R.-C., and E.A. D'Asaro, "The Kolmogorov constant for the Lagrangian velocity spectrum and structure function," Phys. Fluids, 14, 4456-4459.

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1 Dec 2002

The inertial subrange Kolmogorov constant for the Lagrangian velocity structure function C0 is related to the inertial subrange constant for the Lagrangian acceleration spectrum β by C0 = πβ. However, Rλ must be greater than about 105 for the inertial subrange of the structure function to be sufficiently wide to accurately determine C0, while values of Rλ greater than 102 are sufficient to determine π. Taking these Rλ limitations into account, the only two known high-quality independent measurements of C0 are 5.5 and 6.4.

Internal waves and turbulence in the upper central equatorial Pacific: Lagrangian and Eulerian observations

Lien, R.-C., E.A. D'Asaro, and M.J. McPhaden, "Internal waves and turbulence in the upper central equatorial Pacific: Lagrangian and Eulerian observations," J. Phys. Oceanogr., 32, 2619-2639, 2002.

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1 Sep 2002

In the shear stratified flow below the surface mixed layer in the central equatorial Pacific, energetic near-N (buoyancy frequency) internal waves and turbulence mixing were observed by the combination of a Lagrangian neutrally buoyant float and Eulerian mooring sensors. The turbulence kinetic energy dissipation rate ε and the thermal variance diffusion rate χ were inferred from Lagrangian frequency spectral levels of vertical acceleration and thermal change rate, respectively, in the turbulence inertial subrange. Variables exhibiting a nighttime enhancement include the vertical velocity variance (dominated by near-N waves), ε, and χ. Observed high levels of turbulence mixing in this low-Ri (Richardson number) layer, the so-called deep-cycle layer, are consistent with previous microstructure measurements. The Lagrangian float encountered a shear instability event. Near-N waves grew exponentially with a 1-h timescale followed by enhanced turbulence kinetic energy and strong dissipation rate. The event supports the scenario that in the deep-cycle layer shear instability may induce growing internal waves that break into turbulence. Superimposed on few large shear-instability events were background westward-propagating near-N waves. The floats' ability to monitor turbulence mixing and internal waves was demonstrated by comparison with previous microstructure measurements and with Eulerian measurements.

Lagrangian analysis of a convective mixed layer

D'Asaro, E.A., K.B. Winters, and R.-C. Lien, "Lagrangian analysis of a convective mixed layer," J. Geophys. Res., 107, doi:10.1029/2000JC000247, 2002.

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14 May 2002

We characterize and quantify the transport of heat (Boussinesq density) in a highly idealized entraining convective mixed layer based on simulations of Lagrangian measurements in a two-dimensional model. The primary objectives are to assess and explore the merits and difficulties in estimating the heat budget from perfect and imperfect Lagrangian floats. A significant advantage of Lagrangian measurements is that the time derivative of temperature along these trajectories gives a direct measure of the diffusive heat flux. Using simulated perfect Lagrangian floats, estimates of the surface buoyancy flux, the depth of the mixed layer, vertical profiles of advective and diffusive heat flux, and the overall rate of cooling are shown to agree accurately with the known results extracted from the Eulerian simulations. The Lagrangian nature of the data is exploited to reveal the structure of the flow within the convective layer and to quantify the heat fluxes associated with the different types of eddies.

Phase plots of Lagrangian trajectories in density-depth space reveal three distinct classes of motions: (1) plumes, which develop in the cold, heavy near-surface thermal boundary layer and plunge into the mixed layer interior carrying heavy water downward; (2) interior turbulence, comprising random motions between the base of the thermal boundary layer and the base of the surface mixed layer; and (3) entrainment of interior water into plumes below the thermal boundary layer, i.e., a transition from class 2 to class 1. Plumes dominate the heat transport. Simulations were also made using slightly buoyant floats; these are not perfectly Lagrangian. Buoyancy concentrates the floats near the surface resulting in an oversampling of the stronger plumes. Making the same heat budget calculations as with the perfect floats results in a nonzero estimated Lagrangian heating rate in the interior and a curved profile of vertical heat flux that is up to 3 times too large. The local time rate change of density is not significantly affected. A correction of the heat transport for oversampling of the plumes removes most of the error. This technique allows the correct heat budget to be measured for this flow using weakly buoyant floats.

Deep convection in the Labrador Sea as observed by Lagrangian floats

Steffen, E.L., and E.A. D'Asaro, "Deep convection in the Labrador Sea as observed by Lagrangian floats," J. Phys. Oceanogr., 32, 475-492, 2002.

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1 Feb 2002

During the winters of 1997 and 1998, a total of 24 Lagrangian floats were deployed in the Labrador Sea. These floats were designed to match the buoyancy and compressibility of seawater. They measured temperature and three-dimensional position (pressure for vertical position and RAFOS acoustic tracking for latitude and longitude) as they followed water motions three-dimensionally. This data provides direct observation of mixed layer depth and excellent estimates of vertical velocity. Floats were repeatedly carried across the convecting layer by vertical velocities averaging several centimeters per second with vertical excursions of up to one kilometer. In the horizontal, several scales of eddy motion were resolved, as was a possible float predilection toward remaining in water preconditioned for convection. Heat flux estimates from this data reveal entrainment and surface heat fluxes similar in magnitude. The mixed layer acts as a vertical conveyor belt of temperature, transporting heat from depth to the surface without requiring a net change in mixed layer temperature, since incorporation of salt from below allows an increase in density without a net change in temperature. Comparison with NCEP reanalysis meteorological heat flux and wind magnitude data shows that the vertical velocity variance can be modeled with 80% skill as a linear function of lagged buoyancy flux (with the atmosphere leading the ocean by ~1/2 day) without using the wind estimates. Mixed layer motions are clearly driven by the surface buoyancy flux, Bo. A nonrotating scaling of vertical velocity variance, (BoH)1/3, provides a marginally better fit than a rotating scaling, (Bo/f)1/2. Horizontal effects appear to play only a weak role during strong convection but result in rapid restratification when convective forcing weakens.

Fully Lagrangian floats in Labrador Sea deep convection: Comparison of numerical and experimental results

Harcourt, R.R., E.L. Steffen, R.W. Garwood, and E.A. D'Asaro, "Fully Lagrangian floats in Labrador Sea deep convection: Comparison of numerical and experimental results," J. Phys. Oceanogr., 32, 493-510, doi: 10.1175/1520-0485(2002)032, 2002.

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1 Feb 2002

Measurements of deep convection from fully Lagrangian floats deployed in the Labrador Sea during February and March 1997 are compared with results from model drifters embedded in a large eddy simulation (LES) of the rapidly deepening mixed layer. The deep Lagrangian floats (DLFs) have a large vertical drag, and are designed to nearly match the density and compressibility of seawater. The high-resolution numerical simulation of deep convective turbulence uses initial conditions and surface forcing obtained from in situ oceanic and atmospheric observations made by the R/V Knorr. The response of model floats to the resolved large eddy fields of buoyancy and velocity is simulated for floats that are 5 g too buoyant, as well as for floats that are correctly ballasted. Mean profiles of potential temperature, Lagrangian rates of heating and acceleration, vertical turbulent kinetic energy (TKE), vertical heat flux, potential temperature variance, and float probability distribution functions (PDFs) are compared for actual and model floats.

Horizontally homogeneous convection, as represented by the LES model, accounts for most of the first and second order statistics from float observations, except that observed temperature variance is several times larger than model variance. There are no correspondingly large differences in vertical TKE, heat flux, or mixed layer depth. The augmented temperature variance may be due to mixing across large-scale temperature and salinity gradients that are largely compensated in buoyancy. The rest of the DLF statistics agree well with the response of correctly ballasted model floats in the lowest 75% of the mixed layer, and are less consistent with results from buoyantly ballasted model floats.

Other differences between observation and simulation in the mean profiles of heat flux, vertical TKE, and Lagrangian heating and vertical acceleration rates are confined to the upper quarter of the mixed layer. These differences are small contributions to layer-averaged quantities, but represent statistically significant profile features. Larger observed values of heat flux and vertical TKE in the upper quarter of the mixed layer are more consistent with model floats ballasted light. Float buoyancy, however, cannot fully account for the observed PDFs, temperature profiles, and Lagrangian rates of heating and acceleration. A test of Lagrangian self-consistency comparing vertical TKE and Lagrangian acceleration also shows that DLF measurements are not significantly affected by excess float buoyancy. These upper mixed layer features may instead be due to the interaction of wind-driven currents and baroclinicity.

Turbulent vertical kinetic energy in the ocean mixed layer

D'Asaro, E.A. "Turbulent vertical kinetic energy in the ocean mixed layer," J. Phys. Oceanorg., 31, 3530-3537, 2001.

1 Dec 2001

Simple suggestions for including vertical physics in oil spill models

D'Asaro, E.A. "Simple suggestions for including vertical physics in oil spill models," Spill Science and Technology, 6, 209-211, 2001.

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21 Mar 2001

Current models of oil spills include no vertical physics. They neglect the effect of vertical water motions on the transport and concentration of floating oil. Some simple ways to introduce vertical physics are suggested here. The major suggestion is to routinely measure the density stratification of the upper ocean during oil spills in order to develop a database on the effect of stratification.

The wave-turbulence transition in stratified flows

D'Asaro, E.A., and R.-C. Lien, "The wave-turbulence transition in stratified flows," J. Phys. Oceanogr., 30, 1669-1678, 2000.

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1 Jul 2000

Mixing in a stratified ocean is controlled by different physics, depending on the large-scale Richardson number. At high Richardson numbers, mixing is controlled by interactions between internal wave modes. At Richardson numbers of order 1, mixing is controlled by instabilities of the large-scale wave modes. A "wave–turbulence (W–T) transition separates these two regimes. This paper investigates the W–T transition, using observed oceanic and atmospheric spectra and parameterizations. Viewed in terms of Lagrangian (intrinsic) frequency spectra, the transition occurs when the inertial subrange of turbulence, confined to frequencies greater than the buoyancy frequency N, reaches the level of the internal waves, confined to frequencies less than N. Viewed in terms of vertical wavenumber spectra, the W–T transition occurs when the bandwidth of internal waves becomes small. Both of these singularities occur when the typical internal wave velocity becomes comparable to the phase speed of the lowest internal wave mode. At energies below that of the W–T transition, the dissipation rate varies as the energy squared; above the transition the dependence is linear. The transition occurs at lower shear and dissipation rates where the phase speed of the lowest mode is smaller, that is, in shallower water for the same stratification. Traditional turbulence closure models, which ignore internal waves, can be accurate only at energies above the W–T transition.

Lagrangian measurements of waves and turbulence in stratified flows

D'Asaro, E.A. and R.-C. Lien, "Lagrangian measurements of waves and turbulence in stratified flows," J. Phys. Oceanogr., 30, 641-655, 2000.

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1 Mar 2000

Stratified flows are often a mixture of waves and turbulence. Here, Lagrangian frequency is used to distinguish these two types of motion.

A set of 52 Lagrangian float trajectories from Knight Inlet and 10 trajectories from below the mixed layer in the wintertime northeast Pacific were analyzed using frequency spectra. A subset of 28 trajectories transit the Knight Inlet sill where energetic internal waves and strong turbulent mixing coexist.

Vertical velocity spectra show a progression from a nearly Garrett–Munk internal wave spectrum at low energies to a shape characteristic of homogeneous turbulence at high energies. All spectra show a break in slope at a frequency close to the buoyancy frequency N. Spectra from the Knight Inlet sill are analyzed in more detail. For "subbuoyant" frequencies (less than N) all 28 spectra exhibit a ratio of vertical-to-horizontal kinetic energy that varies with frequency as predicted by the linear internal wave equations. All spectra have a shape similar to that of the Garrett–Munk internal wave spectrum at subbuoyant frequencies. These motions are much more like waves than turbulence. For "superbuoyant" frequencies (greater than N) all 28 spectra are isotropic and exhibit the –2 spectral slope of inertial subrange homogeneous turbulence. These motions appear to be turbulent.

These data suggest that stratified flows may be modeled as the sum of nearly isotropic turbulence with superbuoyant Lagrangian frequencies and anisotropic internal waves with subbuoyant Lagrangian frequencies. The horizontal velocities are larger than the vertical velocities for the internal wave component but approximately equal for the turbulent component. Vertical kinetic energy is therefore a better indicator of turbulent kinetic energy than is horizontal or total kinetic energy.

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