Boundary currents along the Sri Lankan eastern and southern coasts serve as a pathway for salt exchange between the Bay of Bengal and the Arabian Sea basins in the northern Indian Ocean, which are characterized by their contrasting salinities. Measurements from two pairs of pressure-sensing inverted echo sounders (PIES) deployed along the Sri Lankan eastern and southern coasts as well as satellite measurements are used to understand the variability of these boundary currents and the associated salt transport. The volume transport in the surface (0–200-m depth) layer exhibits a seasonal cycle associated with the monsoonal wind reversal and interannual variability associated with the Indian Ocean dipole (IOD). In this layer, the boundary currents transport low-salinity water out of the Bay of Bengal during the northeast monsoon and transport high-salinity water into the Bay of Bengal during the fall monsoon transition of some years (e.g., 2015 and 2018). The Bay of Bengal salt input increases during the 2016 negative IOD as the eastward flow of high-salinity water during the fall monsoon transition intensifies, whereas the effect of the 2015/16 El Niño on the Bay of Bengal salt input is still unclear. The time-mean eddy salt flux over the upper 200 m estimated for the April 2015–March 2019 period along the eastern coast accounts for 9% of the salt budget required to balance an estimated 0.13 Sv (1 Sv ≡≡ 10⁶ m³ s^-1) of annual freshwater input into the Bay of Bengal. The time-mean eddy salt flux over the upper 200 m estimated for the December 2015–November 2019 period along the southern coast accounts for 27% of that same salt budget.

Submesoscale flows such as fronts, eddies, filaments, and instabilities with lateral dimensions between 100 m and 10 km are ubiquitous features of the ocean. They act as an intermediary between the mesoscale and small-scale turbulence and are thought to have a critical role in closing the ocean's kinetic budget by facilitating a forward energy cascade, where energy is transferred to small scales and dissipated.

The initiative uses a suite of measurements from autonomous platforms and ships combined with regional simulations to characterize the submesoscale flows in the western Pacific Ocean between Luzon and Mariana Island arcs &$151; the ARCTERX region.

Program goals are to characterize the strength and spectral properties of the turbulent cascade of kinetic energy on the submesoscales in the ARCTERX study region and understand the processes that control energy transfers across scales and their seasonal variability.

We present high-resolution sustained, persistent observations of the ocean around Sri Lanka from autonomous gliders collected over several years, a region with complex, variable circulation patterns connecting the Bay of Bengal and the Arabian Sea to each other and the rest of the Indian Ocean. The Seaglider surveys resolve seasonal to interannual variability in vertical and horizontal structure, allowing quantification of volume, heat and freshwater fluxes, as well as the transformations and transports of key water mass classes across sections normal to the east (2014–2015) and south (2016–-2019) coasts of Sri Lanka. The resulting transports point to the importance of both surface and subsurface flows and show that the direct pathway along the Sri Lankan coast plays a significant role in the exchanges of waters between Arabian Sea and Bay of Bengal. Significant section-to-section variability highlights the need for sustained, long-term observations to quantify the circulation pathways and dynamics associated with exchange between the Bay of Bengal and Arabian Sea and provides context for interpreting observations collected as 'snapshots' of more limited duration.

In seasonally ice-free parts of the Arctic Ocean, autumn is characterized by heat loss from the upper ocean to the atmosphere and the onset of freeze up, in which first year sea ice begins to grow in open water areas. The timing of freeze up can be highly spatially variable, complicating efforts to provide accurate sea ice forecasting for marine operations. While melt season anomalies can be used to predict freeze up anomalies in some parts of the Arctic, this one-dimensional view merits further examination in light of recent work demonstrating the importance of three-dimensional flows in setting mixed layer properties in marginal ice zones. In this study, we show that horizontal advection of sea ice meltwater hastens freeze up in areas distant from the ice edge. We use nearly 800 temperature and salinity profiles along with satellite imagery collected in the central Beaufort Sea in autumn 2018 to document the roughly 100 km advection of a cold and fresh surface meltwater layer over several weeks. After the meltwater arrived, the mixed layer was cooler and shallower than the mixed layer in adjacent areas unaffected by the meltwater. The cooler and shallower meltwater-influenced mixed layer promoted earlier ice formation. Within the meltwater-affected area, advection was nearly as important as heat loss to the atmosphere for seasonally integrated mixed layer heat loss.

The NORSE DRI focuses on characterizing the key physical parameters and processes that govern the predictability of upper-ocean rapid evolution events occurring in the ice-free high latitudes. The goal is to identify which observable parameters are most influential in improving model predictability through inclusion by assimilation, and to field an autonomous observing network that optimizes sampling of high-priority fields. The overall goal is to demonstrate improvements in the predictability of the upper ocean physical fields associated with acoustic propagation over the course of the study. This Science Plan describes the specific objectives and implementation plan.

Observations of ocean surface waves at three sites along the northern coast of Alaska show a strong coupling with seasonal sea ice patterns. In the winter, ice cover is complete, and waves are absent. In the spring and early summer, sea ice retreats regionally, but landfast ice persists near the coast. The landfast ice completely attenuates waves formed farther offshore in the open water, causing up to two-month delay in the onset of waves nearshore. In autumn, landfast ice begins to reform, though the wave attenuation is only partial due to lower ice thickness compared to spring. The annual cycle in the observations is reproduced by the ERA5 reanalysis product, but the product does not resolve landfast ice. The resulting ERA5 bias in coastal wave exposure can be corrected by applying a higher resolution ice mask, and this has a significant effect on the long-term trends inferred from ERA5.

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air-sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the US, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air-sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ~20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s^-1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10–12 m s^-1 wind and evaporation of 0.2 mm h^-1. The evolving environmental state included a deepening ocean mixed layer (from ~20 to 50 m), cooling SST (by ~ 1°C), and warming/drying of the lower to mid-troposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air-sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.

Baroclinic seamount-trapped waves are thought to influence their surrounding ecosystem; however, trapped waves are not well-studied in near-equatorial settings, where stratification is strong and Burger numbers are large. Motivated by observations, we use daily output (2005–2009) from the global 0.1° Parallel Ocean Program Model (POP) to examine topographically trapped baroclinic waves around the Seychelles Plateau in the tropical Indian Ocean. These trapped waves are associated with velocity and temperature oscillations at periods of 15–16 days, similar to the dominant period of some equatorial Yanai waves. Energy flux maps using POP output suggest that quasi-biweekly equatorial Yanai waves excite trapped waves on the western and south-western flanks of the Seychelles Plateau, near the surface. The anticyclonic energy flux associated with the trapped wave extends vertically throughout the water column and around most of the plateau circumference, diminishing on the eastern flank of the plateau. This work highlights the role that equatorial planetary waves and trapped waves play in facilitating energy redistribution, dissipation, and mixing in the tropical ocean.

Unprecedented quantities of heat are entering the Pacific sector of the Arctic Ocean through Bering Strait, particularly during summer months. Though some heat is lost to the atmosphere during autumn cooling, a significant fraction of the incoming warm, salty water subducts (dives beneath) below a cooler fresher layer of near-surface water, subsequently extending hundreds of kilometers into the Beaufort Gyre. Upward turbulent mixing of these sub-surface pockets of heat is likely accelerating sea ice melt in the region. This Pacific-origin water brings both heat and unique biogeochemical properties, contributing to a changing Arctic ecosystem. However, our ability to understand or forecast the role of this incoming water mass has been hampered by lack of understanding of the physical processes controlling subduction and evolution of this this warm water. Crucially, the processes seen here occur at small horizontal scales not resolved by regional forecast models or climate simulations; new parameterizations must be developed that accurately represent the physics. Here we present novel high resolution observations showing the detailed process of subduction and initial evolution of warm Pacific-origin water in the southern Beaufort Gyre.

Understanding and predicting sea ice dynamics and ice‐ocean feedback processes requires accurate descriptions of momentum fluxes across the ice‐ocean interface. In this study, we present observations from an array of moorings in the Beaufort Sea. Using a force‐balance approach, we determine ice‐ocean drag coefficient values over an annual cycle and a range of ice conditions. Statistics from high resolution ice draft measurements are used to calculate expected drag coefficient values from morphology‐based parameterization schemes. With both approaches, drag coefficient values ranged from approximately 1–10 x 10^-3, with a minimum in fall and a maximum at the end of spring, consistent with previous observations. The parameterizations do a reasonable job of predicting the observed drag values if the under ice geometry is known, and reveal that keel drag is the primary contributor to the total ice‐ocean drag coefficient. When translations of bulk model outputs to ice geometry are included in the parameterizations, they overpredict drag on floe edges, leading to the inverted seasonal cycle seen in prior models. Using these results to investigate the efficiency of total momentum flux across the atmosphere‐ice‐ocean interface suggests an inter‐annual trend of increasing coupling between the atmosphere and the ocean.

The ocean circulation around and over the Seychelles Plateau (SP) is characterized using 35 months of temperature and velocity measurements along with a numerical model. The results here provide the first documented description of the ocean circulation atop the SP. The SP is an unusually broad (~200 km), shallow (~50 m) plateau, dropping off steeply to the abyss. It is situated in a dynamic location (3.5–5.5°S, 54–57°E) in the south‐western tropical Indian Ocean where northwesterly winds are present during austral summer and become southeasterly in austral winter, following the reversal of the western Indian ocean monsoon winds. Measurements around the Inner Islands, on the SP, have been carried out since 2015. Velocity measurements show that most of the depth‐averaged current variance on the SP arises from near‐inertial oscillations and lower‐frequency variability. Lower‐frequency variability encompasses seasonal and intraseasonal variability, the latter of which includes the effects of mixed Rossby‐gravity waves and mesoscale eddies. A global 0.1° numerical ocean simulation is used in conjunction with these observations to describe the regional circulation around the SP. Atop the SP, circulation is dominated by ageostrophic processes consistent with Ekman dynamics, while around the SP, both geostrophic and ageostrophic processes are important and vary seasonally. Stratification responds to the sea surface height semiannual signal which is due to Ekman pumping‐driven upwelling (related to the Seychelles–‐Chagos Thermocline Ridge) and the arrival of an annual downwelling Rossby wave.

Alaskan Arctic coastlines are protected seasonally from ocean waves by the presence of coastal and shorefast sea ice. This study presents field observations collected during the autumn 2019 freeze up near Icy Cape, a coastal headland in the Chukchi Sea of the Western Arctic. The evolution of the coupled air‐ice‐ocean‐wave system during a four‐day wave event was monitored using drifting wave buoys, a cross‐shore mooring array, and ship‐based measurements. The incident wave field with peak period of 2.5 s was attenuated by coastal pancake and frazil sea ice, reducing significant wave height by 40% over less than 5 km of cross‐shelf distance spanning water depths from 13 to 30 m. Spectral attenuation coefficients are evaluated with respect to wave and ice conditions and the proximity to the ice edge. Attenuation rates are found to be three times higher within 500 m of the ice edge, relative to values farther in the ice cover. Attenuation coefficients are in the range of <2.3,2.7> m^-1, and follow a power‐law dependence on frequency.

Fast-moving synoptic-scale atmospheric disturbances produce large-scale near-inertial waves in the ocean mixed layer. In this paper, we analyze the distortion of such waves by smaller-scale barotropic eddies, with a focus on the evolution of the horizontal wavevector k under the effects of straining and refraction. The model is initialized with a horizontally uniform (k = 0) surface-confined near-inertial wave, which then evolves according to the phase-averaged model of Young and Ben Jelloul. A steady barotropic vortex dipole is first considered. Shear bands appear in the jet region as wave energy propagates downward and toward the anticyclone. When measured at a fixed location, both horizontal and vertical wavenumbers grow linearly with the time t elapsed since generation such that their ratio, the slope of wave bands, is time independent. Analogy with passive scalar dynamics suggests that straining should result in the exponential growth of |k|. Here instead, straining is ineffective, not only at the jet center, but also in its confluent and diffluent regions. Low modes rapidly escape below the anticyclonic core such that weakly dispersive high modes dominate in the surface layer. In the weakly dispersive limit, k = –t∇ζ(x, y, t)/2 provided that (i) the eddy vertical vorticity ζ evolves according to the barotropic quasigeostrophic equation and (ii) k = 0 initially. In steady flows, straining is ineffective because k is always perpendicular to the flow. In unsteady flows, straining modifies the vorticity gradient and hence k, and may account for significant wave–eddy energy transfers.

To successfully operate in a harsh environment like the Arctic Ocean, one must be able to understand and predict how that environment will evolve over different spatial and temporal scales. This is particularly challenging given the on-going and significant environmental changes that are occurring in the region. Access to the most recent environmental information provides timely knowledge that enables ship-based operations to proceed efficiently, effectively and safely in this difficult arena. Knowledge of the evolving environmental conditions during a field campaign is critical for effective planning, optimal execution of sampling strategies, and to provide a broader context to data collected at specific times and places. We describe the collaborations and processes that enabled an operational system to be developed to provide a remote field-team, located on USCGC Healy in the Beaufort Sea, with near real-time situational awareness information regarding the weather, sea ice conditions, and oceanographic processes. The developed system included the punctual throughput of near real-time products such as satellite imagery, meteorological forecasts, ice charts, model outputs, and up to date locations of key sea ice and ocean-based assets. Science and operational users, as well as onshore personnel, used this system for real-time practical considerations such as ship navigation, and to time scientific operations to ensure the appropriate sea ice and weather conditions prevailed. By presenting the outputs of the system within the context of case studies our results clearly demonstrate the benefits that improved situational awareness brings to ship-based operations in the Arctic Ocean, both today and in the future.

Generated at large horizontal scales by winds, near‐inertial waves (NIWs) are inefficient at radiating energy without a shift to smaller wavelengths. The lateral scales of NIWs can be reduced by gradients in the Coriolis parameter (β‐refraction) or in the vertical vorticity (ζ‐refraction) or by strain. Here we present ship‐based surveys of NIWs in a dipole vortex in the Iceland Basin that show, for the first time, direct evidence of ζ‐refraction. Differences in NIW phase across the dipole were observed to grow in time, generating a lateral wavelength that shrank at a rate consistent with ζ‐refraction, reaching ~40 km in 1.5 days. Two days later, a NIW beam with an ~13 km horizontal and ~200 m vertical wavelength was detected at depth radiating energy downward and toward the dipole's anticyclone. Strain, while significant in strength in the dipole, had little direct effect on the NIWs.

The high degree of heterogeneity in the ice–ocean–atmosphere system in marginal ice zones leads to a complex set of dynamics which control fluxes of heat and buoyancy in the upper ocean. Strong fronts may occur near the ice edge between the warmer waters of the ice-free regions and the cold, fresh waters near and under the ice. This study presents observations of a well-defined density front located along the ice edge in the Beaufort Sea. The evolution of the front over a ~3-day survey period is captured by multiple cross-front sections measured using an underway conductivity–temperature–depth system, with simultaneous measurements of atmospheric forcing. Synthetic aperture radar images bookending this period show that the ice edge itself underwent concurrent evolution. Prior to the survey, the ice edge was compact and well defined while after the survey it was diffuse and filamented with coherent vortical structures. This transformation might be indicative of the development an active ocean eddy field in the upper ocean mixed layer. Over the course of hours, increasing wind stress is correlated with changes to the lateral buoyancy gradient and frontogenesis. Frontal dynamics appear to vary from typical open-ocean fronts (e.g., here the frontogenesis is linked to an "up-front" wind stress). Convective and shear-driven mixing appear to be unable to describe deepening at the heel of the front. While there was no measurable spatial variation in wind speed, we hypothesize that spatial heterogeneity in the total surface stress input, resulting from varying ice conditions across the marginal ice zone, may be a driver of the observed behaviour.

The OceanGliders program started in 2016 to support active coordination and enhancement of global glider activity. OceanGliders contributes to the international efforts of the Global Ocean Observation System (GOOS) for Climate, Ocean Health, and Operational Services. It brings together marine scientists and engineers operating gliders around the world: (1) to observe the long-term physical, biogeochemical, and biological ocean processes and phenomena that are relevant for societal applications; and, (2) to contribute to the GOOS through real-time and delayed mode data dissemination. The OceanGliders program is distributed across national and regional observing systems and significantly contributes to integrated, multi-scale and multi-platform sampling strategies. OceanGliders shares best practices, requirements, and scientific knowledge needed for glider operations, data collection and analysis. It also monitors global glider activity and supports the dissemination of glider data through regional and global databases, in real-time and delayed modes, facilitating data access to the wider community. OceanGliders currently supports national, regional and global initiatives to maintain and expand the capabilities and application of gliders to meet key global challenges such as improved measurement of ocean boundary currents, water transformation and storm forecast.

SPURS-2 (Salinity Processes in the Upper-ocean Regional Study 2) used the schooner Lady Amber, a small sailing research vessel, to deploy, service, maintain, and recover a variety of oceanographic and meteorological instruments in the eastern Pacific Ocean. Low operational costs allowed us to frequently deploy floats and drifters to collect data necessary for resolving the regional circulation of the eastern tropical Pacific. The small charter gave us the opportunity to deploy drifters in locations chosen according to current conditions, to recover and deploy various autonomous instruments in a targeted and adaptive manner, and to collect additional near-surface and atmospheric measurements in the remote SPURS-2 region.

A large collaborative program has studied the coupled air‐ice‐ocean‐wave processes occurring in the Arctic during the autumn ice advance. The program included a field campaign in the western Arctic during the autumn of 2015, with in situ data collection and both aerial and satellite remote sensing. Many of the analyses have focused on using and improving forecast models. Summarizing and synthesizing the results from a series of separate papers, the overall view is of an Arctic shifting to a more seasonal system. The dramatic increase in open water extent and duration in the autumn means that large surface waves and significant surface heat fluxes are now common. When refreezing finally does occur, it is a highly variable process in space and time. Wind and wave events drive episodic advances and retreats of the ice edge, with associated variations in sea ice formation types (e.g., pancakes, nilas). This variability becomes imprinted on the winter ice cover, which in turn affects the melt season the following year.

The Arctic ice cover influences the generation, propagation, and dissipation of internal waves, which in turn may affect vertical mixing in the ocean interior. The Arctic internal wavefield and its relationship to the ice cover is investigated using observations from Ice‐Tethered Profilers with Velocity and Seaglider sampling during the 2014 Marginal Ice Zone experiment in the Canada Basin. Ice roughness, ice concentration, and wind forcing all influenced the daily to seasonal changes in the internal wavefield. Three different ice concentration thresholds appeared to determine the evolution of internal wave spectral energy levels: (1) the initial decrease from 100% ice concentration after which dissipation during the surface reflection was inferred to increase, (2) the transition to 70–80% ice concentration when the local generation of internal waves increased, and (3) the transition to open water that was associated with larger‐amplitude internal waves. Ice roughness influenced internal wave properties for ice concentrations greater than approximately 70–80%: smoother ice was associated with reduced local internal wave generation. Richardson numbers were rarely supercritical, consistent with weak vertical mixing under all ice concentrations. On decadal timescales, smoother ice may counteract the effects of lower ice concentration on the internal wavefield complicating future predictions of internal wave activity and vertical mixing.

The collection and processing of shipboard air, ice, and ocean measurements from the Sea State field campaign in the Beaufort/Chukchi Seas in autumn 2015 are described and the data used to characterize the near‐surface freezeup environment. The number of parameters measured or derived is large and the location and time of year are unique. Analysis was done of transits through the new, growing ice and of ice edge periods. Through differential surface energy fluxes, the presence of new, thin sea ice (<50 cm) produces lower tropospheric air temperatures in the ice interior that average ~4°C colder than those over open water near the ice edge, resulting in an ice edge baroclinic zone. This temperature difference doubles by late October and produces thermodynamic and dynamic feedbacks. These include off‐ice, cold‐air advection leading to enhanced surface heat loss averaging ~200 W/m² over the open water, formation of low‐level jets, suppression of the ice edge baroclinic zone, and enhanced ice drift. The interior ice growth rate is thermodynamically consistent with a surface heat loss of ~65 W/m² to the atmosphere and a heat flux of several tens of W/m² from the ocean below. Ice drift at times contributes to the southward advance of the autumn ice edge through off‐ice winds. The ocean thermohaline structure is highly variable and appears associated with bathymetric features, small‐scale upper‐ocean eddies, and the growing ice cover. Lower salinity under the ice interior compared to the nearby ice edge is an upper‐ocean impact of this thin ice cover.

Low-mode internal tides, a dominant part of the internal wave spectrum, carry energy over large distances, yet the ultimate fate of this energy is unknown. Internal tides in the Tasman Sea are generated at Macquarie Ridge, south of New Zealand, and propagate northwest as a focused beam before impinging on the Tasmanian continental slope. In situ observations from the Tasman Sea capture synoptic measurements of the incident semidiurnal mode-1 internal-tide, which has an observed wavelength of 183 km and surface displacement of approximately 1 cm. Plane-wave fits to in situ and altimetric estimates of surface displacement agree to within a measurement uncertainty of 0.3 cm, which is the same order of magnitude as the nonstationary (not phase locked) mode-1 tide observed over a 40-day mooring deployment. Stationary energy flux, estimated from a plane-wave fit to the in situ observations, is directed toward Tasmania with a magnitude of 3.4 ± 1.4 kW m^-1, consistent with a satellite estimate of 3.9 ± 2.2 kW m^-1. Approximately 90% of the time-mean energy flux is due to the stationary tide. However, nonstationary velocity and pressure, which are typically 1/4 the amplitude of the stationary components, sometimes lead to instantaneous energy fluxes that are double or half of the stationary energy flux, overwhelming any spring–neap variability. Despite strong winds and intermittent near-inertial currents, the parameterized turbulent-kinetic-energy dissipation rate is small (i.e., 10^-10 W kg^-1) below the near surface and observations of mode-1 internal tide energy-flux convergence are indistinguishable from zero (i.e., the confidence intervals include zero), indicating little decay of the mode-1 internal tide within the Tasman Sea.

High‐resolution measurements of the air‐ice‐ocean system during an October 2015 event in the Beaufort Sea demonstrate how stored ocean heat can be released to temporarily reverse seasonal ice advance. Strong on‐ice winds over a vast fetch caused mixing and release of heat from the upper ocean. This heat was sufficient to melt large areas of thin, newly formed pancake ice; an average of 10 MJ/m² was lost from the upper ocean in the study area, resulting in ~3–5 cm pancake sea ice melt. Heat and salt budgets create a consistent picture of the evolving air‐ice‐ocean system during this event, in both a fixed and ice‐following (Lagrangian) reference frame. The heat lost from the upper ocean is large compared with prior observations of ocean heat flux under thick, multi‐year Arctic sea ice. In contrast to prior studies, where almost all heat lost goes into ice melt, a significant portion of the ocean heat released in this event goes directly to the atmosphere, while the remainder (~30–40%) goes into melting sea ice. The magnitude of ocean mixing during this event may have been enhanced by large surface waves, reaching nearly 5 m at the peak, which are becoming increasingly common in the autumn Arctic Ocean. The wave effects are explored by comparing the air‐ice‐ocean evolution observed at short and long fetches, and a common scaling for Langmuir turbulence. After the event, the ocean mixed layer was deeper and cooler, and autumn ice formation resumed.

Variability of the Kuroshio east of Taiwan was observed at a cross-stream transect ~ 50 km south of the PCM-1 line with an array of three moored ADCPs measuring for ~ 23 months, supplemented with eleven repeated shipboard surveys. Observations of the Kuroshio's velocity structure reveal the absence of an obvious regular seasonal signal, but significant variability at 70–200 day period for both maximum velocity axis migration and transport due to interactions with mesoscale eddies. Empirical orthogonal function (EOF) analysis shows the migration and transport modes explain 46% and 29% of the total variance, respectively, which is in contrast to the findings at the PCM-1 line where the transport mode explained more variance than did the migration mode. The Kuroshio transport in the upper 500 m across a 150 km section is 17.2 Sv with a standard deviation of 5 Sv. The estimated Kuroshio transport is 4.3 Sv lower than that reported for the PCM-1 line, likely due to the interannual variations related to abundance of mesoscale eddies in the Subtropical Counter Current (STCC) region. Transport variability east of Taiwan is mostly caused by Kuroshio-eddy interactions. When single anticyclonic (cyclonic) eddies encounter the Kuroshio, they enhance (reduce) poleward transport, presumably by increasing (decreasing) the sea level anomaly (SLA) along the eastern flank of the Kuroshio (correlation = 0.82). When a pair of eddies impinges on the Kuroshio, the upstream confluence and diffluence caused by the dipole eddies increases and decreases the Kuroshio transport, respectively. Furthermore, the eastward (westward) currents that result from either the single eddy or the dipole eddy produce flow divergence (convergence) adjacent to the Kuroshio's eastern edge, favoring the offshore (onshore) migration of the Kuroshio axis.

The existence of a seasonally varying undercurrent along 8°N off the east coast of Sri Lanka is inferred from shipboard hydrography, Argo floats, glider measurements, and two ocean general circulation model simulations. Together, they reveal an undercurrent below 100–200 m flowing in the opposite direction to the surface current, which is most pronounced during boreal spring and summer and switches direction between these two seasons. The volume transport of the undercurrent (200–1000 m layer) can be more than 10 Sv in either direction, exceeding the transport of 1–6 Sv carried by the surface current (0–200 m layer). The undercurrent transports relatively fresher water southward during spring, while it advects more saline water northward along the east coast of Sri Lanka during summer. Although the undercurrent is potentially a pathway of salt exchange between the Arabian Sea and the Bay of Bengal, the observations and the ocean general circulation models suggest that the salinity contrast between seasons and between the boundary current and interior is less than 0.09 in the subsurface layer, suggesting a small salt transport by the undercurrent of less than 4% of the salinity deficit in the Bay of Bengal.

The Salinity Processes in the Upper-ocean Regional Study (SPURS) aims to understand the patterns and variability of sea surface salinity. In order to capture the wide range of spatial and temporal scales associated with processes controlling salinity in the upper ocean, research vessels delivered autonomous instruments to remote sites, one in the North Atlantic and one in the Eastern Pacific. Instruments sampled for one complete annual cycle at each of these two sites, which are subject to contrasting atmospheric forcing. The SPURS field programs coordinated sampling from many different platforms, using a mix of Lagrangian and Eulerian approaches. This article discusses the motivations, implementation, and first results of the SPURS-1 and SPURS-2 programs.

Ocean turbulence is inherently episodic and patchy. It is the primary mechanism that transforms water mass properties and drives the exchanges of heat, freshwater, and momentum across the water column. Given its episodic nature, capturing the net impact of turbulence via direct measurements requires sustained observations over extended temporal and/or broad spatial scales.

The Arabian Sea circulation is forced by strong monsoonal winds and is characterized by vigorous seasonally reversing currents, extreme differences in sea surface salinity, localized substantial upwelling, and widespread submesoscale thermohaline structures. Its complicated sea surface temperature patterns are important for the onset and evolution of the Asian monsoon. This article describes a program that aims to elucidate the role of upper-ocean processes and atmospheric feedbacks in setting the sea surface temperature properties of the region. The wide range of spatial and temporal scales and the difficulty of accessing much of the region with ships due to piracy motivated a novel approach based on state-of-the-art autonomous ocean sensors and platforms. The extensive data set that is being collected, combined with numerical models and remote sensing data, confirms the role of planetary waves in the reversal of the Somali Current system. These data also document the fast response of the upper equatorial ocean to monsoon winds through changes in temperature and salinity and the connectivity of the surface currents across the northern Indian Ocean. New observations of thermohaline interleaving structures and mixing in setting the surface temperature properties of the northern Arabian Sea are also discussed.

Autonomous and Lagrangian platforms and sensors (ALPS) have revolutionized the way the subsurface ocean is observed. The synergy between ALPS-based observations and coupled ocean-sea ice state and parameter estimation as practiced in the Arctic Subpolar gyre sTate Estimate (ASTE) project is illustrated through several examples. In the western Arctic, Ice-Tethered Profilers have been providing important hydrographic constraints of the water column down to 800 m depth since 2004. ASTE takes advantage of these detailed constraints to infer vertical profiles of diapycnal mixing rates in the central Canada Basin. The state estimation framework is also used to explore the potential utility of Argo-type floats in regions with sparse data coverage, such as the eastern Arctic and the seasonal ice zones. Finally, the framework is applied to identify potential deployment sites that optimize the impact of float measurements on bulk oceanographic quantities of interest.

Air–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km-wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (~300 km wide) that carries high-salinity water into BoB during the northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden–Julian oscillation (MJO) permit the study of multiscale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air–sea interactions control the ABL and upper-ocean processes.

Vertical and lateral water properties and density structure within the Arctic Ocean are intimately related to the ocean circulation, and have profound consequences for sea ice growth and retreat as well as for propagation of acoustic energy at all scales. Our current understanding of the dynamics governing arctic upper ocean stratification and circulation derives largely from a period when extensive ice cover modulated the oceanic response to atmospheric forcing. Recently, however, there has been significant arctic warming, accompanied by changes in the extent, thickness distribution, and properties of the arctic sea ice cover. The need to understand these changes and their impact on arctic stratification and circulation, sea ice evolution, and the acoustic environment motivate the Office of Naval Research (ONR) Stratified Ocean Dynamics of the Arctic Departmental Research Initiative.

The region surrounding Sri Lanka modulates monsoon-driven exchange between the Bay of Bengal and the Arabian Sea. Here, local circulation impacts the pathways followed by the boundary currents that drive exchange, thereby modulating mixing and water mass transformation. From 2013 to 2016, an international partnership conducted sustained measurements around the periphery of Sri Lanka, with the goal of understanding how circulation and mixing in this critical region modulate exchange between the Bay of Bengal and the Arabian Sea. Observations from satellite remote sensing, surface drifters, gliders, current meter moorings, and Pressure Inverted Echo Sounders capture seasonally reversing monsoon currents off the southern tip of Sri Lanka, trace the wintertime freshwater export pathway of the East India Coastal Current, and document the deflection of currents running along the east coast of Sri Lanka by cyclonic and anticyclonic eddies. Measurements also reveal energetic interleaving, indicative of mixing and stirring associated with these flows. Circulation inferred from satellite remote sensing and drifter tracks sometimes differs from that indicated by in situ sections, pointing to the need for observing systems that employ complementary approaches toward understanding this region.

A global map of open-ocean mode-1 M₂ internal tides is constructed using sea-surface height (SSH) measurements from multiple satellite altimeters during 1992–2012, representing a 20-year coherent internal tide field. A two-dimensional plane wave fit method is employed to (1) suppress mesoscale contamination by extracting internal tides with both spatial and temporal coherence, and (2) separately resolve multiple internal tidal waves. Global maps of amplitude, phase, energy and flux of mode-1 M₂ internal tides are presented. M₂ internal tides are mainly generated over topographic features including continental slopes, mid-ocean ridges and seamounts. Internal tidal beams of 100–300 km width are observed to propagate hundreds to thousands of km. Multi-wave interference of some degree is widespread, due to the M₂ internal tide's numerous generation sites and long-range propagation. The M₂ internal tide propagates across the critical latitudes for parametric subharmonic instability (28.8°S/N) with little energy loss, consistent with field measurements by MacKinnon et al. (2013). In the eastern Pacific Ocean, the M₂ internal tide loses significant energy in propagating across the Equator; in contrast, little energy loss is observed in the equatorial zones of the Atlantic, Indian, and western Pacific oceans. Global integration of the satellite observations yields a total energy of 36 PJ (1 PJ = 10¹⁵ J) for the coherent mode-1 M₂ internal tide. The satellite observed M₂ internal tides compare favorably with field mooring measurements and a global eddy-resolving numerical model.

Given the ubiquity of layering in environmental stratifications, an interesting example being double-diffusive staircase structures in the Arctic Ocean, we present the results of a joint theoretical and laboratory experimental study investigating the impact of multiple layering on internal wave propagation. We first present results for a simplified model that demonstrates the non-trivial impact of multiple layering. Thereafter, utilizing a weakly viscous linear model that can handle arbitrary vertical stratifications, we perform a comparison of theory with experiments. We conclude by applying this model to a case study of a staircase stratification profile obtained from the Arctic Ocean, finding a rich landscape of transmission behaviour.

The dynamics of the wind-generated near-inertial internal wave field in the Canada Basin of the Arctic Ocean are investigated using the drifting Ice-Tethered Profiler dataset for the years 2005 to 2014, during a decade when sea ice extent and thickness decreased dramatically. This time series, with nearly 10 years of measurements and broad spatial coverage, is used to quantify a seasonal cycle and interannual trend for internal waves in the Arctic, using estimates of the amplitude of near-inertial waves derived from isopycnal displacements. The internal wave field is found to be most energetic in summer when sea ice is at a minimum, with a second maximum in early winter during the period of maximum wind speed. Amplitude distributions for the near-inertial waves are quantifiably different during summer and winter, due primarily to seasonal changes in sea ice properties that affect how the ice responds to the wind, which can be expressed through the "wind factor" — the ratio of sea ice drift speed to wind speed. A small positive interannual trend in near-inertial wave energy is linked to pronounced sea ice decline during the last decade. Overall variability in the internal wave field increases significantly over the second half of the record, with an increased probability of larger-than-average waves in both summer and winter. This change is linked to an overall increase in variability in the wind factor and sea ice drift speeds, and reflects a shift in year-round sea ice characteristics in the Arctic, with potential implications for dissipation and mixing associated with internal waves.

The spatial structure of the semidiurnal internal tide in the vicinity of Tasmania is characterized using temperature and salinity data from Seaglider and Slocum glider deployments. Wavelet analysis of isopycnal displacements measured by the gliders was used to isolate the semidiurnal internal tide, with a solid signal observed both to the east and to the south of Tasmania. The signal south of Tasmania was attributed to local forcing, while that to the east of Tasmania was found to have propagated from the south east to the north west — a result which supports previous studies indicating the presence of an internal tidal beam originating over the Macquarie Ridge, south of New Zealand. Displacement amplitudes were observed to be amplified in the vicinity of the continental slope, with the incoming tidal beam shown to be both reflected and scattered on the continental slope and shelf, and energy transferred to higher modes.

Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis1, sediment and pollutant transport2 and acoustic transmission3; they also pose hazards for man-made structures in the ocean4. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking5, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects6, 7. For over a decade, studies8, 9, 10, 11 have targeted the South China Sea, where the oceans’ most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.

One part of the Salinity Processes in the Upper-ocean Regional Study (SPURS) field campaign focused on understanding the physical processes affecting the evolution of upper-ocean salinity in the region of climatological maximum sea surface salinity in the subtropical North Atlantic (SPURS-1). An upper-ocean salinity budget provides a useful framework for increasing this understanding. The SPURS-1 program included a central heavily instrumented mooring for making accurate measurements of air-sea surface fluxes, as well as other moorings, Argo floats, and gliders that together formed a dense observational array. Data from this array are used to estimate terms in the upper-ocean salinity and heat budgets during the SPURS-1 campaign, with a focus on the first several months (October 2012 to February 2013) when the surface mixed layer was becoming deeper, fresher, and cooler. Specifically, we examine the salinity and temperature balances for an upper-ocean mixed layer, defined as the layer where the density is within 0.4 kg m^-3 of its surface value. The gross features of the evolution of upper-ocean salinity and temperature during this fall/winter season are explained by a combination of evaporation and precipitation at the sea surface, horizontal transport of heat and salt by mixed-layer currents, and vertical entrainment of fresher, cooler fluid into the layer as it deepened. While all of these processes were important in the observed seasonal (fall) freshening at this location in the salinity-maximum region, the variability of salinity on monthly-to-intraseasonal time scales resulted primarily from horizontal advection.

Recent receding of the ice pack allows more sunlight to penetrate into the Arctic Ocean, enhancing productivity of a single annual phytoplankton bloom. Increasing river runoff may, however, enhance the yet pronounced upper ocean stratification and prevent any significant wind-driven vertical mixing and upward supply of nutrients, counteracting the additional light available to phytoplankton. Vertical mixing of the upper ocean is the key process that will determine the fate of marine Arctic ecosystems. Here we reveal an unexpected consequence of the Arctic ice loss: regions are now developing a second bloom in the fall, which coincides with delayed freezeup and increased exposure of the sea surface to wind stress. This implies that wind-driven vertical mixing during fall is indeed significant, at least enough to promote further primary production. The Arctic Ocean seems to be experiencing a fundamental shift from a polar to a temperate mode, which is likely to alter the marine ecosystem.

The response of upper ocean currents to Typhoon Fanapi in fall 2010 was studied using an extensive air-deployed drifter array. Separation of the observations into near-inertial and sub-inertial motions quantified the importance of strong advection by the sub-inertial circulation for the evolution of the cold wake formed by Typhoon Fanapi. The near-inertial currents generated during the storm showed the expected rightward bias, with peak magnitudes of up to 0.6 m/s and an e-folding time of about 4 days for the strong currents within the cold wake. The shear of the near-inertial currents is crucial for the storm-induced cooling and deepening of the mixed layer and such instabilities were here directly observed across the base of th — a dominant process for the wake warming — was found to be noticeably reduced when the near-inertial motions were strongest.

Mooring measurements from the Kuroshio Extension System Study (June 2004–June 2006) and from the ongoing Kuroshio Extension Observatory (June 2004–present) are combined with float measurements of the Argo network to study the variability of the North Pacific Subtropical Mode Water (STMW) across the entire gyre, on time scales from days, to seasons, to a decade. The top of the STMW follows a seasonal cycle, although observations reveal that it primarily varies in discrete steps associated with episodic wind events. The variations of the STMW bottom depth are tightly related to the sea surface height (SSH), reflecting mesoscale eddies and large-scale variations of the Kuroshio Extension and recirculation gyre systems. Using the observed relationship between SSH and STMW, gridded SSH products and in situ estimates from floats are used to construct weekly maps of STMW thickness, providing nonbiased estimates of STMW total volume, annual formation and erosion volumes, and seasonal and interannual variability for the past decade. Year-to-year variations are detected, particularly a significant decrease of STMW volume in 2007–10 primarily attributable to a smaller volume formed. Variability of the heat content in the mode water region is dominated by the seasonal cycle and mesoscale eddies; there is only a weak link to STMW on interannual time scales, and no long-term trends in heat content and STMW thickness between 2002 and 2011 are detected. Weak lagged correlations among air–sea fluxes, oceanic heat content, and STMW thickness are found when averaged over the northwestern Pacific recirculation gyre region.

Salinity and temperature profiles from drifting ice-tethered profilers in the Beaufort gyre region of the Canada Basin are used to characterize and quantify the regional near-inertial internal wave field over one year. Vertical displacements of potential density surfaces from the surface to 750-m depth are tracked from fall 2006 to fall 2007. Because of the time resolution and irregular sampling of the ice-tethered profilers, near-inertial frequency signals are marginally resolved. Complex demodulation is used to determine variations with a time scale of several days in the amplitude and phase of waves at a specified near-inertial frequency. Characteristics and variability of the wave field over the course of the year are investigated quantitatively and related to changes in surface wind forcing and sea ice cover.

The vertical isopycnal displacements associated with internal waves generated by the barotropic tidal currents in the vicinity of Luzon Strait are estimated using measurements collected by autonomous underwater gliders. Nearly 23,000 profiles from Seagliders and Spray gliders, collected during 29 different missions since 2007, are used to estimate the amplitude and phase of the linear semidiurnal and diurnal internal waves in this energetic region, particularly in the previously poorly sampled area near the eastern ridge and on the Pacific side of Luzon Strait. The mean and variability of the internal wave field in the upper 1000 m of the water column are described. The phase progression of internal waves as they propagate away from their generation sites is captured directly. The glider-based observations are used to map the mode-1 semidiurnal and diurnal internal wave fields, providing the baroclinic energy flux over a roughly 600 km x 800 km region based strictly on in situ observations.

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.

Data from the Kuroshio Extension Observatory (KEO) surface mooring are used to analyze the balance of processes affecting the upper ocean heat content and surface mixed layer temperature variations in the Recirculation Gyre (RG) south of the Kuroshio Extension (KE). Cold and dry air blowing across the KE and its warm RG during winter cause very large heat fluxes out of the ocean that result in the erosion of the seasonal thermocline in the RG. Some of this heat is replenished through horizontal heat advection, which may enable the seasonal thermocline to begin restratifying while the net surface heat flux is still acting to cool the upper ocean. Once the surface heat flux begins warming the ocean, restratification occurs rapidly due to the low thermal inertia of the shallow mixed layer depth. Enhanced diffusive mixing below the mixed layer tends to transfer some of the mixed layer heat downward, eroding and potentially modifying sequestered subtropical mode water and even the deeper waters of the main thermocline during winter. Diffusivity at the base of the mixed layer, estimated from the residual of the mixed layer temperature balance, is roughly 3x10^-4 m²/s during the summer and up to two orders of magnitude larger during winter. The enhanced diffusivities appear to be due to large inertial shear generated by wind events associated with winter storms and summer tropical cyclones. The diffusivity's seasonality is likely due to seasonal variations in stratification just below the mixed layer depth, which is large during the summer when the seasonal thermocline is fully developed and low during the winter when the mixed layer extends to the top of the thermocline.

The Marginal Ice Zone (MIZ) intensive field program will employ an array of cutting-edge autonomous platforms to characterize the processes that govern Beaufort Sea MIZ evolution from initial breakup and MIZ formation though the course of the summertime sea ice retreat. Instruments will be deployed on and under the ice prior to initial formation of the MIZ along the Alaska coast, and will continue sampling from open water, across the MIZ, and into full ice cover, as the ice edge retreats northward through the summer. The flexible nature of ice-mounted and mobile, autonomous oceanographic platforms (e.g., gliders and floats) facilitates access to regions of both full ice cover and riskier MIZ regions. This approach exploits the extended endurance of modern autonomous platforms to maintain a persistent presence throughout the entire northward retreat. It also takes advantage of the inherent scalability of these instruments to sample over a broad range of spatial and temporal scales.

The Agulhas Current on the east coast of South Africa is a major western boundary current that exchanges heat and salt between the Indian and South Atlantic Oceans. The current retroflects as it deflects away from the African continent at the southern tip of the Agulhas Bank, a biologically productive extension of the continental shelf south of the South African coast. The less energetic Benguela Current borders the Agulhas Bank to the west. Little is known about mesoscale interactions between the Agulhas Current and the shelf waters of the Agulhas Bank or how these processes influence the biology of the bank. In this study, physical and biological data collected during a dedicated cruise in September 2010 allowed the identification of several mesoscale features that indicate a strong effect of the current on the bank, including a Natal Pulse that forced the Agulhas Current onto the Agulhas Bank. While on the bank itself, the current entrained particles that were then transported offshore. We also found evidence of upwelling on the southeast edge of the Agulhas Bank, which is thought to be a source of water for a cold ridge that characterizes the eastern region of the bank. Large fluctuations of the thermocline, consistent with internal waves, were observed inshore of the Agulhas Current, with high phytoplankton concentrations at their crests. We suggest that this is a physical effect, with doming of the waves concentrating plankton at their crests, thereby creating episodic biological hotspots.

Kaena Ridge, Hawaii is a site of energetic conversion of the semi-diurnal barotropic tide. Diffuse baroclinic wave beams emanate from the critical-slope regions near the ridge crest, directed upward and southward from the north flank of the ridge, upward and northward from the south flank. Here we attempt to quantify the momentum fluxes associated with generation at the Ridge. Continuous vertical profiles of density and velocity from 80–800 m were obtained from the Research Platform FLIP over the southern edge of the ridge, as an aspect of the Hawaii Ocean Mixing Experiment. Data are used to estimate the Reynolds stress, Eulerian buoyancy flux, and the combined Eliassen-Palm Flux in the semi-diurnal band. An upward-southward stress maximum of ~ 0.5 10^-4 m² s^-2 appears at depths 300–500 m, generally consistent with beam-like behavior. A strong off-ridge buoyancy flux (~ 0.3 10^-4 m² s^-3) combines with large along-ridge Reynolds stresses to form an Eliassen Palm flux whose along-ridge and across-ridge magnitudes are comparable. The stress azimuth rotates clockwise with increasing altitude above the ridge crest. The principal upward-southward beam is found to be at depths 100–300 m shallower than are predicted by an analytic 2-dimensional model and a 3-D numerical simulation. This discrepancy is consistent with previous observations of the baroclinic energy flux. If these observed tidal momentum fluxes were to diverge in a 100-m thick near-surface layer, the forcing would be comparable to a moderate wind stress. Pronounced lateral gradients of baroclinic tidal stresses can be expected offshore of Hawaiian topography.

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.

Internal tide generation, propagation, and dissipation are investigated in Luzon Strait, a system of two quasi-parallel ridges situated between Taiwan and the Philippines. Two profiling moorings deployed for about 20 days and a set of nineteen 36-h lowered ADCP–CTD time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals. Measurements were concentrated on a northern line, where the ridge spacing was approximately equal to the mode-1 wavelength for semidiurnal motions, and a southern line, where the spacing was approximately two-thirds that. The authors contrast the two sites to emphasize the potential importance of resonance between generation sites. Throughout Luzon Strait, baroclinic energy, energy fluxes, and turbulent dissipation were some of the strongest ever measured. Peak-to-peak baroclinic velocity and vertical displacements often exceeded 2 m s^-1 and 300 m, respectively. Energy fluxes exceeding 60 kW m^-1 were measured at spring tide at the western end of the southern line. On the northern line, where the western ridge generates appreciable eastward-moving signals, net energy flux between the ridges was much smaller, exhibiting a nearly standing wave pattern. Overturns tens to hundreds of meters high were observed at almost all stations. Associated dissipation was elevated in the bottom 500—1000 m but was strongest by far atop the western ridge on the northern line, where >500-m overturns resulted in dissipation exceeding 2 x 10^-6 W kg^-1 (implying diapycnal diffusivity K%u03C1 > 0.2 m2 s%u22121). Integrated dissipation at this location is comparable to conversion and flux divergence terms in the energy budget. The authors speculate that resonance between the two ridges may partly explain the energetic motions and heightened dissipation.

The Arctic Ocean traditionally has been described as an ocean with low variability and weak turbulence levels. Many years of observations from ice camps and ice-based instruments have shown that the sea ice cover effectively isolates the water column from direct wind forcing and damps existing motions, resulting in relatively small upper-ocean variability and an internal wave field that is much weaker than at lower latitudes. Under the ice, direct and indirect estimates across the Arctic basins suggest that turbulent mixing does not play a significant role in the general distribution of oceanic properties and the evolution of Arctic water masses. However, during ice-free periods, the wind generates inertial motions and internal waves, and contributes to deepening of the mixed layer both on the shelves and over the deep basins - as at lower latitudes. Through their associated vertical mixing, these motions can alter the distribution of properties in the water column. With an increasing fraction of the Arctic Ocean becoming ice-free in summer and in fall, there is a crucial need for a better understanding of the impact of direct wind forcing on the Arctic Ocean.

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.

The distribution of deep near-inertial waves (NIWs) is investigated using data mainly from an array of 46 near-bottom acoustic current meter sensors spanning a 600 km x 600 km region as part of the Kuroshio Extension System Study during 2004–2006. The deep NIW distribution is interpreted in the context of both upper-layer and near-bottom mapped circulations. The wintertime-mean mixed-layer NIW energy input, modeled from observed wind stress, has the same range of values north and south of the Kuroshio Extension in this region. Yet, the wintertime-mean deep NIW energy distribution reveals a sharp factor-of-5 decrease from north to south of the Kuroshio jet. This direct observational evidence shows that the Kuroshio Extension blocks the equatorward propagation of NIWs. The NIW energy that does reach the sea floor within the subset of wintertime observations in the subtropical gyre arrives with patchy spatial and temporal distribution. Elevated NIW energy in deep water is associated with anticyclones in the deep barotropic flow and unassociated with upper layer eddies.

Most of the M2 internal tide energy generated at the Hawaiian Ridge radiates away in modes 1 and 2, but direct observation of these propagating waves is complicated by the complexity of the bathymetry at the generation region and by the presence of interference patterns.

Observations from satellite altimetry, a tomographic array, and the R/P FLIP taken during the Farfield Program of the Hawaiian Ocean Mixing Experiment (HOME) are found to be in good agreement with the output of a high-resolution primitive equation model, simulating the generation and propagation of internal tides. The model shows that different modes are generated with different amplitudes along complex topography. Multiple sources produce internal tides that sum constructively and destructively as they propagate. The major generation sites can be identified using a simplified 2D idealized knife-edge ridge model. Four line sources located on the Hawaiian Ridge reproduce the interference pattern of sea surface height and energy flux density fields from the numerical model for modes 1 and 2. Waves from multiple sources and their interference pattern have to be taken into account to correctly interpret in situ observations and satellite altimetry.

The Arctic is generally considered a low energy ocean. Using mooring data from the northern Chukchi Sea, we confirm that this is mainly because of sea-ice impeding input of wind energy into the ocean. When sea-ice is present, even strong storms do not induce significant oceanic response. However, during ice-free seasons, local storms drive strong inertial currents (>20 cm/s) that propagate throughout the water column and significantly deepen the surface mixed layer. The large vertical shear associated with summer inertial motions suggests a dominant role for localized and seasonal vertical mixing in Arctic Ocean dynamics. Our results imply that recent extensive summer sea-ice retreat will lead to significantly increased internal wave generation especially over the shelves and also possibly over deep waters. This internal wave activity will likely dramatically increase upper-layer mixing in large areas of the previously quiescent Arctic, with important ramifications for ecosystems and ocean dynamics.

This paper reports on the strength and structure of the Kuroshio Extension and its recirculation gyres. In the time average, quasi-permanent recirculation gyres are found to the north and south of the Kuroshio Extension jet. The characteristics of these recirculations gyres are determined from the combined observations from the Kuroshio Extension System Study (KESS) field program (June 2004–June 2006) and include current meters, pressure and current recording inverted echo sounders, and subsurface floats. The position and strength of the recirculation gyres simulated by a high-resolution numerical model are found to be consistent with the observations. The circulation pattern that is revealed is of a complex system of multiple recirculation gyres that are embedded in the crests and troughs of the quasi-permanent meanders of the Kuroshio Extension. At the location of the KESS array, the Kuroshio Extension jet and its recirculation gyres transport of about 114 Sv. This represents a 2.7-fold increase in the transport of the current compared to the Kuroshio's transport at Cape Ashizuri before it separates from the coast and flows eastward into the open ocean. This enhancement in the current's transport comes from the development of the flanking recirculation gyres. Estimates from an array of inverted echo sounders and a high-resolution ocean general circulation model are of similar magnitude.

Turbulent-scale temperature and conductivity were measured during the pan-arctic Beringia 2005 Expedition. The rates of dissipation of thermal variance and diapycnal diffusivities are calculated along a section from Alaska to the North Pole, across deep flat basins (Canada and Makarov Basins) and steep ridges (Alpha-Mendeleev and Lomonosov Ridges). The mixing rates are observed to be small relative to lower latitudes but also remarkably non-uniform. Relatively elevated turbulence is found over deep topography, confirming the dominant role of bottom-generated internal waves. Measured patterns of mixing in the Arctic are also associated with other mechanisms, such as double-diffusive structures and deep overflows. A better knowledge of the distribution of mixing is essential to understand the dynamics of the changing Arctic environment.