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Craig McNeil

Principal Oceanographer

Affiliate Assistant Professor, Oceanography





Research Interests

Bubble mediated gas flux at high to extreme wind speeds; Dissolved gas cycling and the potential impacts of ocean warming on biogeochemistry; River and estuarine processes and their effects on suspended sediment distributions


Significant quantities of gases are exchanged between the atmosphere and ocean during storms, even though storms are relatively infrequent. We know this because measured air-sea gas transfer rates are often found to be non-linear (quadratic or cubic) functions of wind speed. However, there is also unexplained scatter between results of different experiments, even those conducted using the same techniques! Unexplained scatter between experiments results in significant uncertainty in global air-sea flux estimates for individual gases, like CO2 and O2. We are actively investigating the effects of wind and waves on gas fluxes and hope to develop improved parameterizations of bubble mediated air-sea gas exchange and better constraints on global CO2 and O2 cycles.

Rising atmospheric CO2 levels and ocean warming represent significant concerns for ocean health and biogeochemistry via altered pH and dissolved O2 concentrations. The Arctic Ocean is particularly susceptible to pH changes. Eastern basin Oxygen Minimum Zones (OMZs) are regions of the oceans that are potentially susceptible to altered global wind and warming patterns. OMZs are also important since they remove nutrients from the world oceans via denitrification processes. We are conducting cruises and deploying gas sensing floats in these regions to better understand how organic carbon export from productive near surface eddies can stimulate the microbial community that lives in the deeper anoxic waters to remove more nutrients from the water column.

Suspended sediments in rivers and estuaries continuously transport eroded materials from the land to the ocean and in doing so locally impact water quality. We study the mechanisms and processes by which sediments (and bubbles) are suspended, concentrated, and redistributed within rivers and estuaries, and ultimately transported out the mouth of the estuary to the adjacent sea. Suspended sediments, bubbles, and actively mixing brackish waters composed of inhomogeneous mixtures of fresh and saltwater show significantly altered optical and acoustical backscatter relative to the surrounding waters. We use this this to our advantage to help find and locate estuarine features such as fronts, internal hydraulic jumps, breaking internal waves, and the salt-wedge intrusion, via their locally anomalous optical and acoustical backscatter then study the dynamics of these features in greater detail to better understand the sediment transport mechanisms.


B.S. Applied Physics with Solid State Electronics, Heriot-Watt University, 1989

Ph.D. Physics, University of Victoria, 1995


APL-UW Involvement in the Coastal Margin Observation and Prediction Science and Technology Center (CMOP)

AUVs will be deployed by a newly formed APL-UW AUV group as part of CMOP's experimental observation network which consists of multiple fixed and mobile platforms equipped with oceanographic sensors.

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15 Jun 2012

The Center for Coastal Margin Observation and Predication (CMOP) has purchased from Hydroid, LLC two Autonomous Underwater Vehicles (AUVs) for its studies. The REMUS (Remote Environmental Measuring Units) 100 (see Figure 1) is a compact, light-weight, AUV designed for operation in coastal environments up to 100 meters in depth. The AUVs will be deployed by a newly formed APL-UW AUV group as part of CMOP's experimental observation network which consists of multiple fixed and mobile platforms equipped with oceanographic sensors. The AUVs will be used, primarily, to study the Columbia River plume and estuary region. The AUVs will be deployed periodically throughout each operational year. We also plan to allow customization of the AUVs by integrating novel biogeochemical sensors to meet specific scientific objectives for the CMOP program.

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.

Parameterization of Gas Flux at High Wind Speed (Hurricane)

This goal of this project is to improve current parameterizations of air-sea gas transfer for high wind speeds.

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This goal of this project is to improve current parameterizations of air-sea gas transfer for high wind speeds. This will involve continued field work in hurricanes during the 2008-2009 seasons. We also participated in the UK SOLAS Deep Ocean Gas Exchange Experiments (DOGEE), which involved two experiments in the North Atlantic (winter 2006 and Spring 2007). The data from these cruises are being used to validate our new water-side O2 covariance measurement technique based on fast-response O2 measurements on the floats.

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Characterizing PNW Seafloor Methane Seeps with a Fleet of Small AUVs

During a pilot study on Lake Washington, researchers used easily deployed and relatively inexpensive AUVs to find and map simulated seafloor methane seeps. The vehicles' imaging sonars were cross-calibrated with shipboard sonars in preparation for extensive mapping and characterization of hundreds of methane seep sites on the Cascadia Margin.

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14 Oct 2022

Methane is a potent greenhouse gas, and is released into the ocean from the seafloor as gas bubbles via methane seeps. Despite the prevalence of seeps along continental margins, data is limited, and their impacts on the ocean and atmosphere, both positive and negative, are poorly understood. To advance our knowledge about methane seeps, we are pursuing an approach to map and characterize methane seeps over wide areas. We are using easily deployable and relatively inexpensive autonomous underwater vehicles (AUVs) equipped with imaging sonars and custom sensors to find and map seeps and measure associated bubbles and dissolved methane right at the source and up through the water column.

Crimson Tide in the Columbia River Estuary

APL-UW experts in autonomous undersea vehicle operation mapped the September 2012 outbreak of the non-toxic phytoplankton Mesodinium rubrum. This study of Columbia River ecology is conducted in collaboration with the Coastal Margin Observation and Prediction (CMOP) Science and Technology Center.

5 Oct 2012


2000-present and while at APL-UW

Autonomous observations of biogenic N2 in the Eastern Tropical North Pacific using profiling floats equipped with gas tension devices

McNeil, C.L., E.A. D'Asaro, M.A. Altabet, R.C. Hamme, and E. Garcia-Robledo, "Autonomous observations of biogenic N2 in the Eastern Tropical North Pacific using profiling floats equipped with gas tension devices," Front. Mar. Sci., 10, doi:10.3389/fmars.2023.1134851, 2023.

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12 Jun 2023

Oxygen Deficient Zones (ODZs) of the world's oceans represent a relatively small fraction of the ocean by volume (<0.05% for suboxic and <5% for hypoxic) yet are receiving increased attention by experimentalists and modelers due to their importance in ocean nutrient cycling and predicted susceptibility to expansion and/or contraction forced by global warming. Conventional methods to study these biogeochemically important regions of the ocean have relied on well-developed but still relatively high cost and labor-intensive shipboard methods that include mass-spectrometric analysis of nitrogen-to-argon ratios (N2/Ar) and nutrient stoichiometry (relative abundance of nitrate, nitrite, and phosphate). Experimental studies of denitrification rates and processes typically involve either in-situ or in-vitro incubations using isotopically labeled nutrients. Over the last several years we have been developing a Gas Tension Device (GTD) to study ODZ denitrification including deployment in the largest ODZ, the Eastern Tropical North Pacific (ETNP). The GTD measures total dissolved gas pressure from which dissolved N2 concentration is calculated. Data from two cruises passing through the core of the ETNP near 17°N in late 2020 and 2021 are presented, with additional comparisons at 12°N for GTDs mounted on a rosette/CTD as well as modified profiling Argo-style floats. Gas tension was measured on the float with an accuracy of < 0.1% and relatively low precision (< 0.12%) when shallow (P< 200 dbar) and high precision (< 0.03%) when deep (P > 300 dbar). We discriminate biologically produced N2 (i.e., denitrification) from N2 in excess of saturation due to physical processes (e.g., mixing) using a new tracer – 'preformed excess-N2'. We used inert dissolved argon (Ar) to help test the assumption that preformed excess-N2 is indeed conservative. We used the shipboard measurements to quantify preformed excess-N2 by cross-calibrating the gas tension method to the nutrient-deficit method. At 17°N preformed excess-N2 decreased from approximately 28 to 12 μmol/kg over σ0 = 24–27 kg/m3 with a resulting precision of ±1 μmol N2/kg; at 12°N values were similar except in the potential density range of 25.7< σ0< 26.3 where they were lower by 1 μmol N2/kg due likely to being composed of different source waters. We then applied these results to gas tension and O2 (< 3 μmol O2/kg) profiles measured by the nearby float to obtain the first autonomous biogenic N2 profile in the open ocean with an RMSE of ± 0.78 μM N2, or ± 19%. We also assessed the potential of the method to measure denitrification rates directly from the accumulation of biogenic N2 during the float drifts between profiling. The results suggest biogenic N2 rates of ±20 nM N2/day could be detected over >>16 days (positive rates would indicate denitrification processes whereas negative rates would indicate predominantly dilution by mixing). These new observations demonstrate the potential of the gas tension method to determine biogenic N2 accurately and precisely in future studies of ODZs.

ΔO2/N2' as a new tracer of marine net community production: Application and evaluation in the Subarctic Northeast Pacific and Canadian Arctic Ocean

Izett, R.W., R.C. Hamme, C. McNeil, C.C.M. Manning, A. Bourbonnais, and P.D. Tortell, "ΔO2/N2' as a new tracer of marine net community production: Application and evaluation in the Subarctic Northeast Pacific and Canadian Arctic Ocean," Front. Mar. Sci., 8, doi:10.3389/fmars.2021.718625, 2021.

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2 Aug 2021

We compared field measurements of the biological O2 saturation anomalies, ΔO2/Ar and ΔO2/N2, from simultaneous oceanographic deployments of a membrane inlet mass spectrometer and optode/gas tension device (GTD). Data from the Subarctic Northeast Pacific and Canadian Arctic Ocean were used to evaluate ΔO2/N2 as an alternative to ΔO2/Ar for estimates of mixed layer net community production (NCP). We observed strong spatial coherence between ΔO2/Ar and ΔO2/N2, with small offsets resulting from differences in the solubility properties of Ar and N2 and their sensitivity to vertical mixing fluxes. Larger offsets between the two tracers were observed across hydrographic fronts and under elevated sea states, resulting from the differential time-response of the optode and GTD, and from bubble dissolution in the ship's seawater lines. We used a simple numerical framework to correct for physical sources of divergence between N2 and Ar, deriving the tracer ΔO2/N2'. Over most of our survey regions, ΔO2/N2' provided a better analog for ΔO2/Ar, and thus more accurate NCP estimates than ΔO2/N2. However, in coastal Arctic waters, ΔO2/N2 and ΔO2/N2' performed equally well as NCP tracers. On average, mixed layer NCP estimated from ΔO2/Ar and ΔO2/N2' agreed to within ~2 mmol O2 m-2 d-1, with offsets typically smaller than other errors in NCP calculations. Our results demonstrate a significant potential to derive NCP from underway O2/N2 measurements across various oceanic regions. Optode/GTD systems could replace mass spectrometers for autonomous NCP derivation under many oceanographic conditions, thereby presenting opportunities to significantly expand global NCP coverage from various underway platforms.

Suppression of CO2 outgassing by gas bubbles under a hurricane

Liang, J.-H., E.A. D'Asaro, C.L. McNeil, Y. Fan, R.R. Harcourt, S.R. Emerson, B. Yang, and P.P. Sullivan, "Suppression of CO2 outgassing by gas bubbles under a hurricane," Geophys. Res. Lett., 47, doi:10.1029/2020GL090249, 2020.

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28 Sep 2020

The role of gas bubbles on the air‐sea CO2 flux during Hurricane Frances (2004) is studied using a large‐eddy simulation model that couples ocean surface boundary layer turbulence, gas bubbles, and dissolved gases. In the subtropical surface ocean where gases are slightly supersaturated, gases in bubbles can still dissolve due to hydrostatic pressure and surface tension exerted on bubbles. Under the simulated conditions, the CO2 efflux with an explicit bubble effect is less than 2% of that calculated using a gas flux formula without explicit inclusion of bubble effect. The use of a gas flux parameterization without bubble‐induced supersaturation contributes to uncertainty in the global carbon budget. The results highlight the importance of bubbles under high winds even for soluble gases such as CO2 and demonstrate that gas flux parameterization derived from gases of certain solubility may not be accurate for gases of very different solubility.

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Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center