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James Girton

Principal Oceanographer

Affiliate Assistant Professor, Oceanography

Email

girton@apl.washington.edu

Phone

206-543-8467

Research Interests

Overflows and Deep-Water Formation, Internal Waves, Mesoscale Eddies, Oceanic Surface and Bottom Boundary Layers, Measurements of Ocean Velocity Through Motionally-Induced Voltages

Biosketch

James Girton's research primarily investigates ocean processes involving small-scale turbulence and mixing and their influence on larger-scale flows. An important part of physical oceanography is the collection of novel datasets to shed new light on important physical processes, and to this end Dr. Girton's research has frequently drawn upon the widely under-utilized electromagnetic velocity profiling technique developed by Tom Sanford (his Ph.D. advisor and frequent collaborator). Instruments utilizing this technique include the expendable XCP, the full-depth free-falling AVP, and the autonomous long-duration EM-APEX. Each of these instruments has a unique role to play in the study of phenomena ranging from deep boundary currents and overflows to upper ocean mixing and internal waves.

In addition to being less well-understood elements of ocean physics, many of these phenomena are potentially important for the behavior of the large-scale ocean circulation, particularly the meridional overturning that transports heat to subpolar and polar regions and sequesters atmospheric gases in the deep ocean. Prediction of future climate change by coupled ocean-atmosphere models requires reliable predictions of ocean circulation, so physically-based improvements to parameterizations of mixing, boundary stresses and internal waves in such models are an ongoing goal.

Department Affiliation

Ocean Physics

Education

B.A. Physics, Swarthmore College, 1993

Ph.D. Oceanography, University of Washington, 2001

Projects

Wave Glider Observations in the Southern Ocean

A Wave Glider autonomous surface vehicle will conduct a summer-season experiment to investigate ocean–shelf exchange on the West Antarctic Peninsula and frontal air–sea interaction over both the continental shelf and open ocean.

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4 Sep 2019

Southern Ocean climate change is at the heart of the ocean's response to anthropogenic forcing. Variations in South Polar atmospheric circulation patterns, fluctuations in the strength and position of the Antarctic Circumpolar Current, and the intertwining intermediate deep water cells of the oceanic meridional overturning circulation have important impacts on the rate of ocean carbon sequestration, biological productivity, and the transport of heat to the melting continental ice shelves.

Submesoscale Mixed-Layer Dynamics at a Mid-Latitude Oceanic Front

SMILE: the Submesoscale MIxed-Layer Eddies experiment

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1 Mar 2017

This experiment is aimed at increasing our understanding of the role of lateral processes in mixed-layer dynamics through a series of ship surveys and Lagrangian array deployments. Instrument deployments and surveys target the upper ocean's adjustment to winter atmospheric forcing events in the North Pacific subtropical front, roughly 800 km north of Hawaii.

This study will improve understanding of 1–10-km scale lateral processes in three-dimensional mixed-layer dynamics in a region of above-average atmospheric forcing, typical mid-ocean mesoscale advection and straining, and typical submesoscale activity. The results will improve the physical basis of mixed-layer parameterizations, leading to better model predictions of air-sea fluxes, gas transfer, and biological productivity.

Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES)

DIMES is a US/UK field program aimed at measuring diapycnal and isopycnal mixing in the Southern Ocean, along the tilting isopycnals of the Antarctic Circumpolar Current.

 

Publications

2000-present and while at APL-UW

A spatial geography of abyssal turbulent mixing in the Samoan Passage

Carter, G.S., G. Voet, M.H. Alford, J.B. Girton, J.B. Miskent, J.M. Kaymak, L.J. Pratt, K.A. Pearson-Potts, J.M. Cusack, and S. Tan, "A spatial geography of abyssal turbulent mixing in the Samoan Passage," Oceanography, 32, 194-203, doi:10.5670/oceanog.2019.425, 2019.

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1 Dec 2019

High levels of turbulent mixing have long been suspected in the Samoan Passage, an important topographic constriction in the deep limb of the Pacific Meridional Overturning Circulation. Along the length of the passage, observations undertaken in 2012 and 2014 showed the bottom water warmed by ~55 millidegrees Celsius and decreased in density by 0.01 kg m-3. Spatial analysis of this first-ever microstructure survey conducted in the Samoan Passage confirmed there are multiple hotspots of elevated abyssal mixing. This mixing was not just confined to the four main sills — even between sills, the nature of the mixing processes appeared to differ: for example, one sill is clearly a classical hydraulically controlled overflow, whereas another is consistent with mode-2 hydraulic control. When microstructure casts were averaged into 0.1°C conservative temperature classes, the largest dissipation rates and diapycnal diffusivity values (>10-7 W kg-1 and 10-2 m2 s-1, respectively) occurred immediately downstream of the northern sill in the eastern and deepest channel. Although topographic blocking is the primary reason that no water colder than Θ = 0.7°C is found in the western channel, intensive mixing at the entrance sills appeared to be responsible for eroding an approximately 100 m thick layer of Θ < 0.7°C water. Three examples highlighting weak temporal variability, and hence suggesting that the observed spatial patterns are robust, are presented. The spatial variability in mixing over short lateral scales suggests that any simple parameterization of mixing within the Samoan Passage may not be applicable.

Flow-topography interactions in the Samoan Passage

Girton, J.B., J.B. Mickey, Z.X. Zhao, M.H. Alford, G. Voet, J.M. Cusack, G.S. Carter, K.A. Pearson-Potts, L.J. Pratt, S. Tan, and J.M. Klymak, "Flow-topography interactions in the Samoan Passage," Oceanography, 32, 184-193, doi:10.5670/oceanog.2019.424, 2019.

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1 Dec 2019

Mixing in the Samoan Passage has implications for the abyssal water properties of the entire North Pacific — nearly 20% of the global ocean's volume. Dense bottom water formed near Antarctica encounters the passage — a gap in a ridge extending from north of Samoa eastward across the Pacific at around 10°S — and forms an energetic cascade much like a river flowing through a canyon. The 2011–2014 Samoan Passage Abyssal Mixing Experiment explored the importance of topography to the dense water flow on a wide range of scales, including (1) constraints on transport due to the overall passage shape and the heights of its multiple sills, (2) rapid changes in water properties along particular pathways at localized mixing hotspots where there is extreme topographic roughness and/or downslope flow acceleration, and (3) diversion and disturbance of flow pathways and density surfaces by small-scale seamounts and ridges. The net result is a complex but fairly steady picture of interconnected pathways with a limited number of intense mixing locations that determine the net water mass transformation. The implication of this set of circumstances is that the dominant features of Samoan Passage flow and mixing (and their responses to variations in incoming or background properties) can be described by the dynamics of a single layer of dense water flowing beneath a less-dense one, combined with mixing and transformation that is determined by the small-scale topography encountered along flow pathways.

Persistent turbulence in the Samoan Passage

Cusack, J.M., G. Voet, M.H. Alford, J.B. Virton, G.S. Carter, L.J. Pratt, S. Tan, and K.A. Pearson-Potts, "Persistent turbulence in the Samoan Passage," J. Phys. Oceanogr., 49, 3179-3197, doi:10.1175/JPO-D-19-0116.1, 2019.

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1 Dec 2019

Abyssal waters forming the lower limb of the global overturning circulation flow through the Samoan Passage and are modified by intense mixing. Thorpe scale based estimates of dissipation from moored profilers deployed on top of two sills for 17 months reveal that turbulence is continuously generated in the Passage. Overturns were observed in a density band where the Richardson number was often smaller than 1/4, consistent with shear instability occurring at the upper interface of the fast flowing bottom water layer. The magnitude of dissipation was found to be stable on long time scales from weeks to months. A second array of 12 moored profilers deployed for a shorter duration but profiling at higher frequency was able to resolve variability in dissipation on time scales of days to hours. At some mooring locations near-inertial and tidal modulation of the dissipation rate was observed. However, the modulation was not spatially coherent across the Passage. The magnitude and vertical structure of dissipation from observations at one of the major sills is compared with an idealised 2D numerical simulation that includes a barotropic tidal forcing. Depth integrated dissipation rates agree between model and observations to within a factor of 3. The tide has a negligible effect on the mean dissipation. These observations reinforce the notion that the Samoan Passage is an important mixing hot spot in the global ocean where waters are being transformed continuously.

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In The News

UW team sending autonomous surfboard to explore Antarctic waters

UW News, Hannah Hickey

The research project will use the Wave Glider to investigate the summer conditions near Palmer Station on the Antarctic Peninsula, to better understand how the warming ocean interacts with ice shelves that protrude from the shore. It will then head across Drake Passage, braving some of the stormiest seas on the planet.

23 Oct 2019

One year into the mission, autonomous ocean robots set a record in survey of Antarctic ice shelf

UW News, Hannah Hickey

A team of ocean robots deployed in January 2018 have, over the past year, been the first self-guided ocean robots to successfully travel under an ice sheet and return to report long-term observations.

23 Jan 2019

Underwater robots survive a year probing climate change's effects on Antarctic ice

GeekWire, Alan Boyle

A squadron of Seagliders and EM-APEX floats was sent to probe the waters beneath the Dotson Ice Shelf in Antarctica one year ago. They have transmitted their data via satellite successfully, proving that these robots and approach can work in this harsh, remote environment.

22 Jan 2019

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