Girton James B.

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Girton

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James B.

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0000-0001-5692-3216

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Upper-ocean response to Hurricane Frances (2004) observed by Profiling EM-APEX floats

(American Meteorological Society, 2011-06) Sanford, Thomas B. ; Price, James F. ; Girton, James B.

Three autonomous profiling Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats were air deployed one day in advance of the passage of Hurricane Frances (2004) as part of the Coupled Boundary Layer Air–Sea Transfer (CBLAST)-High field experiment. The floats were deliberately deployed at locations on the hurricane track, 55 km to the right of the track, and 110 km to the right of the track. These floats provided profile measurements between 30 and 200 m of in situ temperature, salinity, and horizontal velocity every half hour during the hurricane passage and for several weeks afterward. Some aspects of the observed response were similar at the three locations—the dominance of near-inertial horizontal currents and the phase of these currents—whereas other aspects were different. The largest-amplitude inertial currents were observed at the 55-km site, where SST cooled the most, by about 2.2°C, as the surface mixed layer deepened by about 80 m. Based on the time–depth evolution of the Richardson number and comparisons with a numerical ocean model, it is concluded that SST cooled primarily because of shear-induced vertical mixing that served to bring deeper, cooler water into the surface layer. Surface gravity waves, estimated from the observed high-frequency velocity, reached an estimated 12-m significant wave height at the 55-km site. Along the track, there was lesser amplitude inertial motion and SST cooling, only about 1.2°C, though there was greater upwelling, about 25-m amplitude, and inertial pumping, also about 25-m amplitude. Previously reported numerical simulations of the upper-ocean response are in reasonable agreement with these EM-APEX observations provided that a high wind speed–saturated drag coefficient is used to estimate the wind stress. A direct inference of the drag coefficient CD is drawn from the momentum budget. For wind speeds of 32–47 m s−1, CD ~ 1.4 × 10−3.
Diapycnal mixing in the Antarctic Circumpolar Current

(American Meteorological Society, 2011-01) Ledwell, James R. ; St. Laurent, Louis C. ; Girton, James B. ; Toole, John M.

The vertical dispersion of a tracer released on a density surface near 1500-m depth in the Antarctic Circumpolar Current west of Drake Passage indicates that the diapycnal diffusivity, averaged over 1 yr and over tens of thousands of square kilometers, is (1.3 ± 0.2) × 10−5 m2 s−1. Diapycnal diffusivity estimated from turbulent kinetic energy dissipation measurements about the area occupied by the tracer in austral summer 2010 was somewhat less, but still within a factor of 2, at (0.75 ± 0.07) × 10−5 m2 s−1. Turbulent diapycnal mixing of this intensity is characteristic of the midlatitude ocean interior, where the energy for mixing is believed to derive from internal wave breaking. Indeed, despite the frequent and intense atmospheric forcing experienced by the Southern Ocean, the amplitude of finescale velocity shear sampled about the tracer was similar to background amplitudes in the midlatitude ocean, with levels elevated to only 20%–50% above the Garrett–Munk reference spectrum. These results add to a long line of evidence that diapycnal mixing in the interior middepth ocean is weak and is likely too small to dictate the middepth meridional overturning circulation of the ocean.
UCTD and EM/APEX measurements in support of the April 2015 AirSWOT Campaign : cruise and data report

(Woods Hole Oceanographic Institution, 2015-12) Farrar, J. Thomas ; Hodges, Benjamin A. ; Bigorre, Sebastien P. ; Galbraith, Nancy R. ; Girton, James B. ; Chao, Yi

AirSWOT is an aircraft mounted instrument for measuring and imaging sea surface height (SSH), and it is similar to the SWOT (Surface Water Ocean Topography) instrument that will be deployed on a satellite in 2020. A field campaign was conducted in April 2015 to examine the performance of AirSWOT and to better understand how the measurement is affected by surface waves and currents. Supporting measurements were collected from the R/V Shana Rae, the R/V Fulmar, and a second aircraft (a Partenavia P68 operated by Aspen Helicopter, Oxnard,CA for UCSD/SIO). From 17-20 April 2015, the R/V Shana Rae, a 50-foot research vessel, was used for collection of Underway CTD (or UCTD) measurements and for deployment and recovery of three EM/APEX floats in a study area off the central California coast. The UCTD measurements are being used to estimate the sea surface height signal associated with variations in ocean density structure. The EM/APEX floats provide time series of the same, as well as vertical profiles of ocean velocity. The purpose of this report is to document the shipboard operations on the R/V Shana Rae and the resulting UCTD and EM/APEX data sets.
Hydraulic control of flow in a multi-passage system connecting two basins

(Cambridge University Press, 2022-04-05) Tan, Shuwen ; Pratt, Lawrence J. ; Voet, Gunnar ; Cusack, Jesse M. ; Helfrich, Karl R. ; Alford, Matthew H. ; Girton, James B. ; Carter, Glenn S.

When a fluid stream in a conduit splits in order to pass around an obstruction, it is possible that one branch will be critically controlled while the other remains not so. This is apparently the situation in Pacific Ocean abyssal circulation, where most of the northward flow of Antarctic bottom water passes through the Samoan Passage, where it is hydraulically controlled, while the remainder is diverted around the Manihiki Plateau and is not controlled. These observations raise a number of questions concerning the dynamics necessary to support such a regime in the steady state, the nature of upstream influence and the usefulness of rotating hydraulic theory to predict the partitioning of volume transport between the two paths, which assumes the controlled branch is inviscid. Through the use of a theory for constant potential vorticity flow and accompanying numerical model, we show that a steady-state regime similar to what is observed is dynamically possible provided that sufficient bottom friction is present in the uncontrolled branch. In this case, the upstream influence that typically exists for rotating channel flow is transformed into influence into how the flow is partitioned. As a result, the partitioning of volume flux can still be reasonably well predicted with an inviscid theory that exploits the lack of upstream influence.
Constraining Southern Ocean air-sea-ice fluxes through enhanced observations

(Frontiers Media, 2019-07-31) Swart, Sebastiaan ; Gille, Sarah T. ; Delille, Bruno ; Josey, Simon A. ; Mazloff, Matthew R. ; Newman, Louise ; Thompson, Andrew F. ; Thomson, James M. ; Ward, Brian ; du Plessis, Marcel ; Kent, Elizabeth ; Girton, James B. ; Gregor, Luke ; Heil, Petra ; Hyder, Patrick ; Pezzi, Luciano Ponzi ; de Souza, Ronald Buss ; Tamsitt, Veronica ; Weller, Robert A. ; Zappa, Christopher J.

Air-sea and air-sea-ice fluxes in the Southern Ocean play a critical role in global climate through their impact on the overturning circulation and oceanic heat and carbon uptake. The challenging conditions in the Southern Ocean have led to sparse spatial and temporal coverage of observations. This has led to a “knowledge gap” that increases uncertainty in atmosphere and ocean dynamics and boundary-layer thermodynamic processes, impeding improvements in weather and climate models. Improvements will require both process-based research to understand the mechanisms governing air-sea exchange and a significant expansion of the observing system. This will improve flux parameterizations and reduce uncertainty associated with bulk formulae and satellite observations. Improved estimates spanning the full Southern Ocean will need to take advantage of ships, surface moorings, and the growing capabilities of autonomous platforms with robust and miniaturized sensors. A key challenge is to identify observing system sampling requirements. This requires models, Observing System Simulation Experiments (OSSEs), and assessments of the specific spatial-temporal accuracy and resolution required for priority science and assessment of observational uncertainties of the mean state and direct flux measurements. Year-round, high-quality, quasi-continuous in situ flux measurements and observations of extreme events are needed to validate, improve and characterize uncertainties in blended reanalysis products and satellite data as well as to improve parameterizations. Building a robust observing system will require community consensus on observational methodologies, observational priorities, and effective strategies for data management and discovery.
Highly resolved observations and simulations of the ocean response to a hurricane

(American Geophysical Union, 2007-07-07) Sanford, Thomas B. ; Price, James F. ; Girton, James B. ; Webb, Douglas C.

An autonomous, profiling float called EM-APEX was developed to provide a quantitative and comprehensive description of the ocean side of hurricane-ocean interaction. EM-APEX measures temperature, salinity and pressure to CTD quality and relative horizontal velocity with an electric field sensor. Three prototype floats were air-deployed into the upper ocean ahead of Hurricane Frances (2004). All worked properly and returned a highly resolved description of the upper ocean response to a category 4 hurricane. At a float launched 55 km to the right of the track, the hurricane generated large amplitude, inertially rotating velocity in the upper 120 m of the water column. Coincident with the hurricane passage there was intense vertical mixing that cooled the near surface layer by about 2.2°C. We find consistent model simulations of this event provided the wind stress is computed from the observed winds using a high wind-speed saturated drag coefficient.
Flow-topography interactions in the Samoan Passage

(Oceanography Society, 2019-12-11) Girton, James B. ; Mickett, John B. ; Zhao, Zhongxiang ; Alford, Matthew H. ; Voet, Gunnar ; Cusack, Jesse M. ; Carter, Glenn S. ; Pearson-Potts, Kelly A. ; Pratt, Lawrence J. ; Tan, Shuwen ; Klymak, Jody M.

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.
Deepwater overflow through Luzon Strait

(American Geophysical Union, 2006-01-10) Qu, Tangdong ; Girton, James B. ; Whitehead, John A.

This study examines water property distributions in the deep South China Sea and adjoining Pacific Ocean using all available hydrographic data. Our analysis reveals that below about 1500 m there is a persistent baroclinic pressure gradient driving flow from the Pacific into the South China Sea through Luzon Strait. Applying hydraulic theory with assumptions of zero potential vorticity and flat bottom to the Luzon Strait yields a transport estimate of 2.5 Sv (1 Sv=106 m3 s-1). Some implications of this result include: (i) a residence time of less than 30 years in the deep South China Sea, (ii) a mean diapycnal diffusivity as large as 10-3 m2 s-1, and (iii) an abyssal upwelling rate of about 3×10-6 m s-1. These quantities are consistent with residence times based on oxygen consumption rates. The fact that all of the inflowing water must warm up before leaving the basin implies that this marginal sea contributes to the water mass transformations that drive the meridional overturning circulation in the North Pacific. Density distributions within the South China Sea basin suggest a cyclonic deep boundary current system, as might be expected for an overflow-driven abyssal circulation.
The East Greenland Spill Jet as an important component of the Atlantic Meridional Overturning Circulation

( 2014-06-03) von Appen, Wilken-Jon ; Koszalka, Inga M. ; Pickart, Robert S. ; Haine, Thomas W. N. ; Mastropole, Dana M. ; Magaldi, Marcello G. ; Valdimarsson, Héðinn ; Girton, James B. ; Jochumsen, Kerstin ; Krahmann, Gerd

The recently discovered East Greenland Spill Jet is a bottom-intensified current on the upper continental slope south of Denmark Strait, transporting intermediate density water equatorward. Until now the Spill Jet has only been observed with limited summertime measurements from ships. Here we present the first year-round mooring observations demonstrating that the current is a ubiquitous feature with a volume transport similar to the well-known plume of Denmark Strait overflow water farther downslope. Using reverse particle tracking in a high-resolution numerical model, we investigate the upstream sources feeding the Spill Jet. Three main pathways are identified: particles flowing directly into the Spill Jet from the Denmark Strait sill; particles progressing southward on the East Greenland shelf that subsequently spill over the shelfbreak into the current; and ambient water from the Irminger Sea that gets entrained into the flow. The two Spill Jet pathways emanating from Denmark Strait are newly resolved, and long-term hydrographic data from the strait verifies that dense water is present far onto the Greenland shelf. Additional measurements near the southern tip of Greenland suggest that the Spill Jet ultimately merges with the deep portion of the shelfbreak current, originally thought to be a lateral circulation associated with the sub-polar gyre. Our study thus reveals a previously unrecognized significant component of the Atlantic Meridional Overturning Circulation that needs to be considered to understand fully the ocean’s role in climate.
On the hydrography of Denmark Strait

(John Wiley & Sons, 2017-01-10) Mastropole, Dana M. ; Pickart, Robert S. ; Valdimarsson, Héðinn ; Våge, Kjetil ; Jochumsen, Kerstin ; Girton, James B.

Using 111 shipboard hydrographic sections across Denmark Strait occupied between 1990 and 2012, we characterize the mean conditions at the sill, quantify the water mass constituents, and describe the dominant features of the Denmark Strait Overflow Water (DSOW). The mean vertical sections of temperature, salinity, and density reveal the presence of circulation components found upstream of the sill, in particular the shelfbreak East Greenland Current (EGC) and the separated EGC. These correspond to hydrographic fronts consistent with surface-intensified southward flow. Deeper in the water column the isopycnals slope oppositely, indicative of bottom-intensified flow of DSOW. An end-member analysis indicates that the deepest part of Denmark Strait is dominated by Arctic-Origin Water with only small amounts of Atlantic-Origin Water. On the western side of the strait, the overflow water is a mixture of both constituents, with a contribution from Polar Surface Water. Weakly stratified “boluses” of dense water are present in 41% of the occupations, revealing that this is a common configuration of DSOW. The bolus water is primarily Arctic-Origin Water and constitutes the densest portion of the overflow. The boluses have become warmer and saltier over the 22 year record, which can be explained by changes in end-member properties and their relative contributions to bolus composition.
Persistent turbulence in the Samoan Passage

(American Meteorological Society, 2019-12-09) Cusack, Jesse M. ; Voet, Gunnar ; Alford, Matthew H. ; Girton, James B. ; Carter, Glenn S. ; Pratt, Lawrence J. ; Pearson-Potts, Kelly A. ; Tan, Shuwen

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 in which the Richardson number was often smaller than ¼, 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 from 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 idealized 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.
A spatial geography of abyssal turbulent mixing in the Samoan passage

(Oceanography Society, 2019-12-11) Carter, Glenn S. ; Voet, Gunnar ; Alford, Matthew H. ; Girton, James B. ; Mickett, John B. ; Klymak, Jody M. ; Pratt, Lawrence J. ; Pearson-Potts, Kelly A. ; Cusack, Jesse M. ; Tan, Shuwen

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.
Pacific abyssal transport and mixing: Through the Samoan Passage versus around the Manihiki Plateau

(American Meteorological Society, 2019-01-11) Pratt, Lawrence J. ; Voet, Gunnar ; Pacini, Astrid ; Tan, Shuwen ; Alford, Matthew H. ; Carter, Glenn S. ; Girton, James B. ; Menemenlis, Dimitris

The main source feeding the abyssal circulation of the North Pacific is the deep, northward flow of 5–6 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) through the Samoan Passage. A recent field campaign has shown that this flow is hydraulically controlled and that it experiences hydraulic jumps accompanied by strong mixing and dissipation concentrated near several deep sills. By our estimates, the diapycnal density flux associated with this mixing is considerably larger than the diapycnal flux across a typical isopycnal surface extending over the abyssal North Pacific. According to historical hydrographic observations, a second source of abyssal water for the North Pacific is 2.3–2.8 Sv of the dense flow that is diverted around the Manihiki Plateau to the east, bypassing the Samoan Passage. This bypass flow is not confined to a channel and is therefore less likely to experience the strong mixing that is associated with hydraulic transitions. The partitioning of flux between the two branches of the deep flow could therefore be relevant to the distribution of Pacific abyssal mixing. To gain insight into the factors that control the partitioning between these two branches, we develop an abyssal and equator-proximal extension of the “island rule.” Novel features include provisions for the presence of hydraulic jumps as well as identification of an appropriate integration circuit for an abyssal layer to the east of the island. Evaluation of the corresponding circulation integral leads to a prediction of 0.4–2.4 Sv of bypass flow. The circulation integral clearly identifies dissipation and frictional drag effects within the Samoan Passage as crucial elements in partitioning the flow.
Energy and momentum of a density-driven overflow in the Samoan Passage

(American Meteorological Society, 2023-06-01) Voet, Gunnar ; Alford, Matthew H. ; Cusack, Jesse M. ; Pratt, Larry J. ; Girton, James B. ; Carter, Glenn S. ; Klymak, Jody M. ; Tan, Shuwen ; Thurnherr, Andreas M.

Abstract The energy and momentum balance of an abyssal overflow across a major sill in the Samoan Passage is estimated from two highly resolved towed sections, set 16 months apart, and results from a two-dimensional numerical simulation. Driven by the density anomaly across the sill, the flow is relatively steady. The system gains energy from divergence of horizontal pressure work and flux of available potential energy . Approximately half of these gains are transferred into kinetic energy while the other half is lost to turbulent dissipation, bottom drag, and divergence in vertical pressure work. Small-scale internal waves emanating downstream of the sill within the overflow layer radiate upward but dissipate most of their energy within the dense overflow layer and at its upper interface. The strongly sheared and highly stratified upper interface acts as a critical layer inhibiting any appreciable upward radiation of energy via topographically generated lee waves. Form drag of , estimated from the pressure drop across the sill, is consistent with energy lost to dissipation and internal wave fluxes. The topographic drag removes momentum from the mean flow, slowing it down and feeding a countercurrent aloft. The processes discussed in this study combine to convert about one-third of the energy released from the cross-sill density difference into turbulent mixing within the overflow and at its upper interface. The observed and modeled vertical momentum flux divergence sustains gradients in shear and stratification, thereby maintaining an efficient route for abyssal water mass transformation downstream of this Samoan Passage sill.
Is the Faroe Bank Channel overflow hydraulically controlled?

(American Meteorological Society, 2006-12) Girton, James B. ; Pratt, Lawrence J. ; Sutherland, David A. ; Price, James F.

The overflow of dense water from the Nordic Seas through the Faroe Bank Channel (FBC) has attributes suggesting hydraulic control—primarily an asymmetry across the sill reminiscent of flow over a dam. However, this aspect has never been confirmed by any quantitative measure, nor is the position of the control section known. This paper presents a comparison of several different techniques for assessing the hydraulic criticality of oceanic overflows applied to data from a set of velocity and hydrographic sections across the FBC. These include 1) the cross-stream variation in the local Froude number, including a modified form that accounts for stratification and vertical shear, 2) rotating hydraulic solutions using a constant potential vorticity layer in a channel of parabolic cross section, and 3) direct computation of shallow water wave speeds from the observed overflow structure. Though differences exist, the three methods give similar answers, suggesting that the FBC is indeed controlled, with a critical section located 20–90 km downstream of the sill crest. Evidence of an upstream control with respect to a potential vorticity wave is also presented. The implications of these results for hydraulic predictions of overflow transport and variability are discussed.
The scientific and societal uses of global measurements of subsurface velocity

(Frontiers Media, 2019-07-24) Szuts, Zoltan B. ; Bower, Amy S. ; Donohue, Kathleen A. ; Girton, James B. ; Hummon, Julia M. ; Katsumata, Katsuro ; Lumpkin, Rick ; Ortner, Peter B. ; Phillips, Helen E. ; Rossby, H. Thomas ; Shay, Lynn Keith ; Sun, Charles ; Todd, Robert E.

Ocean velocity defines ocean circulation, yet the available observations of subsurface velocity are under-utilized by society. The first step to address these concerns is to improve visibility of and access to existing measurements, which include acoustic sampling from ships, subsurface float drifts, and measurements from autonomous vehicles. While multiple programs provide data publicly, the present difficulty in finding, understanding, and using these data hinder broader use by managers, the public, and other scientists. Creating links from centralized national archives to project specific websites is an easy but important way to improve data discoverability and access. A further step is to archive data in centralized databases, which increases usage by providing a common framework for disparate measurements. This requires consistent data standards and processing protocols for all types of velocity measurements. Central dissemination will also simplify the creation of derived products tailored to end user goals. Eventually, this common framework will aid managers and scientists in identifying regions that need more sampling and in identifying methods to fulfill those demands. Existing technologies are capable of improving spatial and temporal sampling, such as using ships of opportunity or from autonomous platforms like gliders, profiling floats, or Lagrangian floats. Future technological advances are needed to fill sampling gaps and increase data coverage.

Girton James B.

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