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Jessica, M, R. MM, K. CT.  2018.  Interfacial form stress in the Southern Ocean state estimate. Journal of Geophysical Research: Oceans. 123:3368-3385.   10.1029/2018JC013844   Abstract

Abstract The wind stress that drives the Antarctic Circumpolar Current (ACC) exits the fluid via topographic form stress (TFS) at the sea floor; interfacial form stress (IFS) is thought to carry much of this momentum from source to sink. These form stresses combine to help set the strength and structure of the Southern Ocean meridional overturning circulation (MOC), a key nexus of heat and gas exchange between the deep ocean and the atmosphere. For the first time in a general circulation model, we calculate the time‐varying, three‐dimensional IFS field directly from zonal pressure gradients across vertical perturbations in isopycnal layer interfaces. We confirm previous findings that IFS compensates wind stress at the surface and topographic form stress at the seafloor in the Drake Passage latitudes. We find that zonal and time‐mean IFS is primarily responsible for this surface wind stress compensation, with some contribution from transient eddy IFS. Mean, standing eddy, and transient eddy IFS combine to compensate topographic form stress at depth. Both standing and transient eddy IFS concentrate at stationary meanders along the ACC, and transient eddy IFS dominates standing eddy IFS in regions of high eddy kinetic energy. Finally, total IFS changes sign from balancing eastward wind stress to balancing westward topographic form stress around 28.1 kg m−3, close to the upper limit of Antarctic Bottom Water, indicating the role of buoyancy forcing in setting the structure of the IFS field.

Donohue, KA, Tracey KL, Watts DR, Chidichimo MP, Chereskin TK.  2016.  Mean Antarctic Circumpolar Current transport measured in Drake Passage. Geophysical Research Letters. 43:11,760-11,767.   10.1002/2016GL070319   AbstractWebsite

The Antarctic Circumpolar Current is an important component of the global climate system connecting the major ocean basins as it flows eastward around Antarctica, yet due to the paucity of data, it remains unclear how much water is transported by the current. Between 2007 and 2011 flow through Drake Passage was continuously monitored with a line of moored instrumentation with unprecedented horizontal and temporal resolution. Annual mean near-bottom currents are remarkably stable from year to year. The mean depth-independent or barotropic transport, determined from the near-bottom current meter records, was 45.6 sverdrup (Sv) with an uncertainty of 8.9 Sv. Summing the mean barotropic transport with the mean baroclinic transport relative to zero at the seafloor of 127.7 Sv gives a total transport through Drake Passage of 173.3 Sv. This new measurement is 30% larger than the canonical value often used as the benchmark for global circulation and climate models.

Rocha, CB, Gille ST, Chereskin TK, Menemenlis D.  2016.  Seasonality of submesoscale dynamics in the Kuroshio Extension. Geophysical Research Letters. 43:11304-11311.   10.1002/2016gl071349   AbstractWebsite

Recent studies show that the vigorous seasonal cycle of the mixed layer modulates upper ocean submesoscale turbulence. Here we provide model-based evidence that the seasonally changing upper ocean stratification in the Kuroshio Extension also modulates submesoscale (here 10-100 km) inertia-gravity waves. Summertime restratification weakens submesoscale turbulence but enhances inertia-gravity waves near the surface. Thus, submesoscale turbulence and inertia-gravity waves undergo vigorous out-of-phase seasonal cycles. These results imply a strong seasonal modulation of the accuracy of geostrophic velocity diagnosed from submesoscale sea surface height delivered by the Surface Water and Ocean Topography satellite mission.

Watts, DR, Tracey KL, Donohue KA, Chereskin TK.  2016.  Estimates of eddy heat flux crossing the Antarctic Circumpolar Current from observations in Drake Passage. Journal of Physical Oceanography. 46:2103-2122.   10.1175/jpo-d-16-0029.1   AbstractWebsite

The 4-yr measurements by current- and pressure-recording inverted echo sounders in Drake Passage produced statistically stable eddy heat flux estimates. Horizontal currents in the Antarctic Circumpolar Current (ACC) turn with depth when a depth-independent geostrophic current crosses the upper baroclinic zone. The dynamically important divergent component of eddy heat flux is calculated. Whereas full eddy heat fluxes differ greatly in magnitude and direction at neighboring locations within the local dynamics array (LDA), the divergent eddy heat fluxes are poleward almost everywhere. Case studies illustrate baroclinic instability events that cause meanders to grow rapidly. In the southern passage, where eddy variability is weak, heat fluxes are weak and not statistically significant. Vertical profiles of heat flux are surface intensified with similar to 50% above 1000 m and uniformly distributed with depth below. Summing poleward transient eddy heat transport across the LDA of -0.010 +/- 0.005 PW with the stationary meander contribution of -0.004 +/- 0.001 PW yields -0.013 +/- 0.005 PW. A comparison metric, -0.4 PW, represents the total oceanic heat loss to the atmosphere south of 60 degrees S. Summed along the circumpolar ACC path, if the LDA heat flux occurred at six hot spots spanning similar or longer path segments, this could account for 20%-70% of the metric, that is, up to -0.28 PW. The balance of ocean poleward heat transport along the remaining ACC path should come from weak eddy heat fluxes plus mean cross-front temperature transports. Alternatively, the metric -0.4 PW, having large uncertainty, may be high.

Firing, YL, Chereskin TK, Watts DR, Mazloff MR.  2016.  Bottom pressure torque and the vorticity balance from observations in Drake Passage. Journal of Geophysical Research-Oceans. 121:4282-4302.   10.1002/2016jc011682   AbstractWebsite

The vorticity balance of the Antarctic Circumpolar Current in Drake Passage is examined using 4 years of observations from current-and pressure-recording inverted echo sounders. The time-varying vorticity, planetary and relative vorticity advection, and bottom pressure torque are calculated in a two-dimensional array in the eddy-rich Polar Frontal Zone (PFZ). Bottom pressure torque is also estimated at sites across Drake Passage. Mean and eddy nonlinear relative vorticity advection terms dominate over linear advection in the local (50-km scale) vorticity budget in the PFZ, and are balanced to first order by the divergence of horizontal velocity. Most of this divergence comes from the ageostrophic gradient flow, which also provides a second-order adjustment to the geostrophic relative vorticity advection. Bottom pressure torque is approximately one-third the size of the local depth-integrated divergence. Although the cDrake velocity fields exhibit significant turning with depth throughout Drake Passage even in the mean, surface vorticity advection provides a reasonable representation of the depth-integrated vorticity balance. Observed near-bottom currents are strongly topographically steered, and bottom pressure torques grow large where strong near-bottom flows cross steep topography at small angles. Upslope flow over the northern continental slope dominates the bottom pressure torque in cDrake, and the mean across this Drake Passage transect, 3 to 4x10(-9) m s(-2), exceeds the mean wind stress curl by a factor of 15-20.

Rocha, CB, Chereskin TK, Gille ST, Menemenlis D.  2016.  Mesoscale to submesoscale wavenumber spectra in Drake Passage. Journal of Physical Oceanography. 46:601-620.   10.1175/jpo-d-15-0087.1   AbstractWebsite

This study discusses the upper-ocean (0-200 m) horizontal wavenumber spectra in the Drake Passage from 13 yr of shipboard ADCP measurements, altimeter data, and a high-resolution numerical simulation. At scales between 10 and 200 km, the ADCP kinetic energy spectra approximately follow a k(-3) power law. The observed flows are more energetic at the surface, but the shape of the kinetic energy spectra is independent of depth. These characteristics resemble predictions of isotropic interior quasigeostrophic turbulence. The ratio of across-track to along-track kinetic energy spectra, however, significantly departs from the expectation of isotropic interior quasigeostrophic turbulence. The inconsistency is dramatic at scales smaller than 40 km. A Helmholtz decomposition of the ADCP spectra and analyses of synthetic and numerical model data show that horizontally divergent, ageostrophic flows account for the discrepancy between the observed spectra and predictions of isotropic interior quasigeostrophic turbulence. In Drake Passage, ageostrophic motions appear to be dominated by inertia-gravity waves and account for about half of the near-surface kinetic energy at scales between 10 and 40 km. Model results indicate that ageostrophic flows imprint on the sea surface, accounting for about half of the sea surface height variance between 10 and 40 km.

Masich, J, Chereskin TK, Mazloff MR.  2015.  Topographic form stress in the Southern Ocean State Estimate. Journal of Geophysical Research-Oceans. 120:7919-7933.   10.1002/2015jc011143   AbstractWebsite

We diagnose the Southern Ocean momentum balance in a 6 year, eddy-permitting state estimate of the Southern Ocean. We find that 95% of the zonal momentum input via wind stress at the surface is balanced by topographic form stress across ocean ridges, while the remaining 5% is balanced via bottom friction and momentum flux divergences at the northern and southern boundaries of the analysis domain. While the time-mean zonal wind stress field exhibits a relatively uniform spatial distribution, time-mean topographic form stress concentrates at shallow ridges and across the continents that lie within the Antarctic Circumpolar Current (ACC) latitudes; nearly 40% of topographic form stress occurs across South America, while the remaining 60% occurs across the major submerged ridges that underlie the ACC. Topographic form stress can be divided into shallow and deep regimes: the shallow regime contributes most of the westward form stress that serves as a momentum sink for the ACC system, while the deep regime consists of strong eastward and westward form stresses that largely cancel in the zonal integral. The time-varying form stress signal, integrated longitudinally and over the ACC latitudes, tracks closely with the wind stress signal integrated over the same domain; at zero lag, 88% of the variance in the 6 year form stress time series can be explained by the wind stress signal, suggesting that changes in the integrated wind stress signal are communicated via rapid barotropic response down to the level of bottom topography.

Firing, YL, Chereskin TK, Watts DR, Tracey KL, Provost C.  2014.  Computation of geostrophic streamfunction, its derivatives, and error estimates from an array of CPIES in Drake Passage. Journal of Atmospheric and Oceanic Technology. 31:656-680.   10.1175/jtech-d-13-00142.1   AbstractWebsite

Current and pressure-recording inverted echo sounders (CPIES) were deployed in an eddy-resolving local dynamics array (LDA) in the eddy-rich polar frontal zone (PFZ) in Drake Passage as part of the cDrake experiment. Methods are described for calculating barotropic and baroclinic geostrophic streamfunction and its first, second, and third derivatives by objective mapping of current, pressure, or geopotential height anomaly data from a two-dimensional array of CPIES like the cDrake LDA.Modifications to previous methods result in improved dimensional error estimates on velocity and higher streamfunction derivatives. Simulations are used to test the reproduction of higher derivatives of streamfunction and to verify mapping error estimates. Three-day low-pass-filtered velocity in and around the cDrake LDA can be mapped with errors of 0.04 m s(-1) at 4000 dbar, increasing to 0.13 m s(-1) at the sea surface; these errors are small compared to typical speeds observed at these levels, 0.2 and 0.65 m s(-1), respectively. Errors on vorticity are 9 x 10(-6) s(-1) near the surface, decreasing with depth to 3 x 10(-6) s(-1) at 4000 dbar, whereas vorticities in the PFZ eddy field are 4 x 10(-5) s(-1) (surface) to 1.3 x 10(-5) s(-1) (4000 dbar). Vorticity gradient errors range from 4 x 10(-10) to 2 x 10(-10) m (-1) s(-1), just under half the size of typical PFZ vorticity gradients. Comparisons between cDrake mapped temperature and velocity fields and independent observations (moored current and temperature, lowered acoustic Doppler current profiler velocity, and satellite-derived surface currents) help validate the cDrake method and results.

Polton, JA, Lenn YD, Elipot S, Chereskin TK, Sprintall J.  2013.  Can Drake Passage observations match Ekman's classic theory? Journal of Physical Oceanography. 43:1733-1740.   10.1175/jpo-d-13-034.1   AbstractWebsite

Ekman's theory of the wind-driven ocean surface boundary layer assumes a constant eddy viscosity and predicts that the current rotates with depth at the same rate as it decays in amplitude. Despite its wide acceptance, Ekman current spirals are difficult to observe. This is primarily because the spirals are small signals that are easily masked by ocean variability and cannot readily be separated from the geostrophic component. This study presents a method for estimating ageostrophic currents from shipboard acoustic Doppler current profiler data in Drake Passage and finds that observations are consistent with Ekman's theory. By taking into account the sampling distributions of wind stress and ageostrophic velocity, the authors find eddy viscosity values in the range of 0.08-0.12 m(2) s(-1) that reconcile observations with the classic theory in Drake Passage. The eddy viscosity value that most frequently reconciles observations with the classic theory is 0.094 m(2) s(-1), corresponding to an Ekman depth scale of 39 m.

Brannigan, L, Lenn YD, Rippeth TP, McDonagh E, Chereskin TK, Sprintall J.  2013.  Shear at the base of the oceanic mixed layer generated by wind shear alignment. Journal of Physical Oceanography. 43:1798-1810.   10.1175/jpo-d-12-0104.1   AbstractWebsite

Observations are used to evaluate a simple theoretical model for the generation of near-inertial shear spikes at the base of the open ocean mixed layer when the upper ocean displays a two-layer structure. The model predicts that large changes in shear squared can be produced by the alignment of the wind and shear vectors. A climatology of stratification and shear variance in Drake Passage is presented, which shows that these assumptions are most applicable to summer, fall, and spring but are not highly applicable to winter. Temperature, salinity, and velocity data from a high spatial resolution cruise in Drake Passage show that the model does not predict all large changes in shear variance; the model is most effective at predicting changes in shear squared when it arises owing to near-inertial wind-driven currents without requiring a rotating resonant wind stress. The model is also more effective where there is a uniform mixed layer above a strongly stratified transition layer. Rotary spectral and statistical analysis of an additional 242 Drake Passage transects from 1999 to 2011 confirmed the presence of this shear-spiking mechanism, particularly in summer, spring, and fall when stratification is stronger.

Watts, DR, Kennelly MA, Donohue KA, Tracey KL, Chereskin TK, Weller RA, Victoria I.  2013.  Four current meter models compared in strong currents in Drake Passage. Journal of Atmospheric and Oceanic Technology. 30:2465-2477.   10.1175/jtech-d-13-00032.1   AbstractWebsite

Seven current meters representing four models on a stiffly buoyed mooring were placed for an 11-month deployment to intercompare their velocity measurements: two vector-measuring current meters (VMCMs), two Aanderaa recording current meter (RCM) us, two Aanderaa SEAGUARDs, and a Nortek Aquadopp. The current meters were placed 6-m apart from each other at about 4000-m depth in an area of Drake Passage expected to have strong currents, nearly independent of depth near the bottom. Two high-current events occurred in bursts of semidiurnal pulses lasting several days, one with peak speeds up to 67 cm s(-1) and the other above 35 cm s(-1). The current-speed measurements all agreed within 7% of the median value when vector averaged over simultaneous time intervals. The VMCMs, chosen as the reference measurements, were found to measure the median of the mean-current magnitudes. The RCM11 and SEAGUARD current speeds agreed within 2% of the median at higher speeds (35-67 cm s(-1)), whereas in lower speed ranges (0-35 cm s(-1)) the vector-averaged speeds for the RCM11 and SEAGUARD were 4%-5% lower and 3%-5% higher than the median, respectively. The shorter-record Aquadopp current speeds were about 6% higher than the VMCMs over the range (0-40 cm s(-1)) encountered.

Holte, JW, Talley LD, Chereskin TK, Sloyan BM.  2013.  Subantarctic mode water in the southeast Pacific: Effect of exchange across the Subantarctic Front. Journal of Geophysical Research: Oceans. 118:2052-2066.   10.1002/jgrc.20144   AbstractWebsite

This study considered cross-frontal exchange as a possible mechanism for the observed along-front freshening and cooling between the 27.0 and 27.3 kg m − 3 isopycnals north of the Subantarctic Front (SAF) in the southeast Pacific Ocean. This isopycnal range, which includes the densest Subantarctic Mode Water (SAMW) formed in this region, is mostly below the mixed layer, and so experiences little direct air-sea forcing. Data from two cruises in the southeast Pacific were examined for evidence of cross-frontal exchange; numerous eddies and intrusions containing Polar Frontal Zone (PFZ) water were observed north of the SAF, as well as a fresh surface layer during the summer cruise that was likely due to Ekman transport. These features penetrated north of the SAF, even though the potential vorticity structure of the SAF should have acted as a barrier to exchange. An optimum multiparameter (OMP) analysis incorporating a range of observed properties was used to estimate the cumulative cross-frontal exchange. The OMP analysis revealed an along-front increase in PFZ water fractional content in the region north of the SAF between the 27.1 and 27.3 kg m − 3 isopycnals; the increase was approximately 0.13 for every 15° of longitude. Between the 27.0 and 27.1 kg m − 3 isopycnals, the increase was approximately 0.15 for every 15° of longitude. A simple bulk calculation revealed that this magnitude of cross-frontal exchange could have caused the downstream evolution of SAMW temperature and salinity properties observed by Argo profiling floats.

Sprintall, J, Chereskin TK, Sweeney C.  2012.  High-resolution underway upper ocean and surface atmospheric observations in Drake Passage: synergistic measurements for climate science. Oceanography. 25:70-81.   10.5670/oceanog.2012.77   AbstractWebsite

This article highlights recent and ongoing studies that analyze in situ underway upper ocean and surface atmospheric observations from frequently repeated transects by the Antarctic Research and Supply Vessel Laurence M. Gould in Drake Passage. High-resolution measurements of upper ocean temperature, salinity, and velocity, along with concurrent shipboard meteorological, surface water CO2, and nutrient sampling have been routinely acquired aboard the Gould since the late 1990s. There are significant benefits and synergy of air-sea observations when they are measured at similar temporal and spatial scales from the same platform. The multiyear measurements have been used to examine seasonal and spatial variability in upper ocean heat content, Antarctic Circumpolar Current transport variability, eddy heat and momentum fluxes, frontal variability, validation of satellite and model-based air-sea fluxes, and the upper ocean response to climate variability. At present, the Gould provides the only year-round shipboard air-sea measurements in the Southern Ocean. Collectively, the measurements in Drake Passage have provided much insight into the characteristics, mechanisms, and impacts of the processes and changes that are occurring within the Southern Ocean.

Holte, JW, Talley LD, Chereskin TK, Sloyan BM.  2012.  The role of air-sea fluxes in Subantarctic Mode Water formation. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007798   AbstractWebsite

Two hydrographic surveys and a one-dimensional mixed layer model are used to assess the role of air-sea fluxes in forming deep Subantarctic Mode Water (SAMW) mixed layers in the southeast Pacific Ocean. Forty-two SAMW mixed layers deeper than 400 m were observed north of the Subantarctic Front during the 2005 winter cruise, with the deepest mixed layers reaching 550 m. The densest, coldest, and freshest mixed layers were found in the cruise's eastern sections near 77 degrees W. The deep. SAMW mixed layers were observed concurrently with surface ocean heat loss of approximately -200 W m(-2). The heat, momentum, and precipitation flux fields of five flux products are used to force a one-dimensional KPP mixed layer model initialized with profiles from the 2006 summer cruise. The simulated winter mixed layers generated by all of the forcing products resemble Argo observations of SAMW; this agreement also validates the flux products. Mixing driven by buoyancy loss and wind forcing is strong enough to deepen the SAMW layers. Wind-driven mixing is central to SAMW formation, as model runs forced with buoyancy forcing alone produce shallow mixed layers. Air-sea fluxes indirectly influence winter SAMW properties by controlling how deeply the profiles mix. The stratification and heat content of the initial profiles determine the properties of the SAMW and the likelihood of deep mixing. Summer profiles from just upstream of Drake Passage have less heat stored between 100 and 600 m than upstream profiles, and so, with sufficiently strong winter forcing, form a cold, dense variety of SAMW.

Hartin, CA, Fine RA, Sloyan BM, Talley LD, Chereskin TK, Happell J.  2011.  Formation rates of Subantarctic mode water and Antarctic intermediate water within the South Pacific. Deep-Sea Research Part I-Oceanographic Research Papers. 58:524-534.   10.1016/j.dsr.2011.02.010   AbstractWebsite

The formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) significantly contributes to the total uptake and storage of anthropogenic gases, such as CO(2) and chlorofluorocarbons (CFCs), within the world's oceans. SAMW and AAIW formation rates in the South Pacific are quantified based on CFC-12 inventories using hydrographic data from WOCE. CLIVAR, and data collected in the austral winter of 2005. This study documents the first wintertime observations of CFC-11 and CFC-12 saturations with respect to the 2005 atmosphere in the formation region of the southeast Pacific for SAMW and AAIW. SAMW is 94% and 95% saturated for CFC-11 and CFC-12, respectively, and AAIW is 60% saturated for both CFC-11 and CFC-12. SAMW is defined from the Subantarctic Front to the equator between potential densities 26.80-27.06 kg m(-3), and AAIW is defined from the Polar Front to 20 degrees N between potential densities 27.06-27.40 kg m(-3). CFC-12 inventories are 16.0 x 10(6) moles for SAMW and 8.7 x 10(6) moles for AAIW, corresponding to formation rates of 7.3 +/- 2.1 Sv for SAMW and 5.8 +/- 1.7 Sv for AAIW circulating within the South Pacific. Inter-ocean transports of SAMW from the South Pacific to the South Atlantic are estimated to be 4.4 +/- 0.6 Sv. Thus, the total formation of SAMW in the South Pacific is approximately 11.7 +/- 2.2 Sv. These formation rates represent the average formation rates over the major period of CFC input, from 1970 to 2005. The CFC-12 inventory maps provide direct evidence for two areas of formation of SAMW, one in the southeast Pacific and one in the central Pacific. Furthermore, eddies in the central Pacific containing high CFC concentrations may contribute to SAMW and to a lesser extent AAIW formation. These CFC-derived rates provide a baseline with which to compare past and future formation rates of SAMW and AAIW. (C) 2011 Elsevier Ltd. All rights reserved.

Lenn, YD, Chereskin TK, Sprintall J, McClean JL.  2011.  Near-Surface Eddy Heat and Momentum Fluxes in the Antarctic Circumpolar Current in Drake Passage. Journal of Physical Oceanography. 41:1385-1407.   10.1175/jpo-d-10-05017.1   AbstractWebsite

The authors present new estimates of the eddy momentum and heat fluxes from repeated high-resolution upper-ocean velocity and temperature observations in Drake Passage and interpret their role in the regional Antarctic Circumpolar Current (ACC) momentum balance. The observations span 7 yr and are compared to eddy fluxes estimated from a 3-yr set of output archived from an eddy-resolving global Parallel Ocean Program (POP) numerical simulation. In both POP and the observations, the stream-averaged cross-stream eddy momentum fluxes ((u'v') over bar) correspond to forcing consistent with both a potential vorticity flux into the axis of the Subantarctic Front (SAF) and a sharpening of all three main ACC fronts through Drake Passage. Further, the POP analysis indicates that the mean momentum advection terms reflect the steering of the mean ACC fronts and are not fully balanced by the eddy momentum forcing, which instead impacts the strength and number of ACC fronts. The comparison between POP and observed eddy heat fluxes was less favorable partly because of model bias in the water mass stratification. Observed cross-stream eddy heat fluxes ((v'T') over bar) are generally surface intensified and poleward in the ACC fronts, with values up to approximately -290 +/- 80 kW m(-2) in the Polar and Southern ACC Fronts. Interfacial form stresses F(T), derived from observed eddy heat fluxes in the SAF, show little depth dependence below the Ekman layer. Although F(T) appears to balance the surface wind stress directly, the estimated interfacial form stress divergence is only an order of magnitude greater than the eddy momentum forcing in the SAF. Thus, although the eddy momentum forcing is of secondary importance in the momentum balance, its effect is not entirely negligible.

Meredith, MP, Woodworth PL, Chereskin TK, Marshall DP, Allison LC, Bigg GR, Donohue K, Heywood KJ, Hughes CW, Hibbert A, Hogg AM, Johnson HL, Jullion L, King BA, Leach H, Lenn YD, Maqueda MAM, Munday DR, Garabato ACN, Provost C, Sallee JB, Sprintall J.  2011.  SUSTAINED MONITORING OF THE SOUTHERN OCEAN AT DRAKE PASSAGE: PAST ACHIEVEMENTS AND FUTURE PRIORITIES. Reviews of Geophysics. 49   10.1029/2010rg000348   AbstractWebsite

Drake Passage is the narrowest constriction of the Antarctic Circumpolar Current (ACC) in the Southern Ocean, with implications for global ocean circulation and climate. We review the long-term sustained monitoring programs that have been conducted at Drake Passage, dating back to the early part of the twentieth century. Attention is drawn to numerous breakthroughs that have been made from these programs, including (1) the first determinations of the complex ACC structure and early quantifications of its transport; (2) realization that the ACC transport is remarkably steady over interannual and longer periods, and a growing understanding of the processes responsible for this; (3) recognition of the role of coupled climate modes in dictating the horizontal transport and the role of anthropogenic processes in this; and (4) understanding of mechanisms driving changes in both the upper and lower limbs of the Southern Ocean overturning circulation and their impacts. It is argued that monitoring of this passage remains a high priority for oceanographic and climate research but that strategic improvements could be made concerning how this is conducted. In particular, long-term programs should concentrate on delivering quantifications of key variables of direct relevance to large-scale environmental issues: In this context, the time-varying overturning circulation is, if anything, even more compelling a target than the ACC flow. Further, there is a need for better international resource sharing and improved spatiotemporal coordination of the measurements. If achieved, the improvements in understanding of important climatic issues deriving from Drake Passage monitoring can be sustained into the future.

Firing, YL, Chereskin TK, Mazloff MR.  2011.  Vertical structure and transport of the Antarctic Circumpolar Current in Drake Passage from direct velocity observations. Journal of Geophysical Research-Oceans. 116   10.1029/2011jc006999   AbstractWebsite

The structure of the Antarctic Circumpolar Current (ACC) in Drake Passage is examined using 4.5 years of shipboard acoustic Doppler current profiler (ADCP) velocity data. The extended 1000 m depth range available from the 38 kHz ADCP allows us to investigate the vertical structure of the current. The mean observed current varies slowly with depth, while eddy kinetic energy and shear variance exhibit strong depth dependence. Objectively mapped streamlines are self-similar with depth, consistent with an equivalent barotropic structure. Vertical wavenumber spectra of observed currents and current shear reveal intermediate wavenumber anisotropy and rotation indicative of downward energy propagation above 500 m and upward propagation below 500 m. The mean observed transport of the ACC in the upper 1000 m is estimated at 95 +/- 2 Sv or 71% of the canonical total transport of 134 Sv. Mean current speeds in the ACC jets remain quite strong at 1000 m, 10-20 cm s(-1). Vertical structure functions to describe the current and extrapolate below 1000 m are explored with the aid of full-depth profiles from lowered ADCP and a 3 year mean from the Southern Ocean State Estimate (SOSE). A number of functions, including an exponential, are nearly equally good fits to the observations, explaining >75% of the variance. Fits to an exponentially decaying function can be extrapolated to give an estimate of 154 +/- 38 Sv for the full-depth transport.

Chereskin, TK, Talley LD, Sloyan BM.  2010.  Nonlinear vorticity balance of the Subantarctic Front in the southeast Pacific. Journal of Geophysical Research-Oceans. 115   10.1029/2009jc005611   AbstractWebsite

Direct velocity observations from shipboard and lowered acoustic Doppler current profilers are used to examine the velocity and vorticity structure of the Subantarctic Front (SAF) between the East Pacific Rise and Drake Passage from surveys made in 2005 and 2006. The SAF is characterized by meanders of horizontal wavelength approximately 250-300 km in this region of relatively smooth topography. The depth-averaged SAF jet is observed to be closely aligned with the flow at 150 m, as in an equivalent barotropic flow. The barotropic or depth-averaged vorticity exhibits a balance between advection of planetary vorticity and relative vorticity, as would be seen in a Doppler-shifted short barotropic Rossby wave in a mean flow. The implied wave speed is consistent with the observed range of current speeds. An exponential fit to the vertical structure of the current consistent with the vorticity balance suggests a vertical decay scale of about 1900 m. The vorticity balance at 150 m implies a surface divergence which must be balanced at depth by a divergence of the opposite sign. The calculation confirms the tentative conclusions of Hughes (2005) for this region, which were based on a surface climatology but indicates a larger vertical decay scale and wave speed.

Sloyan, BM, Talley LD, Chereskin TK, Fine R, Holte J.  2010.  Antarctic Intermediate Water and Subantarctic Mode Water Formation in the Southeast Pacific: The Role of Turbulent Mixing. Journal of Physical Oceanography. 40:1558-1574.   10.1175/2010jpo4114.1   AbstractWebsite

During the 2005 austral winter (late August-early October) and 2006 austral summer (February-mid-March) two intensive hydrographic surveys of the southeast Pacific sector of the Southern Ocean were completed. In this study the turbulent kinetic energy dissipation rate epsilon, diapycnal diffusivity kappa, and buoyancy flux J(b) are estimated from the CTD/O(2) and XCTD profiles for each survey. Enhanced kappa of O(10(-3) to 10(-4) m(2) s(-1)) is found near the Subantarctic Front (SAF) during both surveys. During the winter survey, enhanced kappa was also observed north of the "subduction front,'' the northern boundary of the winter deep mixed layer north of the SAF. In contrast, the summer survey found enhanced kappa across the entire region north of the SAF below the shallow seasonal mixed layer. The enhanced kappa below the mixed layer decays rapidly with depth. A number of ocean processes are considered that may provide the energy flux necessary to support the observed diffusivity. The observed buoyancy flux (4.0 x 10(-8) m(2) s(-3)) surrounding the SAF during the summer survey is comparable to the mean buoyancy flux (0.57 x 10(-8) m(2) s(-3)) associated with the change in the interior stratification between austral summer and autumn, determined from Argo profiles. The authors suggest that reduced ocean stratification during austral summer and autumn, by interior mixing, preconditions the water column for the rapid development of deep mixed layers and efficient Antarctic Intermediate Water and Subantarctic Mode Water formation during austral winter and early spring.

Lenn, YD, Chereskin TK.  2009.  Observations of Ekman Currents in the Southern Ocean. Journal of Physical Oceanography. 39:768-779.   10.1175/2008jpo3943.1   AbstractWebsite

Largely zonal winds in the Southern Ocean drive an equatorward Ekman transport that constitutes the shallowest limb of the meridional overturning circulation of the Antarctic Circumpolar Current (ACC). Despite its importance, there have been no direct observations of the open ocean Ekman balance in the Southern Ocean until now. Using high-resolution repeat observations of upper-ocean velocity in Drake Passage, a mean Ekman spiral is resolved and Ekman transport is computed. The mean Ekman currents decay in amplitude and rotate anticyclonically with depth, penetrating to similar to 100-m depth, above the base of the annual mean mixed layer at 120 m. The rotation depth scale exceeds the e-folding scale of the speed by about a factor of 3, resulting in a current spiral that is compressed relative to predictions from Ekman theory. Transport estimated from the observed currents is mostly equatorward and in good agreement with the Ekman transport computed from four different gridded wind products. The mean temperature of the Ekman layer is not distinguishable from temperature at the surface. Turbulent eddy viscosities inferred from Ekman theory and a direct estimate of the time-averaged stress were O(10(2)-10(3)) cm(2) s(-1). The latter calculation results in a profile of eddy viscosity that decreases in magnitude with depth and a time-averaged stress that is not parallel to the time-averaged vertical shear. The compression of the Ekman spiral and the nonparallel shear-stress relation are likely due to time averaging over the cycling of the stratification in response to diurnal buoyancy fluxes, although the action of surface waves and the oceanic response to high-frequency wind variability may also contribute.

Gay, PS, Chereskin TK.  2009.  Mean structure and seasonal variability of the poleward undercurrent off southern California. Journal of Geophysical Research-Oceans. 114   10.1029/2008jc004886   AbstractWebsite

The magnitude, location, and extent of the California Undercurrent (CUC) off southern California is investigated using cruises of the California Cooperative Oceanic Fisheries Investigations (CalCOFI) from 1993 to 2003 which provide hydrographic, biochemical, and acoustic Doppler current profiler (ADCP) velocity data on a quarterly basis. This study is the first use of the decade-long ADCP time series; it improves on prior geostrophic calculations by providing an absolute velocity reference for estimates of currents and transport. The long-term mean reveals two undercurrent cores in the region south of Point Conception and north of Baja, California: one in the region of the continental slope within the Southern California Bight (SCB) and a second off the Santa Rosa Ridge (SRR). A single core is observed off Point Conception. Spiciness is found to be a good indicator of the presence of the CUC; however, direct velocity observations or a deep reference level are required to resolve the full strength of the CUC cores. In particular, the core off the SRR would be almost entirely missed by geostrophic calculations relative to 500 m, the maximum depth sampled by CalCOFI. The undercurrent transport off Point Conception is estimated to be about 1.7 +/- 0.1 Sv, slightly less than the sum of that estimated for the SCB (1.0 +/- 0.1 Sv) and that estimated off the SRR (1.1 +/- 0.1 Sv) and consistent with some of the flow turning offshore at Point Conception. The CUC in the SCB is strongest in summer, while that off the SRR is strongest in fall. The CUC off Point Conception is strong in both summer and fall, reflecting the confluence of these two branches. Interannual variability is also present, and the velocity and spiciness of the CUC appear to peak during El Nino periods.

Chereskin, TK, Donohue KA, Watts DR, Tracey KL, Firing YL, Cutting AL.  2009.  Strong bottom currents and cyclogenesis in Drake Passage. Geophysical Research Letters. 36   10.1029/2009gl040940   AbstractWebsite

Observations from 38 bottom-moored Current and Pressure Recording Inverted Echo Sounders (CPIES) deployed in Drake Passage during the 2007-2008 International Polar Year provide unprecedented coverage of near-bottom currents and pressures spanning the entire Antarctic Circumpolar Current. Year-long-mean currents exceed 10 cm s(-1) north of the Polar Front, and mean directions are not, in general, aligned with the surface fronts. Topographic steering is most evident at the continental margins. Deep eddy kinetic energy (EKE) is maximum at about 200 cm(2) s(-2) between the Subantarctic and Polar Fronts, coinciding with the location but about one quarter of the value of a maximum in surface EKE. Multiple high-speed current events, with peak speeds of 60-70 cm s(-1) and lasting 30 to 70 days, are coherent across sites separated by 45 km. The observed spinup of eddies coinciding with meanders in the surface fronts is consistent with deep cyclogenesis. Citation: Chereskin, T. K., K. A. Donohue, D. R. Watts, K. L. Tracey, Y. L. Firing, and A. L. Cutting (2009), Strong bottom currents and cyclogenesis in Drake Passage, Geophys. Res. Lett., 36, L23602, doi:10.1029/2009GL040940.

Lenn, YD, Chereskin TK, Sprintall J.  2008.  Improving estimates of the Antarctic Circumpolar Current streamlines in Drake Passage. Journal of Physical Oceanography. 38:1000-1010.   10.1175/2007jpo3834.1   AbstractWebsite

Accurately resolving the mean Antarctic Circumpolar Current (ACC) is essential for determining Southern Ocean eddy fluxes that are important to the global meridional overturning circulation. Previous estimates of the mean ACC have been limited by the paucity of Southern Ocean observations. A new estimate of the mean surface ACC in Drake Passage is presented that combines sea surface height anomalies measured by satellite altimetry with a recent dataset of repeat high-resolution acoustic Doppler current profiler observations. A mean streamfunction (surface height field), objectively mapped from the mean currents, is used to validate two recent dynamic height climatologies. The new streamfunction has narrower and stronger ACC fronts separated by quiescent zones of much weaker flow, thereby improving on the resolution of ACC fronts observed in the other climatologies. Distinct streamlines can be associated with particular ACC fronts and tracked in time-dependent maps of dynamic height. This analysis shows that varying degrees of topographic control are evident in the preferred paths of the ACC fronts through Drake Passage.

Lenn, YD, Chereskin TK, Sprintall J, Firing E.  2007.  Mean jets, mesoscale variability and eddy momentum fluxes in the surface layer of the Antarctic Circumpolar Current in Drake Passage. Journal of Marine Research. 65:27-58.   10.1357/002224007780388694   AbstractWebsite

High-resolution Acoustic Doppler Current Profiler (ADCP) observations of surface-layer velocities in Drake Passage, comprising 128 sections over a period of 5 years, are used to study the surface-layer circulation of the Antarctic Circumpolar Current (ACC). These observations resolve details of the mean flow including the topographic control of the mean Subantarctic Front (SAF) and the multiple filaments of the Polar Front (PF) and Southern ACC Front (SACCF) that converge into single mean jets as the ACC flows through Drake Passage. Subsurface definitions of the SAF and PF applied to expendable bathythermograph temperatures generally coincide with mean jets, while the SACCF is better defined in velocity than temperature. The mean transport in the top 250-m-deep surface layer, estimated from the cross-track transport along three repeat tracks, is 27.8 +/- 1 Sv. Eddy momentum fluxes were estimated by ensemble averaging Reynolds stresses relative to gridded Eulerian mean currents. Eddy kinetic energy (EKE) is surface intensified in the mixed layer because of inertial currents and decreases poleward in Drake Passage, ranging from similar to 800 cm(2) s(-2) to similar to 200 cm(2) s(-2). ADCP EKE estimates are everywhere significantly higher than altimetric EKE estimates, although the pattern of poleward decrease is the same. Horizontal-wavenumber spectra of velocity fluctuations peak at wavelengths in the 250-330 km range and are significantly anisotropic. Along-passage fluctuations dominate at wavelengths less than 250 km; cross-passage fluctuations dominate at wavelengths greater than 250 km. Mesoscale eddies dominate the variance in northern Drake Passage. Inertial variability is constant with latitude and together with baroclinic tides accounts for some but not all of the discrepancy between the ADCP surface-layer EKE and altimetry-inferred EKE.