Mean jets, mesoscale variability and eddy momentum fluxes in the surface layer of the Antarctic Circumpolar Current in Drake Passage

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 ∼800 cm2 s−2 to ∼200 cm2 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

Can Drake Passage Observations Match Ekman's Classic Theory?

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 m2 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 m2 s−1, corresponding to an Ekman depth scale of 39 m

Bottom pressure torque and the vorticity balance from observations in Drake Passage

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 urn:x-wiley:21699275:media:jgrc21771:jgrc21771-math-0001 m s−2, exceeds the mean wind stress curl by a factor of 15–20

Mesoscale to Submesoscale Wavenumber Spectra in Drake Passage

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

Wind-Driven Processes Controlling Oceanic Heat Delivery to the Amundsen Sea, Antarctica

Variability in the heat delivery by Circumpolar Deep Water (CDW) is responsible for modulating the basal melting of the Amundsen Sea ice shelves. However, the mechanisms controlling the CDW inflow to the region’s continental shelf remain little understood. Here, a high-resolution regional model is used to assess the processes governing heat delivery to the Amundsen Sea. The key mechanisms are identified by decomposing CDW temperature variability into two components associated with 1) changes in the depth of isopycnals [heave (HVE)], and 2) changes in the temperature of isopycnals [water mass property changes (WMP)]. In the Dotson–Getz trough, CDW temperature variability is primarily associated with WMP. The deeper thermocline and shallower shelf break hinder CDW access to that trough, and CDW inflow is regulated by the uplift of isopycnals at the shelf break—which is itself controlled by wind-driven variations in the speed of an undercurrent flowing eastward along the continental slope. In contrast, CDW temperature variability in the Pine Island–Thwaites trough is mainly linked to HVE. The shallower thermocline and deeper shelf break there permit CDW to persistently access the continental shelf. CDW temperature in the area responds to wind-driven modulation of the water mass on-shelf volume by changes in the rate of inflow across the shelf break and in Ekman pumping-induced vertical displacement of isopycnals within the shelf. The western and eastern Amundsen Sea thus represent distinct regimes, in which wind forcing governs CDW-mediated heat delivery via different dynamics

Energetics of M₂ Barotropic-to-Baroclinic Tidal Conversion at the Hawaiian Islands

A high-resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide (M₂) over a region of the Hawaiian Ridge from Niihau to Maui. This region includes the Kaena Ridge, one of the three main internal tide generation sites along the Hawaiian Ridge and the main study site of the Hawaii Ocean Mixing Experiment. The 0.01°–horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations, moored ADCP currents, and notably reasonable agreement with microstructure data. Barotropic and baroclinic energy equations are derived from the model’s sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget. The M₂ barotropic tide loses 2.7 GW of energy over the study region. Of this, 163 MW (6%) is dissipated by bottom friction and 2.3 GW (85%) is converted into internal tides. Internal tide generation primarily occurs along the flanks of the Kaena Ridge and south of Niihau and Kauai. The majority of the baroclinic energy (1.7 GW) is radiated out of the model domain, while 0.45 GW is dissipated close to the generation regions. The modeled baroclinic dissipation within the 1000-m isobath for the Kaena Ridge agrees to within a factor of 2 with the area-weighted dissipation from 313 microstructure profiles. Topographic resolution is important, with the present 0.01° resolution model resulting in 20% more barotropic-to-baroclinic conversion compared to when the same analysis is performed on a 4-km resolution simulation. A simple extrapolation of these results to the entire Hawaiian Ridge is in qualitative agreement with recent estimates based on satellite altimetry data

Stabilization of dense Antarctic water supply to the Atlantic Ocean overturning circulation

The lower limb of the Atlantic overturning circulation is resupplied by the sinking of dense Antarctic Bottom Water (AABW) that forms via intense air–sea–ice interactions next to Antarctica, especially in the Weddell Sea. In the last three decades, AABW has warmed, freshened and declined in volume across the Atlantic Ocean and elsewhere, suggesting an ongoing major reorganization of oceanic overturning. However, the future contributions of AABW to the Atlantic overturning circulation are unclear. Here, using observations of AABW in the Scotia Sea, the most direct pathway from the Weddell Sea to the Atlantic Ocean, we show a recent cessation in the decline of the AABW supply to the Atlantic overturning circulation. The strongest decline was observed in the volume of the densest layers in the AABW throughflow from the early 1990s to 2014; since then, it has stabilized and partially recovered. We link these changes to variability in the densest classes of abyssal waters upstream. Our findings indicate that the previously observed decline in the supply of dense water to the Atlantic Ocean abyss may be stabilizing or reversing and thus call for a reassessment of Antarctic influences on overturning circulation, sea level, planetary-scale heat distribution and global climate

Improving the Quality and Accessibility of Current Profile Measurements in the Southern Ocean

Like most modern oceanographic research vessels, RVIB Nathaniel B. Palmer and ARSV Laurence M. Gould are equipped with acoustic Doppler current profilers (ADCPs) for measuring the structure of ocean currents over a range of several hundred meters below the hull, both on station and while underway. It takes more than the ADCP itself, however, to yield good current measurements. The end result depends on how and where the sonar is installed; on the quality of ancillary information including position, heading, and, for some sonars, speed of sound at the transducer; on the data acquisition and processing techniques; and on ambient conditions of weather, ice, noise, and the availability of acoustic scatterers in the water (Firing and Hummon, 2010). In addition, the value of the measurements depends not only on their accuracy but also on their accessibility to scientific users both in near real time at sea and as a final product ashore