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

Characterizing the transition from balanced to unbalanced motions in the Southern California Current

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124(3), (2019): 2088-2109, doi:10.1029/2018JC014583.As observations and models improve their resolution of oceanic motions at ever finer horizontal scales, interest has grown in characterizing the transition from the geostrophically balanced flows that dominate at large‐scale to submesoscale turbulence and waves that dominate at small scales. In this study we examine the mesoscale‐to‐submesoscale (100 to 10 km) transition in an eastern boundary current, the southern California Current System (CCS), using repeated acoustic Doppler current profiler transects, sea surface height from high‐resolution nadir altimetry and output from a (1/48)° global model simulation. In the CCS, the submesoscale is as energetic as in western boundary current regions, but the mesoscale is much weaker, and as a result the transition lacks the change in kinetic energy (KE) spectral slope observed for western boundary currents. Helmholtz and vortex‐wave decompositions of the KE spectra are used to identify balanced and unbalanced contributions. At horizontal scales greater than 70 km, we find that observed KE is dominated by balanced geostrophic motions. At scales from 40 to 10 km, unbalanced contributions such as inertia‐gravity waves contribute as much as balanced motions. The model KE transition occurs at longer scales, around 125 km. The altimeter spectra are consistent with acoustic Doppler current profiler/model spectra at scales longer than 70/125 km, respectively. Observed seasonality is weak. Taken together, our results suggest that geostrophic velocities can be diagnosed from sea surface height on scales larger than about 70 km in the southern CCS.This research was funded by NASA (NNX13AE44G, NNX13AE85G, NNX16AH67G, NNX16AO5OH, and NNX17AH53G). We thank Sung Yong Kim for providing the high‐frequency radar spectral estimates and the two anonymous reviewers for providing useful comments and suggestions that greatly improved the manuscript. High‐frequency ALES data for Jason‐1 and Jason‐2 altimeters are available upon request (https://openadb.dgfi.tum.de/en/contact/ALES). Both AltiKa and Sentinel‐3 altimeter products were produced and distributed by the Copernicus Marine and Environment Monitoring Service (CMEMS; http://www.marine.copernicus.eu). D. M. worked on the modeling component of this study at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). High‐end computing resources were provided by the NASA Advanced Supercomputing (NAS) Division of the Ames Research Center. The LLC output can be obtained from the ECCO project (ftp://ecco.jpl.nasa.gov/ECCO2/LLC4320/). The ADCP data are available at the Joint Archive for Shipboard ADCP data (JASADCP; http://ilikai.soest.hawaii.edu/sadcp).2019-08-2

Mean Antarctic Circumpolar Current Transport Measured in Drake Passage

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 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.Fil: Donohue, K. A.. University Of Rhode Island; Estados UnidosFil: Tracey, K. L.. University Of Rhode Island; Estados UnidosFil: Watts, D. R.. University Of Rhode Island; Estados UnidosFil: Chidichimo, María Paz. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografía Naval; Argentina. Universidad de Buenos Aires; ArgentinaFil: Chereskin, T. K.. University of California at San Diego. Scripps Institution of Oceanography; Estados Unido

Observations of near-inertial waves in acoustic doppler current profiler measurements made during the mixed layer dynamics experiment

Measurements of upper ocean shear made during the Mixed Layer Dynamics Experiment (MILDEX) provide evidence of large horizontal scale motion at near‐inertial frequency. The measurements consist of shipboard acoustic Doppler current profiles. Four large‐scale spatial surveys of 2–4 days duration were made by the R/V Wecoma as a set of boxes approximately 60 km per side around a drifting current meter buoy. Velocity time series from the drifting buoy and from sonar measurements made from FLIP also indicated the presence of motions at near‐inertial frequency. Horizontal length and time scales of the motion are estimated from the phase of the shear vector measured during the spatial surveys. Estimates of the length scale of the waves range from 500 to 1000 km, and the frequency is approximately 1.1f. The behavior of the phase is found to be consistent with a model of narrow‐band inertial waves with vertical structure such that there is a zero crossing in velocity at the base of the mixed layer (40–60 m)

The role of the turbulent stress divergence in the equatorial Pacific zonal momentum balance

From a comprehensive set of upper ocean measurements made during a moderate El Niño in boreal spring 1987, we reassess the role of turbulence in transporting momentum vertically at the equator. An examination of the terms in the vertically integrated zonal momentum equations indicates that on short time scales the zonal pressure gradient is not balanced by the surface wind stress despite an apparent balance of these terms on longer (seasonal) time scales. The vertical redistribution of zonal momentum is complex. The strength of the wind determines both the magnitude and, likely, the mechanisms of momentum transport between the surface and the core of the undercurrent. During low wind conditions in April 1987 the turbulent stress divergence was significantly different in magnitude and vertical structure from that found during strong winds in November 1984. In November 1984 the turbulent stress divergence was much too large above 40 m to balance the residual term in the zonal momentum budget of Bryden and Brady (1985, 1989) and decayed exponentially with depth from the wind stress value at the surface. In April 1987 the turbulent stress divergence was smaller than that required by Bryden and Brady and decayed linearly from the surface wind stress. For a proper comparison with Bryden and Brady’s zonal momentum balance, it is necessary to determine the annual average turbulent stress divergence

Fine-scale variability at 140°W in the Equatorial Pacific

In November-December 1984 we carried out an intensive 12-day upper ocean sampling program on the equator at 140°W as part of the Tropic Heat Experiment. From our observations we constructed hourly averaged profiles of temperature, salinity, σ₁, turbulent kinetic energy dissipation rate, and horizontal velocity. These data were used to examine the correspondence between hydrographic and velocity fields and to compare the measured turbulent dissipations with the calculated Richardson numbers. We found that the core of the Equatorial Undercurrent tracked a density surface (σ₁ = 25.25) on times as short as 1 hour. The variability in both hydrographic and velocity fields was greatest at the semidiurnal frequency. The supertidal energy was not significantly different from the Garrett-Munk mid-latitude level once latitudinal scaling was removed from the Garrett-Munk model parameters. Horizontal velocity spectra were found to be contaminated by displacement of the background shear. Turbulent dissipation was dominated by a dirunal cycle, with high values of dissipation occurring at night above the undercurrent core. Shear and buoyancy frequency, calculated over 12-m vertical scales, were observed to track each other above the core and were dominated by a diurnal period above 40 m and by a semidiurnal period below 40 m. When shear and buoyancy frequency were combined to form a Richardson number, neither diurnal nor semidiurnal cycles were present. Above the undercurrent core, the Richardson numbers were uniformly small (0.3 to 0.6)

Spatial and temporal characteristics of the mesoscale circulation of the California Current from eddy-resolving moored and shipboard measurements

Moored observations of currents and temperatures made in the upper 600 m on eddy-resolving scales over a 2-year period are used to examine the spatial and temporal characteristics of the California Current mesoscale circulation. The observations were made at three principal longitudes: 124°W, 126°W, and 128°W in the vicinity of Point Arena. They bracket the 600-km-wide band of high mesoscale variability found along the eastern boundary of the North Pacific. At all locations, the mesoscale variability was larger than the mean flow, and the spatial modes of variability as determined from empirical orthogonal function analysis consisted of an alongshore mode, a cross-shore mode, and a rotational mode. Observations made near the continental slope (124°W) were dominated by the poleward flowing California Undercurrent, with mesoscale eddies and meanders superposed. The nearshore eddy kinetic energy peaked in a band centered around 60 days. Observations made at 128°W, near the offshore boundary between the energetic mesoscale band and the "eddy desert" of the northeast Pacific, were characterized by small means, fewer eddy events, and a peak in eddy kinetic energy at 120-180 days. The good horizontal resolution of the current meter arrays allowed us to estimate the relative vorticity, horizontal divergence, and Rossby number and therefore to evaluate the relative strength and occurrence of anticyclones and cyclones. We found the mesoscale eddy field to be strongly nonlinear, with Rossby numbers ranging from 0.1 to 0.5. All of the eddies observed at the offshore site were nonlinear, deep, warm anticyclones. Shipboard hydrography revealed the origin of one of these anticyclones to be the California Undercurrent, and this eddy retained its strong anomalies after several months and several hundred kilometers of propagation. Despite the lower incidence of eddies as one moves west from the coast, the eddies that we observed offshore provide evidence for propagation and transport of properties from the coast to the central North Pacific across the California Current Syste