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Russell, JL, Kamenkovich I, Bitz C, Ferrari R, Gille ST, Goodman PJ, Hallberg R, Johnson K, Khazmutdinova K, Marinov I, Mazloff M, Riser S, Sarmiento JL, Speer K, Talley LD, Wanninkhof R.  2018.  Metrics for the Evaluation of the Southern Ocean in Coupled Climate Models and Earth System Models. Journal of Geophysical Research: Oceans.   10.1002/2017JC013461   AbstractWebsite


Firing, YL, Chereskin TK, Watts RD, Mazloff MR.  2016.  Bottom pressure torque and the vorticity balance from observations in Drake Passage. Journal of Geophysical Research: Oceans. :n/a–n/a.   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 4×10−9 m s−2, exceeds the mean wind stress curl by a factor of 15–20.

Erickson, ZK, Thompson AF, Cassar N, Sprintall J, Mazloff MR.  2016.  An advective mechanism for Deep Chlorophyll Maxima formation in southern Drake Passage. Geophysical Research Letters. :n/a–n/a.   10.1002/2016GL070565   AbstractWebsite

We observe surface and sub-surface fluorescence-derived chlorophyll maxima in southern Drake Passage during austral summer. Backscatter measurements indicate that the Deep Chlorophyll Maxima (DCMs) are also deep biomass maxima, and euphotic depth estimates show that they lie below the euphotic layer. Sub-surface, off-shore and near-surface, on-shore features lie along the same isopycnal, suggesting advective generation of DCMs. Temperature measurements indicate a warming of surface waters throughout austral summer, capping the Winter Water (WW) layer and increasing off-shelf stratification in this isopycnal layer. The outcrop position of the WW isopycnal layer shifts onshore, into a surface phytoplankton bloom. A lateral potential vorticity (PV) gradient develops, such that a down-gradient PV flux is consistent with offshore, along-isopycnal tracer transport. Model results are consistent with this mechanism. Subduction of chlorophyll and biomass along isopycnals represents a biological term not observed by surface satellite measurements which may contribute significantly to the strength of the biological pump in this region.

Rodriguez, AR, Mazloff MR, Gille ST.  2016.  An oceanic heat transport pathway to the Amundsen Sea Embayment. Journal of Geophysical Research: Oceans. 121:3337–3349.   10.1002/2015JC011402   AbstractWebsite

The Amundsen Sea Embayment (ASE) on the West Antarctic coastline has been identified as a region of accelerated glacial melting. A Southern Ocean State Estimate (SOSE) is analyzed over the 2005–2010 time period in the Amundsen Sea region. The SOSE oceanic heat budget reveals that the contribution of parameterized small-scale mixing to the heat content of the ASE waters is small compared to advection and local air-sea heat flux, both of which contribute significantly to the heat content of the ASE waters. Above the permanent pycnocline, the local air-sea flux dominates the heat budget and is controlled by seasonal changes in sea ice coverage. Overall, between 2005 and 2010, the model shows a net heating in the surface above the pycnocline within the ASE. Sea water below the permanent pycnocline is isolated from the influence of air-sea heat fluxes, and thus, the divergence of heat advection is the major contributor to increased oceanic heat content of these waters. Oceanic transport of mass and heat into the ASE is dominated by the cross-shelf input and is primarily geostrophic below the permanent pycnocline. Diagnosis of the time-mean SOSE vorticity budget along the continental shelf slope indicates that the cross-shelf transport is sustained by vorticity input from the localized wind-stress curl over the shelf break.

Mazloff, MR, Boening C.  2016.  Rapid variability of Antarctic Bottom Water transport into the Pacific Ocean inferred from GRACE. Geophysical Research Letters. 43:3822–3829.   10.1002/2016GL068474   AbstractWebsite

Air-ice-ocean interactions in the Antarctic lead to formation of the densest waters on Earth. These waters convect and spread to fill the global abyssal oceans. The heat and carbon storage capacity of these water masses, combined with their abyssal residence times that often exceed centuries, makes this circulation pathway the most efficient sequestering mechanism on Earth. Yet monitoring this pathway has proven challenging due to the nature of the formation processes and the depth of the circulation. The Gravity Recovery and Climate Experiment (GRACE) gravity mission is providing a time series of ocean mass redistribution and offers a transformative view of the abyssal circulation. Here we use the GRACE measurements to infer, for the first time, a 2003–2014 time series of Antarctic Bottom Water export into the South Pacific. We find this export highly variable, with a standard deviation of 1.87 sverdrup (Sv) and a decorrelation timescale of less than 1 month. A significant trend is undetectable.

Peña-Molino, B, Rintoul SR, Mazloff MR.  2014.  Barotropic and baroclinic contributions to along-stream and across-stream transport in the Antarctic Circumpolar Current. Journal of Geophysical Research: Oceans. 119:8011–8028.   10.1002/2014JC010020   AbstractWebsite

The Southern Ocean's ability to store and transport heat and tracers as well as to dissipate momentum and energy are intimately related to the vertical structure of the Antarctic Circumpolar Current (ACC). Here the partition between barotropic and baroclinic flow in the time-mean ACC is investigated in a Southern Ocean state estimate. The zonal geostrophic transport is predominantly baroclinic, with at most 25% of the transport at any longitude carried by the barotropic component. Following surface streamlines, changes in vertical shear and near-bottom velocity are large, and result in changes in the local partition of barotropic/baroclinic vertically integrated transport from 10/90% in the center of the basins, to 50/50% near complex topography. The velocity at depth is not aligned with the surface velocity. This nonequivalent barotropic flow supports significant cross-stream transports. Barotropic and baroclinic mass transport across the ACC is, on average, in opposite directions, with the net barotropic cross-stream transport being poleward and the net baroclinic equatorward. The sum partially cancels out, leaving a net geostrophic poleward transport across the different fronts between −5 and −20 Sv. Temperature is also transported across the fronts by the nonequivalent barotropic part of the ACC, with maximum values across the northern ACC fronts equivalent to −0.2 PW. The sign and magnitude of these transports are not sensitive to the choice of stream-coordinate. These cross-stream volume and temperature transports are variable in space, and dependent on the interactions between deep flow and bathymetry, thus difficult to infer from surface and hydrographic observations alone.

van Sebille, E, Spence P, Mazloff MR, England MH, Rintoul SR, Saenko OA.  2013.  Abyssal connections of Antarctic Bottom Water in a Southern Ocean State Estimate. Geophysical Research Letters. 40:2177–2182.   10.1002/grl.50483   AbstractWebsite

Antarctic Bottom Water (AABW) is formed in a few locations around the Antarctic continent, each source with distinct temperature and salinity. After formation, the different AABW varieties cross the Southern Ocean and flow into the subtropical abyssal basins. It is shown here, using the analysis of Lagrangian trajectories within the Southern Ocean State Estimate (SOSE) model, that the pathways of the different sources of AABW have to a large extent amalgamated into one pathway by the time it reaches 31°S in the deep subtropical basins. The Antarctic Circumpolar Current appears to play an important role in the amalgamation, as 70% of the AABW completes at least one circumpolar loop before reaching the subtropical basins. This amalgamation of AABW pathways suggests that on decadal to centennial time scales, changes to properties and formation rates in any of the AABW source regions will be conveyed to all three subtropical abyssal basins.

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:n/a–n/a.   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.