Transport and dynamics of the ACC in Drake Passage from observations made during cDrake

CHERESKIN, T.; DONOHUE, K. A.; D. R. WATTS; CHIDICHIMO, M. P.; FIRING, Y.; TRACEY, K. L.

Boulder

Workshop; National Center for Atmospheric Research (NCAR) Southern Ocean Workshop; 2017

National Center for Atmospheric Research (NCAR)

The cDrake program deployed a transport line and local dynamics array (LDA) of Current Meters and Pressure-recording Inverted Echo Sounders (CPIES) from 2007 to 2011 in Drake Passage to quantify the transport and dynamics of the Antarctic Circumpolar Current (ACC). The ACC was continuously monitored with a line of moored instrumentation with unprecedented horizontal and temporal resolution. Annual mean near-bottom currents are remarkably stable, yielding an estimated 45.6 Sv of barotropic transport with an uncertainty of 8.9 Sv. Together with the mean baroclinic transport relative to the seafloor of 127.7 Sv, the total mean ACC transport is 173.3 Sv. This transport is about 30% larger than the canonical value often used as a benchmark for numerical models, and the increase is entirely due to the contribution of the deep currents. The near-bottom currents are strongly topographically steered; however, they contribute to large bottom pressure torques where strong currents cross steep topography at small angles. The mean bottom pressure torque estimated for the cDrake line exceeds the wind stress curl by a factor of 15-20. The time-varying deep currents are found to be the prime agents in fluxing heat across the ACC in northern Drake Passage, when baroclinic instability results in barotropic geostrophic currents crossing the baroclinic Subantarctic and Polar Fronts. The average poleward mean-plus-transient eddy heat flux integrated vertically and horizontally across the 250-km length of the cDrake LDA is -0.013 +/- 0.005 PW. This 250-km LDA segment represents about 1.2% of the circumglobal path of the ACC and accounts for about 3.3% of the estimated total heat loss from ocean to atmosphere south of 60S. We estimate that the full Drake Passage would account for about 4-6 times the LDA estimate. About 6 hot spots like Drake Passage could account for the major portion of heat lost to the atmosphere south of 60S, with the balance coming from weaker eddy heat fluxes plus mean cross-front transport.