Dynamics and Transport of the Antarctic Circumpolar Current in Drake Passage
Teresa Chereskin, Kathy Donohue and Randy Watts
The Southern Ocean is especially sensitive to climate change,
responding to winds that have increased over the past 30 years and
warming significantly more than the global ocean over the past 50
years. The ACC is the pulse of the Southern Ocean, and the Drake
Passage chokepoint is not only well suited geographically for measuring
its time-varying transport, but observations and computer models
suggest that dynamical balances which control its transport are
particularly effective through the Drake Passage.
The goal of this program is to quantify the transport and understand the dynamical balances of the Antarctic Circumpolar Current (ACC) in Drake Passage. For this purpose we have proposed a transport line and a local dynamics array of CPIES (Current and Pressure recording Inverted Echo Sounders) spanning the predominant along-stream wavelength of 250 km for 4 years. Data will be collected annually by acoustic telemetry, leaving the instruments undisturbed until recovered.
This project will contribute to the International Polar Year (IPY) through its transport line monitoring of the ACC in Drake Passage. The observations will resolve the seasonal and interannual variability of the total ACC transport, its vertical structure partitioned between barotropic and baroclinic components, and its lateral structure partitioned among the multiple jets comprised by the ACC. Moreover, Drake Passage is a region of high mesoscale variability. The mesoscale eddies are thought to play a mediating role in transferring momentum from the circumpolar winds that drive the ACC, down through the water column to the sea floor, where topographic form stresses regulate its long-term transport. Measurements in the local dynamics array will quantify eddy exchanges with the mean current and density structure, and they will quantify the mean vorticity balance in order to test hypotheses regarding the dynamical balances that govern the ACC.
Figure 1: Top panel: Proposed array of CPIES (black diamonds) and current meters (blue circles) which will measure the ACC across Drake Passage and map the circulation and eddy currents within a local dynamics array. The cross-passage array is coincident with an ENVISAT track (grey lines) and lies east of the FDRAKE79 current meter moorings (magenta stars) and beneath the LMG repeat ADCP/XBT transects (green lines). Nearby observational programs include the DRAKE Mooring Line, PI C. Provost (red), SR1b repeat hydrographic section (blue line) and bottom pressure recorders (blue circles at terminations of SR1b). The inset to left of the map illustrates the configuration of two deep current meters moorings (blue circles) above the northern continental slope. Each mooring has current meters 100, 300 and 600 meters off the bottom. ACC frontal locations (bold black lines) are closely tied to bathymetry (contoured every 1000 m depth; tans hues represent shallow depths transition to blue hues in the deeper parts of the passage). Bottom panel: time line of experiment.
Figure 2: CPIES instrumentation suite
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