Cross-Frontal Fluxes in the Antarctic Circumpolar Current near Udintsev Fracture Zone

University Of Rhode Island, Kingston RI

Investigators

Abstract

Overview: Across the Antarctic Circumpolar Current (ACC), fluxes of heat and momentum and other dynamical quantities establish the heat budget for the whole Southern Ocean, govern the strength of the ACC and its vital inter-ocean exchanges, and arguably govern the main vertical density structure of the world oceans. Yet, theoretical concepts, numerical models, and the sparse observational records give conflicting views of the mechanisms and magnitude of transfers across the ACC. South of about 55° S the puzzle is further confounded, because the heat fluxes associated with wind-driven Ekman transport and its counterbalancing meridional overturning circulation, that are strong and dominant at lower latitudes, become weak. At that latitude, it falls to cross-frontal processes arising from mean and eddying flow to complete the heat budget. It is particularly puzzling how fluxes cross the several ocean fronts, e.g., from Subantarctic Front to Polar Front to Southern ACC Front. Observations of heat and momentum fluxes and mixing are vitally needed at additional sites around the ACC to inform global climate modeling efforts. Of particular interest is how the oceanic meridional overturning circulation absorbs and redistributes heat and carbon that is critical to the earth's climate. In this study, an array of Current and Pressure recording Inverted Echo Sounders (CPIES) will be deployed for a period of two years to observe and quantify exchanges of buoyancy, heat, momentum and potential vorticity across the ACC as it flows near the Udintsev Fracture Zone in the central South Pacific sector of the Southern Ocean. The CPIES have been shown to be particularly well suited for these measurements. The Udintsev Fracture Zone has been selected because the three main fronts (Subantarctic Front, Polar Front, and Southern ACC Front) converge closely together as they deflect equatorward where they encounter complex shallower ridges and exhibit strong eddy variability in the lee. Therefore, the Udintsev Fracture Zone emerges as a likely location for mean and eddy fluxes to combine to cross the entire ACC system. This project is an international collaboration with a team of French and Korean scientists with complementary goals to deploy tall current meter moorings and conduct CTD surveys in this region. Intellectual Merit: Mesoscale eddies are thought to play a key role in ACC dynamics, with cross-frontal exchange by eddies forming the underpinning of the meridional overturning circulation. Recent studies suggest that this exchange occurs locally in "hot spots" found in the lee of bathymetric obstacles, yet direct estimates of eddy fluxes from ocean measurements are scarce, both globally and especially within the Southern Ocean. Two Current and Pressure recording Inverted Echo Sounders (CPIES) arrays are proposed. The northern array is within the strong eddy region between the Subantarctic Front and Polar Front (where they neighbor closely), and the southern array is located near the Southern ACC Front where historically few measurements have been made. These sites have been selected as candidates where eddy and mean cross-frontal fluxes are likely to be strongest. The CPIES arrays will quantify the role of eddies in redistributing heat, momentum and energy throughout the full water column. The French and Korean tall moorings will provide measurements of currents and temperature at fixed levels, providing complementary estimates of mean and eddy fluxes. The knowledge gained regarding eddy fluxes is crucially important for conceptual understanding of the dynamical balances that control the ACC and drive the overturning circulation and to quantitatively guide future modeling efforts. Broader Impacts: A quality-controlled daily time series of density and currents determined from the CPIES measurements will be disseminated broadly. Combined with data from the current meter moorings, data products suitable for validating numerical models and improving prediction of the cross-frontal flux of momentum, heat, and potential vorticity will be produced. New technology for remote transmission of CPIES data via pop-up data capsules will be tested for use in the strong current, high sea state remote Southern Ocean. Dual pressure sensors will be installed in two CPIES to investigate the calibration-drift of these sensors. An engineering undergraduate student will join the University of Rhode Island technical team and assist CPIES upgrades and preparations.