Award: OCE-0622630

The two polar oceans on the earth, the Arctic Ocean and the Southern Ocean, are unique in having sub-regions in which no land boundaries block flow along latitude lines for a full 360 degrees of longitude. In the latter, this domain is home to the Antarctic Circumpolar Current (ACC), one of the major circulation features on the globe. Significant watermass transformations forced by air-sea/ice-sea exchange in the Southern Ocean produce major bottom and intermediate waters that are traceable as they spread throughout much of the world's oceans. Owing to the lack of zonal boundaries in the Southern Ocean, the dynamics governing the meridional exchange of these newly-modified waters northward, and their parent waters southward across the ACC are dominated by temporally-/spatially- variable eddy motions having horizontal scales of order 10 km and time scales of days to weeks. On space of order 1-m or smaller, turbulent mixing processes exchange water properties vertically and potentially short-circuit some of the eddy-driven meridional exchange in the Southern Ocean with internal waves providing a dynamical connection between these disparate scales of motion, extracting energy from the eddy motions and supplying it to the small-scale turbulence. Importantly, models used to assess climate states of the earth are unable to resolve eddy or smaller-scale motions; the effects of these motions must be parameterized. But to do so, one must first develop physical understanding of the processes involved and their influences on larger-scale motions. Building understanding of lateral eddy stirring processes and vertical turbulent mixing processes in the ocean that will, in turn, hopefully lead to climate model improvements constitute the main goals of the international Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). (The isopycnal direction is defined to lie along density surfaces, which in the ocean are very nearly horizontal, thus the "I" piece of DIMES is chiefly concerned with quasi-horizontal eddy stirring. The "D" in DIMES, denoting the diapycnal - across density surfaces - direction is concerned with "nearly-vertical" turbulent mixing.) Focal point for the DIMES field program is tracking the spread of a chemical tracer injected into the Southeast Pacific Ocean around 1500 m depth in the ACC as it spreads laterally and vertically while being carried east by the current. The spreading rates yield time-average estimates of the eddy stirring and turbulent mixing rates. Research funded by the present grant, in concert with a companion program led by Dr. Louis St. Laurent and field work conducted by U.K. colleagues, focused on the turbulent mixing processes active in the Southern Ocean. Our approach was to join the tracer survey cruises and repeatedly deploy profiling instrumentation to sample the turbulent fluctuations responsible for diapycnal mixing, as well as sensors to document the motions that supply the energy to support the turbulence. Notably, the profiler sampling extended from the ocean surface to well below the level of the tracer, thus providing a broader view of the turbulent mixing distribution in the Southern Ocean than obtained by analysis of the tracer spreading. In total, grant OCE-0622630 supported turbulence sampling on four cruises to the Southern Ocean. DIMES was initiated the Southeast Pacific where the sea floor is relatively flat and the expectation was that the internal wave field in the region and the turbulent mixing it supports were at background intensity. Observations taken during the "US-2" cruise in early 2010 (approximately one year after the tracer was injected into the ocean) confirmed these hypotheses. In contrast, data taken at the end of US-2 in Drake Passage revealed several sites with greatly enhanced mixing. These mixing "hot-spots" corresponded to regions where deep-reaching cores of the ACC impinged on tall, rough bathymetric features. Mec...