Physics Department Colloquium - “Observing and quantifying fluid fluxes across the Antarctic Circumpolar Current” - Ali Johnson - University of Rhode Island Graduate School of Oceanography

The study of physical oceanography is primarily a description of density patterns found in the ocean, the processes that account for those distributions and the associated movement of fluid on a rotating planet. In this talk, I plan to introduce you to the field of study and build some of the fundamental equations we use to describe large scale ocean circulation. I will discuss some of the tools we use to measure various properties of the ocean and life as a seagoing observational oceanographer. This will lead to a description of the Antarctic Circumpolar Current (ACC) which, established by prevailing westerly winds over the Southern Ocean, flows clockwise around Antarctica as the strongest current in the world. 

While extremely effective at interbasin exchange of heat and other physical tracers, the ACC acts as a natural barrier for moving fluid poleward across the major jets of the current. This meridional transport of fluid however is key to close the mass budget on our global system of overturning, the primary conduit of heat from the equator to the poles. Recent work has discovered that eddies, O(100 km) rotating bodies of fluid, facilitate the southward exchange of water across the ACC but only in a few key locations downstream of major topographic features of the Southern Ocean.

Despite the outsize importance of these eddies on our climate system, we lack an accurate estimation of fluxes across the ACC due to the challenges of observing fluctuations in the Southern Ocean on the temporal and spatial scales required. For this work, I use 12 years of autonomous profiling float data to quantify the exchange of fluid across major jets of the ACC in the Southeast Indian Ridge system, a known hot spot of exchange. Elevated exchange is observed downstream the ridge system in nearly every jet and is particularly prominent in the core of the ACC. Additionally, exchange peaks progressively downstream at each poleward jet suggesting a north-to-south handoff across nearly the full breadth of the ACC. Observed lateral stirring is parameterized as an eddy diffusivity, a term essential to modeling and predicting how this arm of the Southern Ocean circulation will respond to climate change.