Collaborative Research: The upper branch of the Southern Ocean overturning in the Southern Ocean State Estimate: water mass transformation and the 3-D residual circulation

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Overview: The Southern Ocean plays a pivotal role in the global circulation and climate. The absence of land barriers in the Drake Passage latitude band makes it possible for westerly winds to drive the strong eastward Antarctic Circumpolar Current (ACC) that encircles the globe in the Southern Ocean, connecting the individual ocean basins. Due to its steeply sloping density surfaces with upwelling from great depths to the sea surface, the Southern Ocean's overturning circulation is extremely different from the equally vigorous North Atlantic/Nordic Seas overturn. Near the sea surface, the upwelled deep waters split into waters that become lighter (more buoyant) and eventually circulate into the subtropical thermocline (upper cell), and waters that become denser and feed the global bottom waters (lower cell). The Southern Ocean's overturning circulation is responsible for a large portion of the global redistribution of heat, freshwater, carbon and nutrients. Warming in the Southern Ocean over the past 50 years is weaker than in the Northern Hemisphere, possibly because Drake Passage limits southward oceanic flux of heat, hence maintaining cold, ice-covered waters. However, because of its very large volume, the Southern Ocean is absorbing a significant fraction of the climate system?s excess heat and about 60% of the total oceanic anthropogenic carbon dioxide inventory is stored in the Southern Hemisphere oceans. Nutrients from the upwelling deep waters in the Southern Ocean enter the thermocline via the upper cell, where they support 75% of primary productivity north of 30°S. Intellectual Merit : The zonally averaged Southern Ocean overturning circulation is commonly hypothesized to have an upper cell and a lower cell, fed by inflowing Indian, Pacific and Atlantic Deep Waters, that upwell to the ocean surface where surface buoyancy fluxes convert them to lighter and denser waters, respectively. The degree of separation (or not) of these cells will be examined as part of this work. Observations indicate that the upper cell is most likely fed by nutrient-rich deep waters that originate in the Indian and Pacific rather than from the Atlantic, a hypothesis that will be examined in detail.A second hypothesis is that the two-dimensional, zonally averaged meridional overturning circulation hides significant zonal asymmetries that are essential to the circulation. Using a residual circulation framework with proposed new online diagnostics of isopycnal and diapycnal volume transport in neutral density coordinates, the investigators will quantitatively examine regional contributions to the residual circulation in the Southern Ocean State Estimate (SOSE), including the role of the ACC, topographic features, and subtropical and Antarctic gyre systems. They will also quantify the relative contributions of eddy-driven and steady flow in the three-dimensional residual circulation pathways.Water mass transformation and formation processes in the upper cell are also three-dimensional. The hypothesis that air-sea fluxes dominate with nearly equal importance of freshwater and heat, but that diapycnal mixing, particularly in isopycnal outcrop regions, can also be important will be tested. Using the proposed new online SOSE diagnostics, the relative, localized contributions of heat and salinity forcing to transformation will be quantified at every model time step. Regionally, with SOSE and these new diagnostics, the investigators will examine the balance of processes that lead to coherent net heating regions, find the most important upwelling/air-sea exchange sites, and quantify the role of sea ice processes in the essential freshwater inputs to the upper cell.Broader Impacts: The project will inform understanding of Southern Ocean response to climate change, including changes in surface temperature, upper ocean heat content and sea ice cover. The results will be published in scientific journals and presented at major meetings. Existing public outreach efforts (schools, teacher groups, libraries, university clubs) will be enhanced by the project, including visualizations. The proposed development of online diagnosis of the 3D residual (i.e. isopycnally-averaged) circulation will benefit MITgcm users as well as SOSE users through the inclusion of the contributions to the diapycnal velocity from all diabatic processes. The code implementing online neutral density calculation will be made available to all MITgcm users. State estimates are increasingly a tool of choice for synthesizing data. This project supports the rapidly growing user base for SOSE, including many students. SOSE will be a broadly used tool for understanding Southern Ocean dynamics, thermodynamics, and biogeochemistry for years to come, as the numbers of in situ observations under sea ice and of biogeochemical parameters soar. Crucial verification of its water mass structure and air-sea fluxes will be undertaken. A graduate student will be mentored.