This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
In the Southern Ocean, air-sea fluxes play a critical role in transforming water at the ocean surface. They determine how water properties change when water is carried northward by surface Ekman currents as part of the meridional overturning circulation. They also determine the properties of Intermediate and Mode Waters that descend to mid-depth from the northern flank of the Antarctic Circumpolar Current, carrying with them fingerprints of their contact with the atmosphere. A number of gridded air-sea flux products are available from numerical weather prediction (NWP) models or detailed analyses of satellite and in situ data. These products differ substantially, often by 50 Watts per square meter or more in the time-averaged mean. In addition, the existing flux products are not accurate enough to explain fully the observed seasonal to interannual variations in the mixed-layer heat content. Moreover, while air-sea fluxes are thought to vary over small spatial scales associated with mesoscale ocean features such as fronts or eddies, small-scale variations are typically not resolved in air-sea flux products.
Intellectual Merit: This research project will investigate the mixed-layer heat budget and water mass transformation in the Southern Ocean. The work is predicated on the hypothesis that improving air-sea flux estimates is an essential step towards an improved understanding of Southern Ocean mixed-layer processes. Heat and freshwater flux estimates will be used to evaluate three key ideas: (a) Surface water mass transformation in the Southern Ocean can be explained primarily through air-sea heat and freshwater fluxes, without requiring that any subsurface processes be invoked. (b) The upper ocean heat balance should close, both regionally and on a system-wide scale. Seasonal to interannual variations in upper ocean temperature should be explainable largely by air-sea heat fluxes, with horizontal advective processes making up most of the difference. (c) Small-spatial scale, rapid variations in surface fluxes are significant, contributing much of the existing uncertainty in air-sea heat and freshwater fluxes, and resolving these small-scale fluxes will reduce the uncertainties in upper ocean heat budgets. In essence, these three hypotheses translate into a single concept, that improving our estimates of heat fluxes should improve our physical understanding of the climate-scale processes that occur in the Southern Ocean. Processes that matter on the scale of the Southern Ocean will be evaluated by considering two metrics of air-sea flux performance: the closure of a time-evolving mixed-layer heat budget and time-averaged balance between northward Ekman advection and water mass conversion driven by air-sea fluxes. A suite of existing flux products will be evaluated, and a new, hybrid flux estimate will be developed by testing possible parameters to find an optimal set of satellite and numerical weather prediction products to feed into the bulk algorithms used to compute air-sea fluxes. On a regional scale, shipboard meteorological data will be used to assess the small-scale structure of air-sea fluxes.
Braoder Impacts: The project will provide partial support in order to train one postdoctoral researcher in the important area of air-sea exchange. Estimates of Southern Ocean air-sea heat and freshwater fluxes derived as part of this research will be released via a website. Research findings will help to lay the groundwork for future advances in air-sea fluxes and their role in climate.
