Identifying Drivers of the Meridional Overturning Circulation from Observations in Drake Passage

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The Antarctic Circumpolar Current (ACC) in the Southern Ocean links all the major oceans of the world. The ACC is therefore very important for the transferring and exchanging of heat, freshwater and other properties like carbon and nutrients that impact the marine ecosystem. The Southern Ocean is particularly sensitive to climate variability, responding to winds that have increased and shifted over the past several decades and a significant warming in the core of the ACC. Yet we still have little understanding about what controls the overall strength of the current as it flows around the Southern Ocean or how the flow and the properties it carries change with depth in the ocean. The proposed work aims to study the processes that might control the changes in strength and depth distribution over a range of temporal and spatial scales using a 20-year time series of currents, temperature and salt in the Drake Passage sector of the Southern Ocean. This work will improve our predictions of how this critical region will respond to future climate change. Given the sparsity of data, the Southern Ocean's role in the climate system in relationship to the impact of global warming and the uptake of carbon dioxide are generally examined using global climate models. Eddy effects must be parameterized in coarse-resolution models, and the parameterizations are generally tested on a very limited observational database. The fine-scale observations in Drake Passage allow for an evaluation of these parameterizations and provide clues for identifying model errors or missing physics so as to build confidence in the detailed model representation of processes and dynamics within the ACC system. The project increases the participation of underrepresented groups in science and technology. It promotes training and learning through the involvement of a graduate student.The research aims to understand the physical processes that contribute to the circulation in the Southern Ocean, focusing on three central research themes within Drake Passage: (1) Mixing: Characterize the internal wave field and infer mixing from velocity, temperature and salinity observations to understand what determines the vertical and horizontal distribution of mixing; (2) Energy and transport: Determine the mean and time-varying transport of the ACC in the upper 1000 m and determine how the variability relates to changes in wind forcing and presence of eddies; and (3) Property stirring: Understand the eddy energy distribution and its relationship to topographic features, ACC fronts, wind and buoyancy forcing. The proposed analysis will take advantage of underway shipboard measurements of temperature, salinity and velocity profiles that have been routinely collected year-round from the United States Antarctic Supply research vessels in the Drake Passage over the past two decades. Contemporaneous measurements at similar high-resolution time and space scales from the same platform offer significant benefit for exploring the statistical characteristics, features, mechanisms and impacts of the processes that drive the Southern Ocean circulation. Additional measurements from Argo floats and satellite wind and sea surface height data will provide regional context to help interpret the shipboard time series.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.