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Southern Ocean, water masses, circulation, carbon cycle, sea ice, biogeochemical floats

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Biogeochemical Argo floats, profiling to 2,000-m depth, are being deployed throughout the Southern Ocean by the Southern Ocean Carbon and Climate Observations and Modeling program (SOCCOM). The goal is 200 floats by 2020, to provide the first full set of annual cycles of carbon, oxygen, nitrate, and optical properties across multiple oceanographic regimes. Building from no prior coverage to a sparse array, deployments are based on prior knowledge of water mass properties, mean frontal locations, mean circulation and eddy variability, winds, air-sea heat/freshwater/carbon exchange, prior Argo trajectories, and float simulations in the Southern Ocean State Estimate and Hybrid Coordinate Ocean Model (HYCOM). Twelve floats deployed from the 2014–2015 Polarstern cruise from South Africa to Antarctica are used as a test case to evaluate the deployment strategy adopted for SOCCOM's 20 deployment cruises and 126 floats to date. After several years, these floats continue to represent the deployment zones targeted in advance: (1) Weddell Gyre sea ice zone, observing the Antarctic Slope Front, and a decadally-rare polynya over Maud Rise; (2) Antarctic Circumpolar Current (ACC) including the topographically steered Southern Zone chimney where upwelling carbon/nutrient-rich deep waters produce surprisingly large carbon dioxide outgassing; (3) Subantarctic and Subtropical zones between the ACC and Africa; and (4) Cape Basin. Argo floats and eddy-resolving HYCOM simulations were the best predictors of individual SOCCOM float pathways, with uncertainty after 2 years of order 1,000 km in the sea ice zone and more than double that in and north of the ACC.

The Southern Ocean, which surrounds Antarctica, is a huge, very sparsely observed region. But we know from the decades of observations and from computer models that the Southern Ocean is a major player in the Earth's climate, taking up a significant fraction of the excess carbon dioxide from the atmosphere and absorbing a large fraction of the extra heat that results from that extra carbon dioxide. The extra carbon dioxide is also acidifying the Southern Ocean waters. To observe these, we are expanding one of the major ocean-observing systems: Argo profiling floats that measure temperature and salinity to 2,000-m depth every 10 days. SOCCOM has (1) added extra sensors that measure the ocean's pH, oxygen, nitrate, chlorophyll, and particles; (2) developed algorithms to derive the rest of the carbon budget terms, and (3) vigorously expanded Argo into the enormous seasonal ice zone. We are deploying about 30 floats per year all around Antarctica. We describe our strategy for float deployments in 2014 in the large region between Africa and Antarctica, describing the circulation, water properties, and forcing of this region. We evaluate how well we met our goal of sampling each of the distinct oceanographic regimes.

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Journal of Geophysical Research: Oceans, v. 124, issue 1, p. 403-431

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