Upper ocean state estimation in the Southern Ocean Gas Exchange Experiment region using the four-dimensional variational technique

Abstract

[1] A prototypical ocean circulation model, and associated 4-D variational data assimilation system, is configured in support of the Southern Ocean Gas Exchange Experiment (Southern Ocean GasEx). The regional circulation model has 4 km horizontal resolution and 5 m vertical resolution in the upper ocean. In situ temperature and salinity observations and remotely sensed, gridded sea surface temperature and sea surface height observations are combined with the circulation model between 6 March and 4 April 2008. The benefits of customized data assimilation in support of a single oceanographic cruise are illustrated using Southern Ocean GasEx as a case study. The assimilated fields fit the observations within their respective uncertainties and fit 4–6 times better than the initial guess solution before assimilation. The fit of the assimilated fields is up to 100 times closer to observations than various global state estimate products using a cost function variance metric. The space-time variability in mixed layer depth is consistent with the Southern Ocean GasEx observations and shows a deepening through March 2008. This variability is correlated with sea level height anomalies, sea surface temperature, and air temperature, during the assimilation window. The mixed layer depth estimate between 21 March and 4 April 2008 at the location of the Southern Ocean GasEx deliberate tracer release is 41–54 ± 6 m. The average zonal volume flux estimate is 16 ± 1.9 Sv (1 Sv is 106 m3 s−1) for latitudes 50.7°S–51.8°S, longitude 38.5°W. The zonal volume flux approximately halves during 6–16 March; temporal changes in volume flux are associated with geostrophic circulation but not with the Ekman circulation.