Gopalakrishnan, G., A. C. Subramanian, A. J. Miller, H. Seo and D. Sengupta, 2020:

Estimation and prediction of the upper ocean circulation in the Bay of Bengal.

Deep-Sea Research II, 172, 104721.

Abstract. The upper ocean stratification and circulation in the Bay of Bengal (BoB) plays a key role in the northward propagating monsoon intraseasonal oscillation during the months of June - August. This region is highly influenced by strong, seasonal atmospheric forcing and the oceanic circulation is characterized by dominant mesoscale variability and strong horizontal gradients in salinity and temperature during the monsoon period. Given the role of the ocean in the monsoon circulation, it is important to investigate accurate ocean state estimates and forecasts of the BoB ocean circulation in preparation for coupled ocean-atmosphere modeling and predictions. Hence, we use a mesoscale-permitting regional implementation of Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint-based four-dimensional variational (4DVAR) system to assimilate satellite-derived Sea Surface Height (SSH) and Sea Surface Temperature (SST) data in the BoB for a period of one month (June 1 - 30, 2017). It is shown that the MITgcm-BoB 4DVAR assimilation system is able to significantly improve the model consistency with the assimilated observations in the BoB region, reducing the model-data misfit by 50% and provided a dynamically-consistent BoB ocean circulation for the one-month hindcast period. We performed forecasting experiments using the state estimate to initialize two forecasts for a period of 30-days (July 1 - 30, 2017) from the end of the hindcast period. These forecasts used either atmosphere reanalysis and ocean analysis forcings or monthly climatology of atmosphere reanalysis and ocean analysis forcings. They therefore do not represent a "true" regional ocean forecast, forced using actual atmosphere and ocean forecasts, but bound the performance between climatological and nearly perfect forecasts. The model forecast is a crossvalidation against future observations and showed that the initial conditions from the state estimate improves the prediction of the three-dimensional circulation in the BoB. The model hindcast and forecasts were also cross-validated against independent Argo temperature and salinity observations in the BoB. Additional state estimation and forecast experiments for other periods showed similar model performance with improved hindcasts and forecasts for the BoB region.

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