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
Deep-Sea Research II, 172, 104721.
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