Gopalakrishnan, G., A. C. Subramanian, A. J. Miller, H. Seo and D. Sengupta, 2019:
Estimation and prediction of the upper ocean circulation in the Bay of
Deep-Sea Research II, sub judice.
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 the strong, seasonal atmospheric forcing and the oceanic
circulation is characterized by dominant mesoscale variability and strong gradients in salinity and
temperature during the monsoon period. Given the key role of the BoB ocean state in the monsoon
circulation, it is important to produce an accurate state estimate of the BoB ocean circulation.
Hence, we use a high-resolution, 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-4DVAR assimilation system is able to significantly improve the model consistency with
the assimilated observations in the BoB region. The state estimation reduced the model-data misfit
by 50% and provided a reasonably accurate and dynamically-consistent BoB ocean circulation for
the one-month hindcast period. We further tested the state estimate by performing forecasting experiments
using optimized solutions to initialize the forecast 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,
therefore do not represent a “true” regional ocean forecast forced using actual atmosphere
and ocean forecasts. The model forecast cross-validation showed that the initial conditions from
the state estimate improves the prediction of sea surface state in the BoB. The model hindcast
and forecast were also cross-validated with independent, three-dimensional Argo temperature and
salinity observations in the BoB. Additional state estimation experiments for other periods showed
similar model performance with improved hindcasts and forecasts for the BoB region.