Upper Ocean Four-Dimensional Variational Data Assimilation in the Arabian Sea and Bay of Bengal

ABSTRACT

A regional ocean circulation model with four-dimensional variational data assimilation scheme is configured to study the ocean state of the Indian Ocean region (65°E–95°E; 5°N–20°N) covering the Arabian Sea (AS) and Bay of Bengal (BoB). The state estimation setup uses 10 km horizontal resolution and 5 m vertical resolution in the upper ocean. The in-situ temperature and salinity, satellite-derived observations of sea surface height, and blended (in-situ and satellite-derived) observations of sea surface temperature alongwith their associated uncertainties are used for data assimilation with the regionally configured ocean model. The ocean state estimation is carried out for 61 days (1 June to 31 July 2013). The assimilated fields are closer to observations compared to other global state estimates. The mixed layer depth (MLD) of the region shows deepening during the period of assimilation with AS showing higher MLD compared to the BoB. An empirical forecast equation is derived for the prediction of MLD using the air–sea forcing variables as predictors. The surface and sub-surface (50 m) heat and salt budget tendencies of the region are also investigated. It is found that at the sub-surface, only the advection and diffusion temperature and salt tendencies are important.

KEYWORDS:

• Arabian Sea and Bay of Bengal
• heat budget
• hydrography
• Indian Ocean modeling
• mixed layer depth
• Ocean state estimation
• Ocean General Circulation Model (OGCM)
• satellite altimete

Acknowledgments

Authors thank two anonymous reviewers for their useful suggestions on an earlier version of the manuscript. Thanks are also due to AVISO, NCEP, ECCO-JPL, OSCAR communities for providing their datasets freely available. TMI data are produced by Remote Sensing Systems and sponsored by the NASA Earth Sciences Program.

Funding

AKS and AKM are thankful to ISRO and MoES for providing Junior Research Fellowship. SD is thankful to ISRO/DST/MoES, Govt. of India for financial assistance in the form of research projects.