Impact of excess and deficit river runoff on Bay of Bengal upper ocean characteristics using an ocean general circulation model

Abstract

The present study investigates the impact of excess river runoff (eRR) and deficit river runoff (dRR) on the Bay of Bengal (BoB) upper ocean physics and circulation patterns using high resolution Modular Ocean Model version 5 (MOM5) simulations. The ocean model MOM5 was forced by 3-hourly surface fluxes and interannual river runoff from the Japanese 55-year Reanalysis (JRA-55). Sensitivity experiments are carried out with interannual and climatological river discharges from 1965 to 2016. Four positive river runoff anomaly years (1980, 1988, 1990, 2007) and four negative river runoff anomaly years (1979, 1986, 1992, 2009) during the satellite era, which are not influenced by the equatorial wind anomalies, are considered for the detailed analysis of eRR and dRR years. The study reveals that the distribution of salinity and the freshwater plume in eRR years differ significantly in the northern and western BoB from that of dRR years. The sea surface salinity difference is ~2 psu in the northern BoB and the advection of freshwater by the southward East India Coastal Current (EICC) reaches the southern part of the BoB and enters the Arabian Sea in the eRR years. A strong cooling is observed in eRR years over the plume distributed area during post-monsoon and the excess freshwater amplifies the EICC as well. The large inflow of freshwater during eRR years is closely associated with a shallow mixed layer depth (MLD), formation of a thick barrier layer (BL), and strong circulation and temperature inversions, while the dRR years accumulate more heat compared to the eRR years in the upper 100 m of the plume area. This study, therefore, provides evidence for the impact of freshwater discharge on the BoB upper ocean processes. These may have a large impact on the local air-sea coupled processes and so representing accurate river discharge in climate models is crucial. It is also found that the EICC and the basin-wide cyclonic gyre advect the barrier layer along the Indian east coast into the southeastern Arabian Sea. This is one of the first studies to incorporate interannual runoff of all the major rivers flowing into the BoB in an ocean general circulation model for investigations of BoB processes.