Dynamics of river plumes in the Bay of Bengal

Abstract

The Bay of Bengal (BoB) receives a large amount of freshwater through runoff from sev- eral rivers along its border. This large freshwater gain has severe implications on the dynamics and thermodynamics of the BoB as well as on the regional climate. Several studies have indicated that the freshwater from rivers is an important parameter in de- termining the observed structure of the Sea Surface Salinity (SSS) and its variability in the northern BoB. There are several studies dedicated to understand the movement and export pathways of freshwater in the bay. However, there have been no attempts to in- vestigate the fate of the discharge from individual rivers in the bay. Seasonal changes in the BoB circulation take place by virtue of both direct forcing from the seasonally reversing winds and remote forcing from the equator. The effects of wind forcing (both direct and remote) on the movement of river plumes in the bay have not yet been re- ported. This thesis explores various aspects of the dynamics of river plumes due to four largest rivers in the BoB namely, Ganga-Brahmaputra (GB), Irrawaddy (IR), Godavari (GD) and Mahanadi-Brahmani (MB). A numerical ocean circulation model was setup to conduct idealized experiments in which a river plume was isolated from other major sources of salinity (Other rivers and surface freshwater flux from the atmosphere) and was subjected to different wind forcing scenarios namely wind forcing over the entire model domain, no wind forcing, wind forcing over the equatorial region and wind forcing over the Bay of Bengal. Results from experiment with Ganga-Brahmaputra river and wind forcing applied over the entire model domain were used to validate the model against observations of sea surface temperature, SSS and surface currents. Dynamics of GB plume was investigated using experiments with only GB river and four different wind forcing scenarios. Experiment with only GB river and winds over the entire model domain revealed that GB plume drifts upstream (upstream) during spring. With the onset of summer monsoon in June, freshwater from GB flows southward along the east coast of India. As the monsoon progresses, the plume detaches from the coast and spreads eastward over the open ocean. By the end of summer monsoon, GB plume occupies all of the head bay, but remains confined to the north of 16◦ N. During winter monsoon (November-February), the plume flows southward, assisted by the East India Coastal Current (EICC) along the east coast of India. In the central bay, plume recedes northward during winter monsoon. In the absence of winds, GB plume flows southward along the coast of India throughout the year. Remote forcing from the equator channelizes the GB discharge southward along the coast of India throughout the year. Response of GB plume to winds over only BoB is similar to the winds over the entire model domain. Geostrophy controls the behavior of the plume during spring and winter. During summer monsoon, downstream expansion of the plume is due to geostrophy whereas its expansion over the open ocean occurs under the influence of winds. Experiments with only IR river and wind forcing over the entire model domain show that IR plume expands southward during spring. During summer monsoon, IR plume flows rapidly towards southeast in the form of a narrow jet of freshwater that accumu- lates over the shelf along the eastern boundary of the Andaman Sea. During winter monsoon, IR plume flows northward along the coast of Myanmar and spreads westward over the open ocean in the northern BoB. In the absence of winds, IR discharge flows downstream throughout the year. In response to equatorial forcing, IR plume flows down- stream throughout the year with accelerated flow occurring in April-May and November- December. Wind over only BoB invoke a response from IR plume that is similar when the winds are present over the entire model domain. Analysis of momentum balances inside the plume shows that IR plume is completely driven by winds during summer monsoon. Geostrophy controls the movement of the plume during spring and winter monsoon. This is also the first time that the investigations of IR plume are being reported. Experiments with only Godavari (GD) river show that the GD discharge drifts north- ward (upstream) along the coast of India throughout February-June because of the pole- ward EICC. Throughout July-September, GD discharge spreads offshore (first southward and then eastward) because of the breakdown of EICC into two opposing flows and the presence of eddies along the coast of India. During October-January, GD plume expands southward along the coast of India due to the equatorward EICC. In the absence of winds, GD discharge flows downstream (southward) throughout the year. In response to equatorial forcing, GD plume expands southward during April-August and November-January and northwards during September-October and February-March. BoB winds invoke a response similar to the winds over entire domain. Dispersion of the GD plume during summer monsoon occurs under the influence of geostrophy with an added assistance of wind friction whereas that during spring and winter monsoon is largely controlled by only geostrophy. Experiments conducted with only MB river show that MB plume expands upstream due to poleward EICC during spring. During summer monsoon, EICC splits into two opposite flows and the convergence of these two opposing legs carries MB discharge south- ward and then pushes it offshore. During winter monsoon, EICC carries MB discharge southward. In the absence of winds, MB discharge flows downstream throughout the year. In response to equatorial winds, MB discharge drifts upstream during February-March and downstream during April-December. In September, reversal of EICC spreads MB discharge offshore. Response of MB plume to winds only over the BoB closely matches to the response to winds over all of the model domain. Winds augment the offshore expan- sion of the plume during spring and summer whereas geostrophy controls the downstream expansion during winter monsoon. This study makes several key contributions to our understanding of the behavior of freshwater plumes due to four largest rivers in the BoB. The principle contribution of the study is the characterization of the seasonal orientation of the freshwater plumes due to these rivers. This helps us better understand the fate of the discharge from each of these rivers in the bay. This study also reports the response of these river plumes to different wind forcing scenarios of the bay and highlights the importance of each of these scenarios. Our experiments provide an insight into the mechanisms responsible for the seasonal dispersion of the freshwater from these rivers in the bay. The results obtained from this study are relevant to our understanding of the salinity structure of the BoB and to the dynamical processes resulting from this structure.