Subseasonal Dispersal of Freshwater in the Northern Bay of Bengal in the 2013 Summer Monsoon Season

Abstract

Near-surface salinity in the north Bay of Bengal (BoB) is very low for nearly three seasons (July-January), due to freshwater input from summer monsoon rainfall and seasonal discharge from several major rivers, including the Ganga-Brahmaputra-Meghna and the Irrawady. The low-salinity surface layer leads to a very shallow density mixed layer, profoundly influencing air-sea interaction on diurnal to seasonal time scales. We use mooring and satellite observations to study the mechanisms responsible for lateral dispersal of river water in the north BoB on subseasonal time scales (days to weeks) during the summer monsoon season of 2013. A new ocean current data set, the BoB near-surface current and advection estimation (BoBcat), is developed to account for the influence of near-surface stratification on directly wind-forced currents. A salt balance based on Aquarius sea surface salinity, BoBcat currents and moored observations shows that subseasonal sea surface salinity variability is mainly due to lateral advection. A shallow Ekman current, forced by enhanced wind stress during an active spell of the monsoon in mid-August, plays a dominant role in moving freshwater from the western boundary to the interior. During subseasonal spells of weak monsoon winds, stirring by mesoscale eddies is the main mechanism of dispersal of low-salinity water. The dispersal of river water in the BoB is very sensitive to surface wind stress. SREE LEKHA ET AL. 6330

Observed Upper Ocean Seasonal and Intraseasonal Variability in the Andaman Sea

“…Moreover, the temporal evolution of MLS tendency shows reasonable good agreement with respect to horizontal advection term particularly during those periods of predominant subseasonal variability of SSS observed (August--February of 2016-17 and 2017-2018; Figures 9 and S12). The prominent role of horizontal advection to determine MLS is consistent with earlier observational studies in the BoB (Akhil et al, 2016;Pant et al, 2015;Rao & Sivakumar, 2003;Sree Lekha et al, 2018;Wilson & Riser, 2016). Consistent with the earlier studies, our analysis based on OMNI+ also shows similar result with respect to OMNI data (Figures 9 and S12).…”

“…This annual cycle of SSS field leads to a strong lateral salinity gradient, and it is oriented along the southeast to northwest direction near to the BD12 mooring location during August-February. Based on the moorings observations in the northern BoB, Sree Lekha et al (2018) shows that large lateral gradient of SSS leads to large amplitude in SSS variability at the mooring location in subseasonal time scale. Consistent with their results, our study also shows the presence of strong lateral salinity gradient during August-February with large amplitude intraseasonal variability in the near-surface salinity at the mooring location in the AS ( Figure S3).…”

Monsoon Mission: A Targeted Activity to Improve Monsoon Prediction across Scales

“…A key science question is how the enormous amount of freshwater introduced at the surface in the north BoB from local rain as well as from the major rivers like Ganga-Brahmaputra discharge gets transported to the south by the mesoscale eddies and gets mixed with deeper layers in the highly stratified background. Using the data from the observational campaigns funded under OMM, new insights have emerged from a recent study (Sree Lekha et al 2018) indicating the key role of the monsoon subseasonal oscillations in this process. The annual freshwater input from monsoonal rains and rivers creates a stable upper-ocean stratification which is known to influence the air-sea interaction in the region (Shenoi et al 2002;Vinayachandran et al 2002;Rao et al 2011).…”

Impact of Continental Freshwater Runoff on Coastal Sea Level

“…They concluded that this was explained by reversing cross-shore currents, well consistent with Ekman drift. Similarly, Sree Lekha et al (2018) analyzed the offshore export of Ganges-Brahmaputra freshwater in the northern Bay of Bengal. They concluded that the overall export is largely driven by Ekman drift, under the favorable conditions of along-shore winds.…”