The Dynamics of the Southwest Monsoon Current in 2016 from High-Resolution In Situ Observations and Models

Abstract

The strong stratification of the Bay of Bengal (BoB) causes rapid variations in sea surface temperature (SST) that influence the development of monsoon rainfall systems. This stratification is driven by the salinity difference between the fresh surface waters of the northern bay and the supply of warm, salty water by the Southwest Monsoon Current (SMC). Despite the influence of the SMC on monsoon dynamics, observations of this current during the monsoon are sparse. Using data from high-resolution in situ measurements along an east–west section at 8°N in the southern BoB, we calculate that the northward transport during July 2016 was between 16.7 and 24.5 Sv (1 Sv ≡ 106 m3 s−1), although up to ⅔ of this transport is associated with persistent recirculating eddies, including the Sri Lanka Dome. Comparison with climatology suggests the SMC in early July was close to the average annual maximum strength. The NEMO 1/12° ocean model with data assimilation is found to faithfully represent the variability of the SMC and associated water masses. We show how the variability in SMC strength and position is driven by the complex interplay between local forcing (wind stress curl over the Sri Lanka Dome) and remote forcing (Kelvin and Rossby wave propagation). Thus, various modes of climatic variability will influence SMC strength and location on time scales from weeks to years. Idealized one-dimensional ocean model experiments show that subsurface water masses advected by the SMC significantly alter the evolution of SST and salinity, potentially impacting Indian monsoon rainfall.

The Railroad Switch Effect of Seasonally Reversing Currents on the Bay of Bengal High‐Salinity Core

“…A comparison of the cross sections of the vertical structure of salinity at 7°N from NEMO (Figures d and e) and the Argo OI (not shown) show that the model agrees well with the observations. Both NEMO (Figure d) and the Argo OI show the structure of the HSC (>35 g/kg) centered between 75 and 130 m and 85–90°E for summer 2014 (Figure d) and 83–87°E for the summer of 2015 (Figure e), consistent with previous studies (Vinayachandran et al, ; Webber et al, ). Webber et al () found that the NEMO ocean model provides an accurate representation of the velocity, salinity, and density structure in the BoB, compared to observations made during the 2016 monsoon season that were not assimilated into the model (Figure 2 of Webber et al, ).…”

“…The structure of the subsurface (90–130 m) salinity across the wider BoB during the summer of 2014 (Figure b) shows the intrusion of a high‐salinity tongue alongside the eastward flowing SMC (surface current vectors from OSCAR; Bonjean & Lagerloef, ), in agreement with Vinayachandran et al (, ) and Webber et al (). The SMC is a surface‐intensified current with northward flow extending to a depth of 300 m (Vinayachandran et al, ; Webber et al, ). As the HSC is advected north by the SMC, it is subducted under the fresher, lighter waters of the northern BoB (Vinayachandran et al, , ).…”

“…Zero or negative salinity anomalies are observed during the summers of 2010 and 2015 ( Figure 1a). The structure of the subsurface (90-130 m) salinity across the wider BoB during the summer of 2014 ( Figure 1b) shows the intrusion of a high-salinity tongue alongside the eastward flowing SMC (surface current vectors from OSCAR; Bonjean & Lagerloef, 2002), in agreement with Vinayachandran et al (2013Vinayachandran et al ( , 2018 and Webber et al (2018). The SMC is a surface-intensified current with northward flow extending to a depth of 300 m Webber et al, 2018).…”

“…The high-salinity core (HSC) plays an important role in maintaining the BoB's salinity balance, as it is the main source of salinity available to balance the BoB's net outflow of freshwater (Schott et al, 2009;Vinayachandran et al, 1999Vinayachandran et al, , 2013. To date, several studies have linked the HSC to Arabian Sea High-Salinity Water (ASHSW), which originates from the northern Arabian Sea and is transported to the BoB via the SMC (Jensen, 2001;Jensen et al, 2016;Schott & McCreary, 2001;Vinayachandran et al, 1999;Webber et al, 2018).…”

Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal

“…(2018) and Webber et al (2018). In response to the prevailing atmospheric conditions, the upper ocean in the southern bay exhibited large spatial variability at seasonal and synoptic timescales.…”