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Interannual variability of the Chlorophyll-a concentration over Sri Lankan Dome in the Bay of Bengal

ABSTRACT This study investigates the interannual variability of the surface and subsurface Chlorophyll-a Concentration (CC) over the Sri Lankan Dome (SLD) in the Bay of Bengal (BoB) and its possible linkages to local wind-induced upwelling and advection-driven CC transport via southwest monsoon current (SMC) from surrounding waters. A regional ocean modelling system (ROMS) coupled with a nitrogen-based nutrient, phytoplankton, zooplankton, and detritus (NPZD) ecosystem model has been used to simulate the three-dimensional SMC and the upwelling system over the SLD region (5–10 N, 82–87 E) during 2003–2016. A deep chlorophyll maximum (DCM: 0.8–1.8 mg m−3) is observed at the intermediate depths of 10 to 55 m. The model-simulated CC in the SLD region shows a seasonal variation with maximum value during June through September (JJAS), compares well with Bio-Argo observations available nearby, and significantly correlates (r = 0.78) with satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS). The subsurface chlorophyll distribution demonstrated strong interannual variability, especially during JJAS and is associated with concurrent upwelling. The Ekman Pumping correlates well with the thermocline depth (r = −0.70) and with the deep chlorophyll maximum (r = −0.74). The simulated zonal transport of chlorophyll across 80 E is moderately correlated with the surface chlorophyll on SLD from MODIS observation (0.69) and from the biophysical simulation (0.79), which signifies the role of SMC intrusion from the Arabian Sea to the southwestern BoB. A data-model composite analysis showed that a favourable wind stress curl and shallower thermocline enhanced the Chlorophyll concentration in the SLD region in the high-chlorophyll years (2004, 2014, and 2016). In contrast, reduced advection from the Arabian Sea, deeper thermocline, and the weakening of the deep chlorophyll maxima are responsible for lowering the chlorophyll concentration during the low-chlorophyll years (2008, 2009, and 2011).