On the eastward shift of winter surface chlorophyll‐a bloom peak in the Bay of Bengal

Abstract

Analysis of satellite-derived surface layer phytoplankton chlorophyll-a concentration (Chl-a) in the Bay of Bengal revealed an eastward shift of peak of Chl-a bloom during northeast monsoon period (November-March) from western to the eastern Bay. The winter Chl-a bloom in the western Bay is shorter duration ($1-2 months) while that in the eastern Bay is longer duration ($3-4 months). Unlike other oceans, the eastward bloom peak shift found in the Bay of Bengal is opposite to the direction of propagation of Rossby wave and seasonal mean zonal currents. During winter, sufficient light is available in the Bay of Bengal for phytoplankton growth. Herein, we studied the processes controlling the eastward winter bloom peak shift and the zonal depiction of bloom duration in the Bay of Bengal. There is no single dominant process that drives the eastward bloom peak shift everywhere in the Bay of Bengal. Rather, different physical processes operate in different regions. The physical mechanisms governing the eastward winter Chl-a bloom peak shift reported here include open ocean Ekman pumping, coastal upwelling, upwelling driven by Rossby wave, wind-induced vertical mixing, nutrients supply from river input, and westward advection through prevailing zonal currents.

Examining features of enhanced phytoplankton biomass in the Bay of Bengal using a coupled physical‐biological model

“…This surface feature of enhanced phytoplankton biomass seen in the model‐derived Chl a is also seen in ocean color images for the same time period (Figures d–f) where a large and more defined cyclonic eddy‐like structure of higher Chl a concentrations (0.5–0.6 mg m −3 ) than the surrounding areas is seen at about 15°N, persistently from February to April. In open ocean waters, where Chl a rarely exceeds 0.4 mg m −3 [ Martin and Shaji , ], these values are relatively high. The earlier shipboard study of Gomes et al .…”

“…North of 15°N, Chl a values less than 0.4 mg m −3 prevailed. Although the model‐derived Chl a concentrations show the bloom at approximately 9°N, its location can vary annually between 9 and 20°N [ Girishkumar et al ., ; Martin and Shaji , ]. The remotely sensed OCTS image for December 1996 (Figure e) also shows a bloom, with a distinct high Chl a region, although its extent was scattered more northward than eastward as seen in the model outputs.…”

“…North of 15°N, both model (Figure c) and satellite imagery (Figure f) show that Chl a values had increased in January to 0.6–0.8 mg m −3 , a change from the previous oligotrophic state of November–December. Examination of model output and ocean color images (not shown) for later years show larger and more distinct blooms extending eastward and with considerable interannual variability (see also climatology of Chl a in Martin and Shaji []). Comparatively, the bloom of 1996–1997 was a weak one.…”

Marine Dynamics and Productivity in the Bay of Bengal

“…14.2). Observational evidence on chlorophyll and nutrient levels is limited (Narvekar and Kumar 2014;Kumar et al 2010), but satellite data shows evidence of two chlorophyll peaks, in July-August and in DecemberFebruary, with the second being higher (Martin and Shaji 2015). The seasonal cycle of phytoplankton growth can be explained by a combination of nutrient and light limitation.…”

“…The waters clear during the autumn and the north-east winds of winter lead to mixing of the water column and a new supply of nutrients, leading to the second and stronger bloom (Narvekar and Kumar 2014;Kumar et al 2010). The winter bloom starts later and persists longer in the north-east than the north-west; this has been attributed to a combination of the eastward advection of riversourced nutrients and an upwelling of nutrients due to subsurface currents in the east in winter (Martin and Shaji 2015). IOD/ENSO events can influence phytoplankton distribution patterns in the southern Bay of Bengal through changes in circulation and hence nutrient distribution (Martin and Shaji 2015;Currie et al 2013).…”