Summertime phytoplankton blooms and surface cooling in the western south equatorial Indian Ocean

Abstract

Chlorophyll-a (Chla) concentration derived from the Sea viewing Wide field of View sensor (SeaWiFS) data (January 1998 to December 2010) shows phytoplankton blooms in the western south equatorial Indian Ocean (WSEIO) during the summer monsoon. The mechanism that sustains the blooms is investigated with the high-resolution Ocean General Circulation Model for the Earth Simulator (OFES) products. The summer blooms in the WSEIO are separated from the coast; they occur in June, reach their maximum in August, and decay in October. With summer monsoon onset, cross-equatorial wind induces open-ocean upwelling in the WSEIO, uplifting the nutricline. The mixed layer heat budget analysis reveals that both thermal forcing and ocean processes are important for the seasonal variations of SST, especially wind-driven entrainment plays a significant role in cooling the WSEIO. These processes cause nutrient enrichment in the surface layer and trigger the phytoplankton blooms. As the summer monsoon develops, the strong wind deepens the mixed layer; the entrainment thus increases the nutrient supply and enhances the bloom. Horizontal advection associated with the Southern Gyre might also be an important process that sustains the bloom. This large clockwise gyre could advect nutrient-rich water along its route, allowing Chla to bloom in a larger area.

Wintertime Phytoplankton Blooms in the Western Equatorial Indian Ocean Associated With the Madden‐Julian Oscillation

“…Readers are referred to Sasaki et al () for a more complete model description. The OFES outputs have been extensively analyzed by a number of earlier studies in various regions, and they can adequately capture the general characteristics of surface currents in the tropical oceans (e.g., Du et al, ; Liao et al, ; Masumoto, ; Qiu et al, ; Qu et al, ).…”

“…During boreal summer, cross‐equatorial wind induces open‐ocean upwelling and the deepening of the mixed layer in the southwestern equatorial Indian Ocean. Under the effects of the horizontal advection of the near‐equatorial clockwise gyres (i.e., the Southern Gyre), phytoplankton blooms occur in the WEIO (Liao et al, ). High Chl a concentrations are also observed in this region during boreal winter.…”

Merging Process of the Great Whirl and the Socotra Gyre in 2019

“…Meanwhile, the second upwelling cold wedge formed immediately north of the GW. Located between two upwelling wedges, the GW continuously transports nutrient‐rich water from the coast to the central Arabian Sea, substantially augmenting the summertime phytoplankton blooms in the Arabian Sea (Lévy et al., 2007; Liao et al., 2014; Wiggert et al., 2005). The third eddy, located northeast of the GW and southeast of the Socotra Island, is known as the Socotra Gyre (SoG) (Bruce, 1983; Jensen, 1991).…”

Intensification of upwelling along Oman coast in a warming scenario

“…During the southwest monsoon season the northward flowing Somali Current (SC) favors upwelling along these coasts bringing nutrient rich subsurface waters to the surface and thereby fostering the productivity [Cabarcos et al, 2014;De Wilde and Helder, 1997;Smith and Codispoti, 1980]. The offshore Ekman transport further connects this upwelled waters to the open ocean making this region a zone of high marine productivity [Liao et al, 2014;Piontkovski et al, 2012;Valsala, 2009;Goes et al, 2005].…”

“…Within the anticyclonic gyre the water converges and downwells [ Song et al , ] while intense upwelling and advection of upwelled waters are seen along the northern flank of the GW. This feature (upwelling and spreading of upwelled water) is visible in various variables like sea surface temperature (SST), chlorophyll a (Chl a ) and air‐sea CO 2 fluxes [ Liao et al , ; Valsala and Murtugudde , ].…”