Biophysical Interactions in Driving the Summer Monsoon Chlorophyll Bloom Off the Somalia Coast

Abstract The biophysical interactions along the Somali coast are discussed during summer monsoon based on a numerical model study. The Somali coast is known to be the fifth largest upwelling region of the world ocean and one of the most productive regions of the globe. Based on scattered observations, mostly dated back to 1960s, it is widely accepted that the strong chlorophyll bloom along the Somali coast during summer is driven by upwelled nutrient flux. We show that the upwelling‐driven productivity is primarily limited to the northern part of the coast when examined along the shelf break off the Somalia coast. In contrast, productivity in the south of 9°N is driven by weaker upwelling in the early half of the summer monsoon but later dominated by the wind‐based mixing induced entrainment and therefore shows much weaker chlorophyll concentration than the north. Further, the strong poleward alongshore currents advect the upwelled nutrients away from the southern and central parts of the coast to the north, thereby controlling the biological community over the Somali region. This abundant locally upwelled and remotely advected nutrients support the enhanced growth of diatoms in the northern part of the transect as earlier observed from in situ measurements. However, limited nutrients allow smaller phytoplankton communities to grow in the southern and central sections. We show that nitrate is the primary limiting nutrient for the phytoplankton growth in the central and southern parts of the Somalia coast.