Shelf-break upwelling and productivity over the North Kenya Banks: the importance of large-scale ocean dynamics

The North Kenya Banks (NKB) have recently emerged as a new frontier for food security and could transform into an economically important fishery for Kenya with improved resources providing accessibility. Little research has been done on the mechanisms supporting high fish productivity over the NKB with information on annual and interannual environmental variability lacking. Here we use a high-resolution, global, biogeochemical ocean model with remote sensing observations to demonstrate that the ocean circulation exerts an important control on the productivity over the NKB. During the Northeast Monsoon (NEM), from December to February, upwelling occurs along the Kenyan coast, which is topographically enhanced over the NKB. Additionally, enhanced upwelling events, associated with widespread cool temperatures, elevated chlorophyll, primary production, nutrients and phytoplankton biomass, can occur over this region. Eight such modelled events, characterized by primary production exceeding 1.3 g C/m^-2/day, were found to occur in either January or February from 1993-2015. Even though the upwelling is always rooted to the NKB, the position, spatial extent and intensity of the upwelling exhibits considerable interannual variability. The confluence zone between the Somali Current and East African Coastal Current (referred to as the Somali Zanzibar Confluence Zone [SZCZ]) forms during the NEM and is highly variable. We present evidence that when the SZCZ is positioned further south, it acts to enhance shelf-edge upwelling and productivity over the NKB. This finding provides the first indication of the environmental controls that need to be considered when developing plans for future sustainable management of the NKB fishery.