Regionalization of the Red Sea Based on Phytoplankton Phenology: A Satellite Analysis

The current average state of Red Sea phytoplankton phenology needs to be resolved in order to study future variations that could be induced by climate change. Moreover, a regionalization of the Red Sea could help to identify areas of interest and guide in situ sampling strategies. Here, a clustering method used 21 years of satellite surface chlorophyll‐a concentration observations to characterize similar regions of the Red Sea. Four relevant phytoplankton spatiotemporal patterns (i.e., bio‐regions) were found and linked to biophysical interactions occurring in their respective areas. Two of them, located in the northern part the Red Sea, were characterized by a distinct winter‐time phytoplankton bloom induced by mixing events or associated with a convergence zone. The other two, located in the southern regions, were characterized by phytoplankton blooms in summer and winter which might be under the influence of water advected into the Red Sea from the Gulf of Aden in response to the seasonal monsoon winds. Some observed inter‐annual variabilities in these bio‐regions suggested that physical mechanisms could be highly variable in response to variations in air‐sea heat fluxes and ENSO phases in the northern and southern half of the Red Sea, respectively. This study reveals the importance of sustaining in situ measurements in the Red Sea to build a full understanding about the physical processes that contribute to phytoplankton production in this basin. Plain Language Summary The Red Sea has been characterized as an area sensitive to climate change, which may affect the Red Sea phytoplankton phenology. A better understanding on the factors controlling the phytoplankton phenology is needed. We thus performed a regionalization of the Red Sea using 21 years of satellite surface chlorophyll‐a concentration observations to characterize similar regions of the Red Sea. We identified four relevant phytoplankton spatiotemporal patterns named “bio‐regions” linked to the physical characteristics within each region. The northern Red Sea bio‐regions are marked by a winter phytoplankton bloom due to mixing events that may uplift nutrients towards the surface layer. The southern Red Sea bio‐regions, marked by summer and winter phytoplankton blooms, are mainly related to the summer southwest monsoon as well as the winter northeast monsoon, which favors the intrusion of sub‐surface and surface nutrient‐rich waters from the Gulf of Aden, respectively. These findings provide a