Monthly mean sea surface Chlorophyll-a concentration (in mg.m^-3 (log10) at 4km resolution - Satellite remote sensing Ocean color data): Chlorophyll is a photosynthetic pigment commonly present in all phytoplankton species. It is used as a proxy for phytoplankton biomass. Chlorophyll concentration is a standard product from satellite-based optical sensors, usually retrieved from empirical algorithms using reflectance ratios at two or more wavebands