“Extreme Highest” and “extreme Anomalous”: Proposed Indices for Chlorophyll-a Extreme Events in European Seas Between 2003 and 2021

This study addresses the occurrence of extreme values of chlorophyll-a concentrations in European seas, using the satellite's OBGP MODIS AQUA v2018 dataset, for the 2003–2021 period. Two novel and complementary statistical indices based on the combination of the overall period's and the monthly 90th percentiles (P90 and mP90, respectively) are proposed: (i) the Extreme Highest exceedances (EH) include the observations with highest chlorophyll-a magnitudes in each location (larger than P90 and mP90); (ii) the Extreme Anomalous exceedances (EA) address the observations larger than mP90 but lower than P90, thus representing relevant anomalies during low phytoplankton growing seasons. Given the important differences of available observations per pixel, the EH and EA exceedances are normalized and also presented in percentage units.