Extreme event waves in marine ecosystems: method and application to the surface chlorophyll in Mediterranean Sea

Extreme events can have great impacts on the functioning of marine ecosystems. In this work we describe a novel method that identifies and characterises extreme events in marine ecosystems at the basin scale, by accounting for the persistence of the events within a certain impacted area and over a specific time duration. Extreme events are firstly identified as peaks over a predefined threshold (i.e. the 99th percentile) computed from a local timeseries. Then, a series of extreme events that are connected over space and time is defined as an extreme event wave (EEW) and characterised by a set of indexes describing their initiation, extent, duration and strength.  
We applied the method to surface chlorophyll concentration, that is an essential ocean variable representative of the marine ecosystem state and evolution. Since high frequency and seamless data in time and space are mandatory to properly detect extreme events on the basin scale, we used daily data provided by a numerical model. In particular, we employed data of winter-spring surface chlorophyll concentration provided by the coupled hydrodynamic-biogeochemical model MITgcm-BFM at 1/12° horizontal resolution, validated in the Mediterranean Sea in the 1994-2012 period. Our method allowed us to identify and characterise surface chlorophyll EEWs occurring in the considered period and also to provide a bio-regionalisation of the Mediterranean Sea associated to different regimes of chlorophyll dynamics, by means of a fuzzy classification of EEW indexes.