Chlorophyll bloom triggered by tropical storm Chedza at the southern tip of Madagascar Island

Abstract

A phytoplankton bloom during the passage of Tropical Storm (TS) Chedza was observed at the southern tip of Madagascar on January 28, 2015. The mechanisms of the chlorophyll bloom were researched with satellite remote sensing data, reanalysis data and Argo buoy data. The results show that there was horizontal transport of chlorophyll-a (Chl-a) with the western coastal current of Madagascar and the South East Madagascar Current (SEMC). At the southern tip of Madagascar, there was a tilted anticyclonic eddy moving westward, which promoted the flow of nitrate at depths from 60 m to 100 m northward into Box A for a month. Simultaneously, the SEMC formed another anticyclonic eddy entering Box C. Following the passage of TS Chedza, cyclone vorticity in Box A increased continuously, and the upwelling became stronger with Ekman pumping. At the same time, the barrier layer weakened, the mixed layer deepened, and the thermocline decreased, which can uplift the nitrate to the upper ocean. After the passage of TS Chedza, sufficient photosynthetically active radiation (PAR) facilitated the Chl-a bloom in Box A. This study contributes to the assessment of the ecological impact of ocean eddies at the southern tip of Madagascar.