The role of Rossby waves and Indonesian throughflow waters in shaping the phytoplankton bloom over the Seychelles Chagos thermocline ridge: a biogeochemical Argo case study

Abstract

The Seychelles Chagos Thermocline Ridge (SCTR) is a biologically productive region within the tropical South Indian Ocean. The wind-driven upwelling system is strongly impacted by westward propagating Rossby waves on interannual timescales; however, concomitant in situ biogeochemical observations of this process are scarce. Observations from two Biogeochemical Argo (BGC-Argo) floats captured the interaction between a downwelling Rossby wave and the SCTR through 2023. Simultaneously a third BGC-Argo not impacted by this interaction acted as a control. Easterly winds anomalies, typical of a positive Indian Ocean Dipole, drove a downwelling Rossby wave deepening the thermocline and reducing chl-a concentrations through the water column in the eastern extreme of the SCTR. The deepening of the thermocline allowed the warm, less saline waters from the Indonesian Throughflow (ITF) to penetrate farther westward into the upwelling region. The low salinity of these waters is shown to play a dominant role in stratifying the water column, further preventing the entrainment of nutrients into the euphotic zone. Both the depression of the thermocline, through the Rossby wave interaction, and subsequent stratification of the water column, via the intrusion of ITF waters, resulted in a truncated phytoplankton surface bloom. Although the role of downwelling Rossby waves in suppressing upwelling is well described, the role of stratification, via ITF waters, in suppressing the chl-a bloom is novel. Additional case studies were identified using reanalysis, satellite, and mooring data sets, confirming that the described salinity-driven stratification is a recurrent process and may be associated with positive Indian Ocean Dipole events.