Vertical Structure and Seasonal Variability of Chlorophyll Concentrations in the Southern Tropical Indian Ocean Revealed by Biogeochemical Argo Data

The variability of chlorophyll (Chla) in the Southern Tropical Indian Ocean (STIO) is not fully understood. This study utilized biogeochemical Argo (BGC-Argo) and satellite observations to investigate the seasonal Chla variations in the upper layer (above 200 m) and their relationships to physical dynamics. The results indicate the existence of a well-developed deep Chla maximum (DCM) layer situated between depths of 50 and 150 m. The shallowest DCM was at the Seychelles-Chagos thermocline ridge because of permanent upwelling. Both the northern (4°S–12°S, 52°E−92°E) and southern (12°S–25°S, 52°E−92°E) regions experience surface blooms during July–August. However, they exhibit distinct Chla changes in response to different physical processes and nitrate concentrations below the mixed layer. In the northern region, the thermocline plays a critical role in regulating DCM depth and intensity. From April to June, subsurface upwelling and near-surface stratification processes promote nutrient and Chla accumulation in the subsurface layer, resulting in elevated surface Chla levels in the subsequent months. In contrast, the southern region is characterized by oligotrophic conditions, where light availability primarily governs Chla variability below the mixed layer. Specifically, from November to January, when light intensity intensifies, Chla increases below the mixed layer. Furthermore, BGC-Argo data revealed a long-lived cyclonic eddy that facilitated the westward transport of Chla, significantly contributing to surface Chla blooms through eddy-pumping and eddy-trapping mechanisms. This research elucidates the fundamental characteristics of Chla distribution from a three-dimensional perspective and furthers our understanding of the complex biophysical interactions within the STIO.