A phenoregion approach to understanding chlorophyll‐a seasonal cycles and interannual variability in the South‐West Indian Ocean

Abstract

The South-West Indian Ocean (SWIO) displays marked spatial variations in physical forcings, driving in turn distinct seasonal biogeochemical regimes. Mesoscale eddies in the Mozambique Channel drive large-scale redistribution of nutrients and plankton, while eastern Madagascar waters are highly oligotrophic. However, the spatial organization and diversity of biogeochemical seasonal cycles across the SWIO remain insufficiently characterized on the basin scale. This study examines the seasonal and interannual variability of surface chlorophyll-a (Chl-a), used as a proxy for primary production. A phenoregion approach is employed, involving the grouping of areas exhibiting similar Chl-a phenology, that is, bloom timing (austral winter and/or summer) and the number of blooms per year (0, 1, or 2). 22-year of weekly satellite-derived Chl-a from the OC-CCI data set was used to generate normalized climatological seasonal cycles, which were subsequently clustered using a k-means algorithm. This approach identifies six distinct Chl-a phenoregions across the SWIO. Winter blooms dominate the basin and are likely associated with mixed-layer deepening and enhanced vertical nutrient supply, leading to temporally stable phenoregions. In contrast, summer blooms are restricted to coastal regions and south-eastern Madagascar, coinciding with enhanced terrestrial nutrient inputs during the wet season. In the Mozambique Channel, Chl-a exhibits limited seasonal variability, likely reflecting strong eddy-driven exchanges between coastal and offshore waters. Interannual analysis reveals stable phenoregion cores with consistent seasonality, while their boundaries exhibit variability, highlighting contrasts between persistent and highly intermittent biogeochemical regimes. The results obtained provide a basin-scale framework for linking physical dynamics to ecosystem variability in the SWIO.