Drivers of Chlorophyll-a Variability and Trends in the Agulhas Region: Insights From 25 Years of Satellite Observations

Abstract

The Agulhas Current system in the Indian sector of the Southern Ocean is significant for the climate as well as marine ecosystems. Global warming and alteration in wind patterns are altering the Agulhas region. However, it remains unresolved whether phytoplankton blooms in the Agulhas region show a robust trend in the satellite data era, and if so, what physical mechanisms account for this trend. We used high-resolution satellite-derived chlorophyll-a (Chl-a) data, a proxy for phytoplankton biomass, to examine seasonal and long-term trends and their relationship to sea surface temperature(SST), mixed layer depth (MLD), wind speed, sea surface height, and stratification in the Agulhas Current system between 1998 and 2022. Using Empirical Orthogonal Function analysis, we identified dominant spatial and temporal patterns associated with Chl-a variability. Notable results indicate a significant increase in Chl-a concentrations in the Agulhas Return Current (ARC) and Agulhas Retroflection regions, with increasing trends of 26% and 15% per decade, respectively. In the ARC region, decreased SST, deepened MLD, and intensified wind speeds promoted vertical mixing and nutrient entrainment, supporting increased Chl-a levels. This effect is largely driven by the Southern Annular Mode (SAM), which enhances westerly winds, promoting these physical changes. In contrast, the influence of SAM in the Agulhas Retroflection zone is moderated by the stabilizing presence of Indian Ocean waters. These results highlight the complex interaction between local physical processes and broader climatic variability in driving phytoplankton dynamics in the Agulhas region.