Seasonal and interannual variations of Chlorophyll-a concentrations in Agulhas return current region

Abstract

The Agulhas Return Current (ARC) is one of the current systems in the Southern Indian Ocean (SIO) associated with high Chlorophyll-a Concentrations (CC). This work presents the seasonal and inter-annual variability of the CC during 2003–2017 in three different regions along the ARC pathway, which are selected based on the circulation pattern, bathymetry, and local dynamics. The parts are the retroflection region (R1), middle (R2), and end (R3) of the ARC. The analysis of the CC variability revealed the highest variability of CC in the R3 region, followed by R1, and the least variability in R2 for the entire study period. The CC is high during the austral summer in both R1 and R2 compared to the other seasons. However, at the end of the ARC region (R3), higher CC is observed during austral spring. The CC is high (low) during the negative (positive) phase of Southern Annular Mode (SAM) due to increased (decreased) westerlies. A comparative analysis was carried during the spring of 2004, 2008, and 2016, when the CC value was moderate, low, and high, respectively. It shows that the retroflection happened more eastward than its normal retroflection position during 2008. The dominance of anticyclonic eddies in the R3 reduced the CC during 2008. The study also identified the effect of the polar water in the variation of the chlorophyll.