Warming of the Indian Ocean and its impact on temporal and spatial dynamics of primary production

The Indian Ocean, the third largest among the world's oceans, is experiencing unprecedented changes in sea surface temperature (SST). We present temporal and spatial dynamics of phytoplankton and their response to warming in the Indian Ocean (similar to 25 degrees N to 30 degrees S) during 1998-2019 using remote sensing data. Our study revealed that the area of the Indian Ocean Warm Pool (IOWP), defined as waters with SST values >28 degrees C, is significantly expanding in most regions, particularly in the most recent decade. The increase in IOWP area was greatest (similar to 74%) in the south-central basin. Furthermore, SST increased significantly in most areas of the Indian Ocean (10 out of 11 regions explored) over the 22-year study period with the highest increase of 0.7 degrees C observed in the south-central regions. Most other regions showed an average increase in temperature of 0.4-0.5 degrees C. At the same time, net primary production (NPP) showed large interannual variability in northern and central regions of the Indian Ocean, with slightly decreasing trends in a few northern regions. Overall, years of the first decade (1998-2008) showed more often cooler temperatures and higher productivity, except for a few years, whereas years of the last decade (2009-2019) showed more often warmer temperature and lower productivity, except in very recent years (2017-2019) when productivity was high. Mean Chl a concentrations increased in the last decade during the northeast monsoon period in the northwestern regions, suggesting increased NPP in December to March period as a future scenario in this highly productive area of the Indian Ocean. We also observed increasing SST in several major upwelling areas during the study period, whereas Chl a showed high interannual variability with no marked significant trends in most areas. Results from this study corroborate the importance of the southwest monsoon as a key driver of seasonal patterns in Chl a in major upwelling areas of the Indian Ocean.