A persistent change in south Asian monsoon seasonality during the last 580 kyr

Abstract The south Asian monsoon is a complex climatic phenomenon characterized by seasonal shifts in winds, rainfall, and ocean currents. The monsoon is a key driver of regional climate and ecosystems. The reconstruction of the monsoon variations in the past helps in understanding the potential drivers of the monsoon. The eastern Arabian Sea is a dynamic marine region strongly influenced by the advection of nutrient- rich upwelled water from the western Arabian Sea during the summer season, and convective mixing during the winter season. Here, we have used the relative abundance and stable oxygen isotopic ratio of planktic foraminifera in sediments drilled as part of the International Ocean Discovery Program Expedition 355, at Site U1457 in the northeastern Arabian Sea to reconstruct the summer and winter monsoon variability during the last 580 kyr. Globigerinita glutinata was the most abundant species, prior to Marine Isotopic Stage (MIS) 12. Interestingly, its relative abundance as well as the glacial-interglacial difference in abundance consistently decreased post MIS 12, suggesting a consistent weakening of the winter monsoon. At the same time, the relative abundance of Globigerina bulloides was not only higher during the warmer interglacials, but also its peak abundance during the interglacials increased continuously, indicating intensified summer monsoon. From the cumulative relative abundance of summer and winter monsoon assemblages, it is evident that the south Asian summer (winter) monsoon consistently strengthened (weakened) during the last 580 kyr. The several folds higher relative abundance of the winter monsoon assemblage indicates that the northeastern Arabian Sea was dominantly influenced by the winter monsoon throughout the last 580 kyr. The summer monsoon was strongly influenced by eccentricity, whereas both the eccentricity and precession affected the south Asian winter monsoon. The findings will help in improving the long-term monsoon projections, in the wake of global warming.