Sea Surface Temperature over the Bay of Bengal: A key driver for South Asian Summer Monsoon rainfall during past 31 kiloyears

Abstract

Abstract. Warmer Sea Surface Temperature (SST) in the Bay of Bengal (BoB) is crucial for driving deep atmospheric convection, facilitating low-level south-westerly winds, and enhancing moisture transport, thereby intensifying South Asian Summer Monsoon (SASM) rainfall over South Asia. However, the specific impact of BoB SST on SASM rainfall during the Glacial-Interglacial periods remains poorly understood. In this study, we reconstructed SST and evaporation versus rainfall variability over the past 31 kiloyears by simultaneously analyzing the carbonate clumped isotopes and stable oxygen isotopic composition of surface-dwelling planktic foraminifera Globigerinoides ruber from the Central West BoB (CWBoB), a key moisture source region. Additionally, cloud cover index was inferred from the abundance ratio of planktic foraminifera Globigerina bulloides to Neogloboquadrina dutertrei. Our SST reconstruction reveals an 8 °C variability over the past 31 kyr, coinciding with shifts in the G. bulloides to N. dutertrei ratio during the Last Glacial period and deglaciation, suggesting SST regulation by variable cloud cover. The increase in SST from the Early Holocene is attributed to CO2 radiative forcing. The stable oxygen isotope of seawater δ18Osw strongly aligns with a proxy record of SASM wind intensity, indicating that changes in wind patterns drive the variable evaporation versus rainfall dynamics over CWBoB. Furthermore, we examined the temporal variation in SASM continental runoff and rainfall to the Northern BoB (NBoB) by assessing changes in δ18Osw (∆18Osw), a proxy for Sea Surface Salinity (ΔSSS), between the NBoB and CWBoB. Our analysis revealed a significant relationship between SASM rainfall and SST in the CWBoB, indicating a sensitivity of 0.9±0.1 psu drop in ΔSSS across the NBoB per 1 °C rise in SST. These findings enhance our understanding of the relationship between CWBoB SST and SASM rainfall, highlighting the intricate dynamics of monsoon variability and paving the way for improved predictability of SASM rainfall patterns.