Indian Ocean Warm Pool Variability during the Last 100 ka and Its Impact on Summer Monsoon Rainfall over Sri Lanka

Indian Ocean Warm Pool (IOWP ) is a novel topic which came to light with the introduction of ocean-atmospheric climate models. Recent studies have confirmed a positive correlation between the expansion of IOWP and Indian Summer Monsoon Rainfall (ISMR) during the last century. Sri Lanka, as a country with a direct summer monsoon impact on its economy and climate, it is crucial to have a thorough knowledge of the factors that control its rainfall. Hence, this study was carried out to study the long-term variability of IOWP and its impact on ISMR in Sri Lanka.

Sea Surface Temperature (SST) models of the Indian Ocean and ISMR records were constructed for the past 100 ka periods using both published and newly generated data. Accelerator Mass Spectrometry (AMS) 14C-datings were performed on Mollusc shell and Globigerina bulloids samples to develop age models. Globigerinoides ruber was selected as the key species for paleotemperature proxy construction. Published Alkenone data and foraminifera SST data were used to fill the gaps in records where Mg/Ca data are unavailable.

For the last 100 ka, a permanent IOWP above 28 0C first appeared around 15 ka BP. Between 100 ka BP and 15 ka Bp, the warmest water masses observed were around 26 0C and 25 0C. The size and location of these warm water masses were sensitive to warm (interstadials) and cold (stadial) periods but insensitive to abrupt global climate events. We assume the climate events associated with the weakening and strengthening of thermohaline circulation do not considerably impact the Indian Ocean SST since the Indian Ocean circulation is weakly bounded with thermohaline overturning. However, the larger-scale Marine Isotope events triggered by solar radiance have a direct impact on the SST fluctuations in the Indian Ocean. From an orbital scale viewpoint, the expansion of IOWP from Bay of Bengal to the Arabian sea is positively correlated to the increase of ISMR. Compared to Northern ISMR, Monsoon rainfall over Sri Lanka shows a weaker relationship with global climate events due to Low-Level Jet (LLJ) branching. Under strong IOWP, the Arabian Sea warms up, triggering the LLJ branching, which intensifies the ISMR over Sri Lanka and its sensitivity for the global climate events.