Using Past Surface Water Density to Constrain Future South Asian Monsoon Precipitation

The hydrological cycle plays a crucial role in the Earth’s climate and has a direct impact on human populations. Despite advances, the IPCC AR6 report highlights persistent uncertainties concerning future projections of potential changes in the hydrological cycle, in particular for low latitudes monsoonal systems. This is because numerical climate models exhibit significant spread in their projections.

Traditionally, to estimate the future value of a climate variable, the distribution of projections from an ensemble of models is examined. However, this uncertainty is very high for water cycle, and the best estimates may be biased. To improve these projections, observational constraint, or emergent constraint methods, have been developed. These approaches adjust the distribution of projected variables based on observations, helping to reduce uncertainty. Furthermore, some studies show that the spatial pattern of sea surface salinity (SSS) is strongly correlated with the mean spatial pattern of the evaporation-precipitation (E-P) balance. Given that, water surface density is mainly influenced by salinity changes in region with strong precipitation and coastal runoff, both salinity and density could provide a useful tracer of the hydrological cycle.

In this study, we reconstruct past sea surface density based on geochemical analyses (ẟ¹⁸O_c) on foraminifera extracted from marine sediment cores in the Bay of Bengal. Density changes in this dilution basin are mainly related to south Asian monsoon precipitation changes. We used our density reconstructions for the last glacial maximum (LGM) and Mid-Holocene (MH) as a predictor for the observational constraint method. Our goal is to reduce uncertainties in future South Asian monsoon precipitation projections in climate models by linking paleoclimatic information with future climate projections. To do so, we used PMIP and CMIP numerical climate modelling experiments.

Our preliminary results show an underestimation of South Asian monsoon precipitation in the future (2000-2100) in most models, when using historical surface density and salinity (1900-2000) as a predictor. We are currently finalizing the use of LGM and MH surface density as predictor, in order to compare results when past predictors (LGM and MH) are used rather than an historical predictor.