Understanding CMIP6 Inter-model Spread of Projected Change in Tropical Sea Surface Salinity

Here, we analyze projected tropical sea surface salinity (SSS) changes in 32 Coupled Model Intercomparison Projects phase 6 (CMIP6) global climate models historical simulations and representative concentration pathway 8.5 (SSP5-8.5) scenario. A robust “fresh gets fresher” pattern emerges by the end of the twenty-first century, with fresher tropical Indian and Pacific Oceans and saltier tropical Atlantic Ocean. We examine the inter-model diversity in this pattern using Empirical Orthogonal Function (EOF) analysis. The first two EOFs explain 45% of the total variance. EOF2 (22%) is a modulation of the multi-model mean SSS change, associated with the tropical-average warming intensity (r=0.61). Higher climate sensitivity leads to a more pronounced El Niño-like (positive IOD-like) warming pattern and stronger rainfall in the equatorial and north subtropical Pacific (west Indian) Ocean, leading to local freshening. In the equatorial Atlantic, an enhanced warming leads to more evaporation through the Clausius–Clapeyron relation, and a stronger SSS saltening. The “fresh gets fresher” SSS pattern inter-model diversity is thus more a response to the SST pattern diversity through the “warmer gets wetter” mechanism than an evidence of the “wet gets wetter” intensification of the hydrological cycle. EOF1 (25%) is characterized by saltening in the Indian Ocean and freshening in the Pacific Ocean, associated with changes in the inter-hemispheric relative SST gradient (r=0.55). Enhanced warming in the south hemisphere shifts the precipitation south, reducing total rainfall and saltening the Indian Ocean, while increasing rainfall and freshening the south Pacific Ocean. Overall, we find a strong influence of SST changes on the rainfall distribution, which influences SSS with some effects related to transport by the oceanic circulation.