The role of freshwater flux and wind-driven circulation in shaping future tropical salinity

A robust basin-wide “fresh-gets-fresher” response in tropical sea surface salinity (SSS) emerges in CMIP projections, with freshening in the Indo-Pacific and salinification in the Atlantic Ocean. Yet large uncertainties persist due to long-standing mean-state biases and inter-model spread in CMIP simulations, limiting confidence in future SSS projections and their underlying mechanisms. By correcting ocean mean-state biases, we show that CMIP models with a strong equatorial cold tongue bias substantially underestimate future western Pacific freshening. Using a bias-corrected ocean model forced by air–sea flux anomalies from multiple CMIP6 models, we disentangle the respective roles of surface freshwater forcing and ocean dynamics. Freshwater flux changes advected by the climatological circulation dominate the basin-scale Pacific–Atlantic salinity contrast, while changes in wind-driven circulation strongly modulate regional SSS anomalies, particularly in the equatorial Indo-Pacific. The balance between these processes varies markedly across CMIP6 forcing sets. Our results demonstrate that improving the representation of the tropical mean state, equatorial winds, and the Walker circulation—together with their projected changes—is essential for reducing uncertainty in CMIP-based projections of future ocean salinity. More broadly, this work highlights how targeted bias correction and process-based analysis can help bridge CMIP limitations and advance robust projections of the future ocean.