Maritime Continent freshening intensifies the observed La Niña-like warming

Abstract

The Indo-Pacific Maritime Continent (MC) is the site of intense atmospheric convection, associated with the Walker Circulation, which significantly influences global climate. Observations reveal decreasing sea surface salinity (SSS) in the MC region since 1960, which was a consequence of enhanced freshwater flux. The observed sea surface temperature (SST) increasing trends of the tropical Pacific in the past decades exhibit La Niña-like patterns, with enhanced western tropical Pacific warming and eastern cooling, which diverge from many climate model results. This study explores how MC freshening influences the La Niña-like pattern under global warming. Sensitivity experiments demonstrate that imposed MC freshening enhances near-surface stratification, shoaling the mixed layer and amplifying western Pacific warming. The intensified zonal SST gradient strengthens the Walker circulation, enhancing easterly winds and equatorial upwelling, thereby cooling the eastern Pacific. CMIP6 simulations analysis corroborates that salinity-modulated stratification significantly influences Pacific SST patterns, underscoring the need for improved observations and model performance of the MC region water cycle, as well as associated upper-ocean thermohaline stratification patterns.