Observable polar and Atlantic sea surface salinity trends have emerged, while the noisy Indo-Pacific hesitates

Abstract

The sea surface salinity (SSS) trend pattern is widely accepted as a fingerprint of hydrologic cycle intensification, and more tentatively as an indicator of Atlantic Meridional Overturning Circulation (AMOC) weakening. As these systematic changes imply consequential impacts, it is important to know when the signal of forced SSS change emerges above the noise of internal climate variability, and what drives this signal. We estimate time of emergence of global SSS change across four, single-model initial-condition large ensembles (LEs) spanning the 20th and 21st Centuries. We also compare the climatology, variability, and trend patterns in these models to estimates from two observational datasets. Consistently across the LEs, the SSS signal first emerges (as early as 1990s) in the central Arctic and subtropical north Atlantic, and parts of the Southern Ocean. Across most of the Indo-Pacific, the signal does not emerge until after 2050. The global SSS trend pattern broadly follows the ‘fresh get fresher, salty get saltier’ signature of hydrologic cycle intensification, but additional processes—including AMOC weakening, sea ice loss and vertical mixing—boost the signals in the Atlantic basin and polar regions. When attributing the observed trends, independent models demonstrate that greenhouse gases (GHGs) are essential for explaining the global-scale pattern of freshening in the Pacific while the Atlantic gets saltier. According to one of the best-performing models, anthropogenic aerosols have muted the GHG-driven signal, delaying the emergence of the Atlantic salting trend and north Pacific freshening trend by up to three decades. In the Indo-Pacific, SSS trends are affected by large internal variability, the interplay of aerosol-driven and GHG-driven signals, and attenuation by vertical mixing and surface currents. Looking forward, all models project that the established global SSS trend pattern will intensify. Continued SSS observations in conjunction with model evaluation will help distinguish among possible futures in the tropical Indo-Pacific.