Isotopic evidence for an intensified hydrological cycle in the Indian sector of the Southern Ocean

Abstract

The hydrological cycle is expected to intensify in a warming climate. However, observational evidence of such changes in the Southern Ocean is difficult to obtain due to sparse measurements and a complex superposition of changes in precipitation, sea ice, and glacial meltwater. Here we disentangle these signals using a dataset of salinity and seawater oxygen isotope observations collected in the Indian sector of the Southern Ocean. Our results show that the atmospheric water cycle has intensified in this region between 1993 and 2021, increasing the salinity in subtropical surface waters by 0.06 ± 0.07 g kg⁻¹ per decade, and decreasing the salinity in subpolar surface waters by -0.02 ± 0.01 g kg⁻¹ per decade. The oxygen isotope data allow to discriminate the different freshwater processes showing that in the subpolar region, the freshening is largely driven by the increase in net precipitation (by a factor two) while the decrease in sea ice melt is largely balanced by the contribution of glacial meltwater at these latitudes. These changes extend the growing evidence for an acceleration of the hydrological cycle and a melting cryosphere that can be expected from global warming.

Based on concurrent salinity and oxygen isotope observations, the authors find that amplification of the atmospheric water cycle is the main contributor to changes in surface salinity in the Indian Southern Ocean over the past three decades.