ALBERT

Description: 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. To unravel these signals, we leverage a unique dataset of salinity and seawater oxygen isotope (δ〈sup〉18〈/sup〉O) observations collected in the Indian sector of the Southern Ocean. Much of the surface south (north) of 46°S experienced a freshening (salinification) associated with decreases (increases) in δ〈sup〉18〈/sup〉O between 1993 and 2021. Many processes have been proposed to explain surface salinity changes, including intensified freshwater fluxes from the atmosphere or increased glacial meltwater or from intensification of sea ice freshwater transport. However, the contribution of each mechanism that led to observed changes of the Southern Ocean surface waters, remains unclear. Here, we argue that the atmospheric water cycle has intensified in Indian sector of the Southern Ocean between 1993 and 2021, increasing the salinity in subtropical surface waters by 0.06 g kg〈sup〉-1〈/sup〉 per decade, and decreasing it in subpolar surface waters by -0.02 g kg〈sup〉-1〈/sup〉 per decade. In the subpolar region, this salinity decrease is countered by a salinity increase of 0.008 g kg〈sup〉-1〈/sup〉 per decade from reduced sea ice melt, and enhanced by a salinity decrease of -0.005 g kg〈sup〉-1〈/sup〉 per decade from increased glacial melt. These changes extend the growing evidence for an acceleration of the atmospheric water cycle and a melting cryosphere that is expected from global warming.

Description: The diurnally-varying deep cycle of turbulence in the equatorial oceans just below the mixed layer is critical for the heat flux from the mixed layer into the interior ocean. Its presence depends on the downward propagation of shear instabilities which are induced by the diurnal cycle of near-surface stratification and shear, i.e., by the diurnal warm layer and the diurnal jet. Using velocity and hydrographic data taken during two trans-Atlantic cruises along the equator in autumn 2019 and spring 2022, data from three types of surface drifters and data from PIRATA moorings along the equator, we analyse the presence and downward propagation of near-surface diurnal stratification and shear in the upper 20 m of the equatorial Atlantic. These different datasets all indicate that the diurnal warm layer established after sunrise leads to the development of a diurnal surface jet resulting in maximum velocity differences between surface and 15 m depth in the afternoon. The diurnal horizontal velocity pattern depends on the wind speed with vertical shear being stronger, reaching its maximum earlier, and propagating downward faster for stronger winds within the observed wind range from 2-9 ms〈sup〉-1〈/sup〉. Similar downward propagation can be observed for stratification. These diurnal variations are affected by the background shear and stratification associated with seasonal and longitudinal variations of depth and strength of the Equatorial Undercurrent and the thermocline. The results are discussed with respect to recent measurements of deep cycle turbulence obtained from moored observations at the PIRATA sites at the equator, 23°W and 10°W.