Evidence of intensification of the water cycle from SMOS SSS maps

Abstract

Important changes in the Earth’s water cycle can be assessed by analysing sea surface salinity, as this variable on average reflects the balance between precipitation and evaporation over ocean, being the upper layers the most sensitive to atmosphere-ocean interactions. In situ measurements of salinity are relatively scarce, reduced to a limited number of field campaigns, buoys, and drifters, and typically acquired some meters below the sea surface. Thus, they may not necessarily well represent ocean-atmosphere exchanges. Satellite measurements, on the contrary, are synoptic, repetitive and represent the uppermost surface of the ocean. In this work, we show that the dynamics captured by satellite-derived sea surface salinity (SSS) measurements differ from the dynamics shown by in situ near SSS (NSS) measurements. We compare a temporal series of 8 years of the Soil Moisture and Ocean Salinity (SMOS) SSS maps with the output of an ocean model that assimilates in situ salinity measurements, which includes salinity in the first meters of the surface (NSS), as well as the mixed layer depth (MLD) and the sea surface temperature (SST). On the one hand, the satellite SSS measurements present a clear intensification of the water cycle which is somewhat less evident in the NSS. The water cycle is expected to intensify in the context of global warming, according to the Clausius-Clapeyron (CC) relation, which states that the saturation of the water vapor pressure increases at a rate of 7% per degree Celsius of warming. During the analysed 8 years, we observe a positive SST trend ranging between 0.2ºC/year and 0.1ºC/year (depending on the region) and a SSS trend ranging between 0.008 psu/year and 0.015 psu/year, which is consistent with the CC law. On the other hand, we observe that the largest positive differences between the satellite SSS and the NSS trends are in regions that simultaneously present a large positive SST trend, and a negative MLD trend. This suggests that global warming is inducing a stratification over wide open ocean areas, with potentially significant consequences for the Earth’s thermal model, ocean dynamics and life in our oceans