Satellite Salinity for the Characterization of Mode Waters at the Interannual Timescale

Mode waters (MWs) play a major role in the modulation of the Earth climate as they transport a large volume of heat, CO₂and other properties across basins at seasonal to longer time-scales. In the context of anthropogenic global warming, unlocking the understanding of the MWs is critical as CMIP5 climate models still struggle in their representation. MWs are formed in the deep winter mixed layers, where they acquire their homogeneous properties like salinity, temperature, CO2, heat, nutrients as well as low potential vorticity. They are then subducted in the pycnocline in conjunction with the strong tilted density polar fronts and the separation of the western boundary current. MWs subsequently flow and export the characteristics acquired at the surface at subsurface or intermediate depth into the subtropical gyre and the equatorial region.

MWs in the South Pacific Ocean are of particular interest because of their interaction with the El Niño Southern Oscillation (ENSO), which is the first mode of variability of the Pacific Ocean at the interannual time scale with global societal implications. Large anomalies in Sea Surface Salinity (SSS) in the Western Pacific Ocean as well as in theSouth Pacific Convergence Zone are very well correlated with ENSO. They are subsequently advected south by Ekman currents into the MWs formation sites. These SSS anomalies affect the mixed layer density and in consequence the volume and anomalies exported.

10 years of SSS observations from the ESA SMOS (Soil Moisture Ocean Salinity) mission provides an unprecedented dataset to study the interannual variability of SSS in the MWs formation sites of the South Pacific Ocean. MWs pathways are furthermore identified using modelling and observations in order to estimate the characteristics of the ENSO-related interannual anomalies reaching the equatorial region of the Western Pacific Ocean.