Using SMOS to analyze the variability of the South Pacific SSS maximum

Using SMOS to analyze the variability of the South Pacific Sea Surface Salinity maximum

Understanding the variability of high-salinity surface waters, as shown in Fig. 1  for  the south-eastern tropical Pacific,  is important to improve our interpretation of climate and hydrological cycle changes at different time scales. SMOS CATDS-CEC LOCEAN SSS products have been used , in complement  to Voluntary Observing Ships (VOS) thermo-salinograph data obtained from the French SSS Observation Service,  to validate and understand the seasonal variability of the South Pacific Sea Surface Salinity maximum simulated by an ocean general circulation model with no direct SSS relaxation.

All products reveal a consistent seasonal cycle of the displacement of the 36-isohaline barycenter (Fig. 2; about +/-400 km in longitude) in response to changes in the South Pacific Convergence Zone location and Easterly winds intensity respectively associated with changes in precipitation and evaporation

The SSS from 8 VOS transects compare remarkably well with collocated SMOS SSS averaged over 100km, 18 days  (std difference=0.2), as exemplified in Fig. 3 along a shipping track running from New Zealand to Panama ; the comparison with simulated SSS is slightly degraded  due to a few degrees latitudinal shift of the simulated SSS maximum (std difference=0.26).

Model results and in situ measurements further indicate a low frequency westward shift of the 36-isohaline barycenter (about 1400 km since 1992) that could not be linked to ENSO and may reflect the signature of decadal changes and/or global warming.