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 Sea Surface Salinity (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 SSS maximum simulated by an ocean general circulation model with no direct SSS relaxation.

Fig. 1. Mean 1990-2011 modelled mixed-layer salinity. The blue lines represent the Matisse ship routes of 2010 and 2011.

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 associated with changes in precipitation and evaporation respectively.

Fig.2. Location of isohaline 36 (simulated) and of its barycentre (dots: model; stars: SMOS) for various months (colours).

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 ship 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).

Fig. 3. Example of comparison between SMOS (dots), VOS (straight line), and simulated (dashed line) SSS as a function of latitude.

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.

Details can be found in: Hasson, A., T. Delcroix, and J. Boutin (2013), Formation and variability of the South Pacific Sea Surface Salinity maximum in recent decades, J. Geophys. Res. Oceans, 118, doi:10.1002/jgrc.20367.