Detecting the SSS 2015 El Nino signature with SMOS

Thierry Delcroix (LEGOS) and Jacqueline Boutin (LOCEAN)

El Nino Southern Oscillation is the strongest climate signal of the planet at the interannual time scale. It is characterized by strong oceanic and atmospheric anomalies in the tropical Pacific, and induces many environmental disturbances across the globe. A major event began in early 2015. The ongoing related changes suggest that it is probably the strongest El Nino recorded since 1997-98, the four intermediate events being much lower.

The current event is remarkable in terms of sea surface salinity (SSS) anomalies, as deducted from SMOS measurements. There is a very sharp drop of the order of – 1 pss in SSS in the western-central equatorial Pacific Ocean (large blue pattern in Figure 1), in accord with sporadic in situ measurements. Qualitative analysis suggests these huge anomalies result from a reversal of the ocean surface current during El Nino. The resulting anomalous eastward current transports the warm and low salinity waters of the so-called western Pacific warm pool to the East, while moving heavy precipitation located above these warm waters. Both horizontal advection and precipitation would then be responsible for the observed SSS anomalies. A quantitative analysis will help us better understand the exact role of the various mechanisms responsible for the strong anomalies observed by SMOS.

el nino 2015

Figure 1. Sea surface salinity anomalies deducted from SMOS measurements in December 2015. The anomalies are related to one month’s average December.

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