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Rectification of the ENSO variability by interdecadal changes in the equatorial background mean state in a CGCM simulation

Boris Dewitte, Sang-Wook Yeh, Byung-Kwon Moon, Carole Cibot and Laurent Terray

A 260-yr long coupled general circulation model (CGCM) simulation and an Intermediate Coupled Model (ICM) of the tropical Pacific are used to investigate the link between the changes in equatorial background stratification and the ENSO (El Niño Southern Oscillation) modulation. The focus is on the role of non-linearities associated to equatorial wave dynamics. As a first step, the change in mean states is diagnosed and documented from the “shallow-water” parameters derived from a vertical mode decomposition of the CGCM low frequency varying mean stratification. The parameters exhibit a different variability according to the baroclinic modes which can explain why a flattening thermocline does not necessary lead to reduced ENSO activity. Estimated baroclinic mode contribution to zonal current anomalies indicates that the decadal variability projects differently on the baroclinic modes as compared to the interannual variability. In particular, the high-order modes associated with decadal variability have more signature in the western Pacific whereas that associated with interannual variability (i.e., ENSO) have more energy in the eastern Pacific.

In the light of the results of the CGCM vertical mode decomposition, we test if the non-linearities associated to change in the baroclinic mode energy distribution can lead to coherent ENSO modulation in the ICM. The results indicate that rectification of the interannual variability (ENSO timescales) by the interdecadal variability associated to changes in the oceanic mean states takes place. The rectified effect results mostly in an increased variability and skewness of the zonal advection that tends to produce a zonal seesaw of sea surface temperature anomaly. A tropical mechanism for producing ENSO modulation is then proposed, which conceals both the rectified effect due to non-linearities associated to equatorial wave dynamics and the tropical decadal mode of thermocline depth arising from Ekman pumping anomalies located in the central South Pacific.

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