Source of low frequency modulation of ENSO amplitude in a CGCM

Abstract We study the relationship between changes in equatorial stratification and low frequency El Niño/ Southern Oscillation (ENSO) amplitude modulation in a coupled general circulation model (CGCM) that uses an anomaly coupling strategy to prevent climate drifts in the mean state. The stratification is intensified at upper levels in the western and central equatorial Pacific during periods of high ENSO amplitude. Furthermore, changes in equatorial stratification are connected with subsurface temperature anomalies originating from the central south tropical Pacific. The correlation analysis of ocean temperature anomalies against an index for the ENSO modulation supports the hypothesis of the existence of an oceanic ‘‘tunnel’’ that connects the south tropical Pacific to the equato-rial wave guide. Further analysis of the wind stress projection coefficient onto the oceanic baroclinic modes suggests that the low frequency modulation of ENSO amplitude is associated with a significant con-tribution of higher-order modes in the western and central equatorial Pacific. In the light of these results, we suggest that, in the CGCM, change in the baroclinic mode energy distribution associated with low fre-quency ENSO amplitude modulation have its source in the central south tropical Pacific.