Interdecadal change in the lagged relationship between the Pacific-South American pattern and ENSO

A significant interdecadal change in the lagged relationship between the austral summer Pacific-South American (PSA) pattern and the El Nio-Southern Oscillation (ENSO) in the following austral summer (the PSA serving as a precursor signature to ENSO events) has been detected by analysis of a 91-year historical record. Strong correlations between the PSA and ENSO occurred during the periods 1956-1975 and 1990-2004 [referred to as the high correlation (HC) periods], but the correlations were weak for the periods 1928-1956 and 1975-1990 [referred to as the low correlation (LC) periods]. Both the processes of surface air-sea coupling in the extratropical/tropical Pacific, and subsurface ocean temperature evolution along the equator associated with the PSA, were found to be stronger during the HC periods than during the LC periods, thereby resulting in a stronger influence of the PSA on the subsequent ENSO during the HC periods. Changes in the PSA-ENSO relationship can be attributed mainly to interdecadal changes in the intensity of the austral summer PSA. The latter was found to have contributions from both the modulation of the Pacific decadal oscillation and long-term variations in the Southern Annular Mode.