A mechanism for the ENSO amplitude modulation associated with the Atlantic Multi-decadal Oscillation

Abstract

The modulation of the El Niño-Southern Oscillation (ENSO) amplitude caused by the Atlantic Multi-decadal Oscillation (AMO) is investigated in this study by using observational data and a Coupled General Circulation Model (CGCM). The observational data for the period 1900-2013 show that the ENSO variability weakened during the positive phase of the AMO and strengthened in the negative phase. In particular, the weakening of the ENSO amplitude for the past 15 years is related to changes in the Pacific mean climate: e.g., the SST cooling in the eastern Pacific and the easterly wind anomalies in the equatorial central Pacific. The observed changes were reproduced reasonably well by CGCM simulations that were performed with the Atlantic Ocean SST nudged perpetually with the observed SST representing the positive phase of the AMO and free integration in the other oceans. Using a hybrid coupled model, it was determined that the mechanism associated with the weakening of the ENSO amplitude is related to the westward shift of the ENSO zonal wind stress anomalies in the positive phase of the AMO. This westward shift of the zonal wind stress anomalies results from the westward shift of precipitation anomalies associated with the relatively cold background mean SST over the central Pacific in the positive phase of the AMO. Thus, the present study demonstrates that the AMO plays an important role in modulating the amplitude of ENSO through mean state changes over the tropical Pacific.