Interdecadal Change in the Relationship Between the El Nino-Southern Oscillation and the North/South Pacific Meridional Mode

The North and South Pacific Meridional Modes (NPMM and SPMM) are subtropical ocean-atmospheric coupling variabilities that are significantly related to the El Nino-Southern Oscillation (ENSO). In this study, we show that there has been a clear interdecadal change in the lagged relationship between NPMM/SPMM and ENSO by analyzing observations and reanalysis data. There is a significant and close relation between SPMM and ENSO during 1956-1980 (PD1), with SPMM leading by 6-10 months; while the significant and close correlation between NPMM and ENSO appears during 1981-2005 (PD2), with the former leading by 8-11 months. The SPMM coupling strength shows a character that is similar to that of the SPMM-ENSO relationship. An intensified sea surface temperature variability results in a stronger air-sea coupling strength for SPMM before 1980 than that after 1980, in which wind-evaporation-SST feedback plays an important role. Consequently, the influence of the SPMM on the following ENSO is stronger in PD1 than in PD2. Mixed-layer heat budget analysis shows that the stronger zonal advective and thermocline feedbacks associated with the NPMM contribute to ENSO onset in PD2. At the same time, subsurface ocean temperature evolution induced by the trade wind charging mechanism in the equatorial Pacific associated with NPMM in PD2 is more significant than that in PD1, which is responsible for the better relation between NPMM and ENSO in PD2. Moreover, we infer that the relationship between NPMM/SPMM and ENSO are influenced by the interdecadal change of mean thermocline depth over equatorial Pacific.