Relationship between the Pacific Meridional Mode and El Nino-Southern Oscillation on different timescales

Previous studies have shown that the Pacific Meridional Mode (PMM) has a crucial impact on the central Pacific El Ni & ntilde;o-Southern Oscillation (ENSO). In this study, we redefined the PMM index used previously by focusing on the sea surface temperature in the subtropical northeastern Pacific (NEP), and found that the correlations between the PMM and ENSO are distinct on different timescales. We refer to this redefined index as PMMNEP. Results show that a positive PMM is weakly related to El Ni & ntilde;o, while a negative PMM is more convincingly related to La Ni & ntilde;a, specifically, the equatorial negative SSTAs possibly affecting negative PMM events. Moreover, the cause-effect relation between the PMMNEP and ENSO is much more significant on a decadal timescale than on an interannual timescale, and their interaction may be related to the equatorial zonal wind field. Our finding indicates that a La Ni & ntilde;a-like decadal pattern is established through the combined actions of the PMM and ENSO, rather than an El Ni & ntilde;o-like decadal pattern.