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Using the ensemble hindcasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) coupled model for the period of 1980–2005, spatiotemporal evolution in the covariability of sea surface temperature (SST) and low-level winds in the ensemble mean and spread over the tropical Atlantic is investigated with the month-reliant singular value decomposition (SVD) method, which treats the variables in a given monthly sequence as one time step. The leading mode of the ensemble mean represents a coevolution of SST and winds over the tropical Atlantic associated with a phase transition of El Niño from the peak to decay phase, while the second mode is related to a phase transition from El Niño to La Niña, indicating a precursory role of the north tropical Atlantic (NTA) SST warming in La Niña development. The leading mode of ensemble spread in SST and winds further illustrates that an NTA SST anomaly acts as a precursor for El Niño–Southern Oscillation (ENSO). A north-tropical pathway for the delayed effect of the NTA SST anomaly on the subsequent ENSO event is identified; the NTA SST warming induces the subtropical northeast Pacific SST cooling through the modulation of a zonal–vertical circulation, setting off a North Pacific meridional mode (NPMM). The coupled SST–wind anomalies migrate southwestward to the central equatorial Pacific and eventually amplify into a La Niña event in the following months due to the equatorial Bjerknes feedback. Ensemble spread greatly increases the sample size and affords insights into the interbasin interactions between the tropical Atlantic and Pacific, as demonstrated here in the NTA SST impact on ENSO. [ABSTRACT FROM AUTHOR]

Several studies have discussed the interannual influences of the North Pacific Meridional Mode (PMM) on the tropical cyclone (TC) in the western North Pacific (WNP). However, most of them either mixed the contributions of the PMM and Pacific decadal modes or failed to separate the PMM and El Niño-Southern Oscillation (ENSO) impacts. Here, we systematically revisited the year-to-year relationship between the PMM and WNP TC activity, including TC genesis frequency (TCGF) and TC track density (TCTD), by removing decadal variability. The results show that positive (negative) PMM events can induce anomalous basin-uniform cyclonic (anticyclonic) circulation and lead to a significant increase (decrease) in TCGF and TCTD over the WNP. The relationship between TCGF and the PMM is modulated by the ENSO events. When the positive PMM events are synchronized with the El Niño phase, they can enhance southeastern quadrant TC genesis over the WNP and increase TCTD in the whole WNP. However, when the positive PMM events are in phase with La Niña events, it shows an insignificant impact on WNP TCs due to the reverse influence of PMM and ENSO. This study emphasizes the important influence of the PMM on WNP TC activity on interannual time scale, especially during neutral ENSO years. [ABSTRACT FROM AUTHOR]

Previous studies have shown that the Pacific Meridional Mode (PMM) has a crucial impact on the central Pacific El Niño–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 PMM NEP. Results show that a positive PMM is weakly related to El Niño, while a negative PMM is more convincingly related to La Niña, specifically, the equatorial negative SSTAs possibly affecting negative PMM events. Moreover, the cause–effect relation between the PMM NEP 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 Niña-like decadal pattern is established through the combined actions of the PMM and ENSO, rather than an El Niño-like decadal pattern. • The PMM index defined in this study removes the signals of ENSO more effectively. • A positive PMM is weakly related to El Niño, while a negative PMM is more convincingly related to La Niña. • The relation between the PMM and ENSO is much more significant on a decadal timescale than on an interannual timescale. [ABSTRACT FROM AUTHOR]