Cross-Basin Interactions between the Tropical Atlantic and Pacific in the ECMWF Hindcasts

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 Nino from the peak to decay phase, while the second mode is related to a phase transition from El Nino to La Nina, indicating a precursory role of the north tropical Atlantic (NTA) SST warming in La Nina development. The leading mode of ensemble spread in SST and winds further illustrates that an NTA SST anomaly acts as a precursor for El Nino-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 Nina 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.