The ENSO-induced South Pacific Meridional Mode

The meridional modes (MM) in the Pacific are the conduit by which mid to high-latitudes external forcing (NPO/SPO) can trigger or influence ENSO; While for the Northern Hemisphere the MM (NPMM) is considered a precursor of ENSO, the MM-ENSO relationship in the Southern Hemisphere (SH) is more uncertain. Here we show that, rather than acting as a precursor, strong MMs of the SH (SPMM) are dominantly (~66%) triggered by strong El Niño events in observations and the historical simulations of the Large Ensemble CESM (LENS-CESM). In the LENS-CESM simulations, strong ENSO-induced SPMMs are associated with a precursor signal (warm SST anomalies) of the coast off northern central Chile (20°S-35°S) resulting from the combined effect of the propagation of oceanic downwelling coastal Kelvin waves and the reduction in upwelling favorable winds due to an activated Pacific South American (PSA) pattern during the development of coincident ENSO cycle. The analysis of the simulations of the Coupled Intercomparison Project phases 5 and 6 (CMIP5/6) indicate a large diversity in terms of the ENSO-SPMM relationship, which can be interpreted as resulting from the spread in the meridional location of the center of action of the SPMM and of the seasonality of the SPO variance. We further discuss how ENSO-induced SPMM interferes with the coincident ENSO cycle and contributes to its asymmetry.