The South Pacific atmospheric internal variability and its role in the El Niño-Southern Oscillation
The Pacific Meridional Mode, a thermodynamically-coupled mode of variability, links extratropical Pacific oceanic and atmospheric anomalies to the tropics, contributing to tropical Pacific variability across multiple timescales. While mechanistic studies on the role of the North Pacific Meridional Mode (NPMM) are prevalent in the literature, the South Pacific Meridional Mode (SPMM) has only recently been examined. This work explores further the dynamics associated with SPMM in both reanalysis and models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Akin to the NPMM, the SPMM exhibits anomalous subtropical South Pacific sea surface temperature (SST) warming in response to a weakened South Pacific High and weakened trade winds. However, as opposed to the NPMM, the seasonality of the SPMM SST and wind is out-of-phase. Results indicate that the South Pacific mixed layer depth acts as an important regulator for the equatorward propagation of SPMM-related anomalies by modulating the wind-evaporation-SST feedback. Moreover, the SPMM interacts with the NPMM and contributes significantly to the occurrence, amplitude, and evolution of El Niño-Southern Oscillation (ENSO) events. Parallel analyses with the CMIP5 models indicate that the models capture fairly well the spatial and temporal characteristics of the SPMM, including is physical mechanisms. However, ENSO predictability within the models is less coherent and rely on a different set of dynamics than those related to the meridional modes. Overall, this study reveals how the SPMM energizes components of tropical Pacific variability across multiple timescales, providing an additional benchmark for model evaluation of Pacific climate variability.