The South Pacific Meridional Mode and Its Role in the Tropical Pacific Interannual-to-Decadal Variability: Observations and Models

The Pacific Meridional Mode, an intrinsic thermodynamically coupled mode of variability, links extratropical North or South Pacific oceanic and atmospheric anomalies to the tropics, where those anomalies are amplified by the Bjerknes feedback and contribute to tropical Pacific variability. 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 expands upon this preliminary work by examining the SPMM in both observations and coupled climate models used in the Coupled Model Intercomparison Project Phase 5 (CMIP5). Akin to the NPMM, the SPMM is characterized by anomalous subtropical South Pacific sea surface temperature (SST) warming corresponding to weakened southeasterly trade winds. However, as opposed to the NPMM, the seasonality of the SPMM SST and wind is out-of-phase; i.e. the winds peaks in the austral winter while SST peaks in the summer. Results indicate that the South Pacific mixed layer depth, which is shallowest in the austral summer, acts as an important regulator for the equatorward propagation of the SPMM-related anomalies by modulating the excitation of the wind-evaporation-SST (WES) feedback. The authors also find that the first leading internal atmospheric mode, termed the South Pacific Oscillation (SPO), initiates the summertime SPMM. Although these relationships are not clearly reproduced in several CMIP5 models, the multi-model ensemble indeed captures realistic aspects of the SPMM. Further, the SPMM energizes both tropical Pacific interannual and decadal variability, adding an additional benchmark by which to test the models’ ability to reproduce Pacific decadal variability dynamics. Acting in concert with the NPMM, the SPMM regulates the occurrence, amplitude, zonal phase propagation, and potentially the type of El Niño-Southern Oscillation (ENSO) events.