On the Differences Between the South Pacific Meridional and Quadrupole Modes

The South Pacific quadrupole (SPQ) and South Pacific meridional mode (SPMM) are the dominant modes of South Pacific sea surface temperature (SST) variability. Both SST modes have been found to affect the occurrence of El Nino-Southern Oscillation (ENSO) events. In this study, the authors examine the differences between the SPQ and SPMM in terms of their spatial structure, temporal variations, and impacts on ENSO using observational and Coupled Model Intercomparison Project Phase 5 (CMIP5) model data. As a local SST mode defined in the subtropical southeast Pacific, the SPMM has large-amplitude SST anomalies (SSTAs) mainly confined in the southeast Pacific. In contrast, as a basin-scale SST mode defined in the South Pacific basin, the SPQ has large-amplitude SSTAs over the southeast Pacific as well as over the southwest Pacific and high-latitude South Pacific. In addition, the SPQ has its temporal variability independent of the SPMM, and the occurrence of strong SPQ events is not always simultaneous with strong SPMM events. The SPQ is more closely linked than the SPMM to ENSO, possibly because the SPQ includes an additional component of SSTAs in the southwest Pacific and high-latitude South Pacific. These findings suggest that compared with the SPMM, the SPQ may act as an improved precursor signal for ENSO events. The results have implications for understanding the respective roles of SSTAs over different regions of the South Pacific in influencing ENSO. Plain Language Summary El Nino-Southern Oscillation (ENSO) is known to have profound impacts on worldwide climate systems, and a better understanding of the effect of the South Pacific sea surface temperature (SST) modes on ENSO may help to improve the skill of climate predictions. In this study, we show that two dominate modes of South Pacific SST variability, the South Pacific quadrupole (SPQ) and South Pacific meridional mode (SPMM), have different impacts on ENSO. The SPQ is more closely linked than the SPMM to ENSO for lead times of up to a year, suggesting that the SPQ may represent a more reliable precursor signal than the SPMM for ENSO events. The reason for this is may be that the SPQ includes an additional component of SST anomalies (SSTAs) in the southwest Pacific and high-latitude South Pacific.