Interannual variability of the South Pacific Ocean in observations and simulated by the NCEP Climate Forecast System, version 2

The mechanism of the South Pacific Ocean Dipole (SPOD) mode is examined, using a 50-year simulation of the Climate Forecast System, version 2 (CFSv2) and 50-year observation-based ocean-atmosphere analyses (1961-2010). It is shown that the SPOD, a sea surface temperatures (SST) seesaw between the subtropics and extratropics, is the dominant mode of the interannual variability in the South Pacific in both observations and CFSv2 simulation. CFSv2 also reproduces the seasonal phase-locking of the observed SPOD, with the anomaly pattern developing in austral spring, peaking in summer, and decaying in autumn. Composite analyses based on both observational and model data suggest that in the warm phase of SPOD, positive SST anomaly (SSTA) is initiated by weakened westerly winds over the central South Pacific in austral spring, which suppress the surface evaporative heat loss and reduce the oceanic mixed layer depth, both contributing to the SST warming. The wind-SST-mixed layer anomalies then evolve coherently over the next two seasons while the cold SSTA develops to the north. The wind perturbations are in turn a response to El Nio-Southern Oscillation (ENSO), which forces an atmospheric planetary wave train, the Pacific-South American pattern, emanating from an anomalous heat source in the tropical western Pacific. Moreover, SPOD is significantly correlated with the southern annular mode (SAM) while the latter is also significantly correlated with the ENSO index. This suggests that ENSO's influence on the SPOD may be partially conveyed through SAM.