Seasonal Temperature Variability in the Upper Southwest Pacific Ocean

Climatological monthly upper-ocean temperature anomalies from the annual mean in the subtropical southwest Pacific Ocean show a characteristic out-of-phase relationship between the mixed layer and the underlying water. The mixed layer temperature anomalies in the subtropical gyre and midlatitudes are consistent in the spatial distribution and phase expected from solar radiation. However, below the mixed layer, the temperature anomalies between 10°S and 30°S are coherent throughout the water column to 450-m depth and are almost 180° out of phase with the mixed layer temperatures. This pattern of temperature anomalies describes vertical movements of the thermocline more closely linked to the seasonal variations in the wind stress curl. To test this hypothesis, a one-dimensional linear vorticity model was forced using the Hellerman and Rosenstein monthly wind stresses across the entire width of the South Pacific Ocean. This simple wind-driven model has considerable skill in predicting the gyre-scale pattern of change in the phase and amplitude associated with thermocline variations in the subtropical gyre. Experiments, varying the Rossby wave speed, showed that a better representation is achieved with speeds of 2 to 2.5 times that observed from altimeter observations. Overall, the inclusion of long Rossby waves appears to be a very important contribution to the amplitude of the thermocline depth variations in the southwest Pacific. Furthermore, this important Rossby wave contribution is supported by the large-scale anomaly patterns obtained from more sophisticated three-dimensional dynamical ocean models.