Seasonal temperature variability in the upper southwest Pacific Ocean

Climatological monthly upper-ocean temperature anomalies from the annual me an in the subtropical southwest Pacific Ocean show a characteristic out-of- phase relationship between the mixed layer and the underlying water. The mi xed layer temperature anomalies in the subtropical gyre and midlatitudes ar e consistent in the spatial distribution and phase expected from solar radi ation. However, below the mixed layer, the temperature anomalies between 10 degrees S and 30 degrees S are coherent throughout the water column to 450 -m depth and are almost 180 degrees out of phase with the: mixed layer temp eratures. This pattern of temperature anomalies describes vertical movement s 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 force d using the Hellerman and Rosenstein monthly wind stresses across the entir e width of the South Pacific Ocean. This simple wind-driven model has consi derable skill in predicting the gyre-scale pattern of change in the phase a nd amplitude associated with thermocline variations in the subtropical gyre . Experiments, varying the Rossby wave speed, showed that a better represen tation is achieved with speeds of 2 to 2.5 times that observed from altimet er observations. Overall, the inclusion of long Rossby waves appears to be a very important contribution to the amplitude of the thermocline depth var iations in the southwest Pacific. Furthermore, this important Rossby wave c ontribution is supported by the large-scale anomaly patterns obtained from more sophisticated three-dimensional dynamical ocean models.