Rossby Waves in the Tropical North Pacific and Their Role in Decadal Thermocline Variability

A band of enhanced thermocline variability at 108–158N in the Pacific found in nature also occurs in an ocean general circulation model forced with observed fluxes of momentum, heat, and freshwater over the period 1958– 97. The variability in the model is primarily associated with long baroclinic Rossby waves characterized by periods in the decadal range (7–10 yr). The waves are forced by westward propagating Ekman pumping anomalies east of the date line and propagate at a speed of ;13 cm s21, which is slower than the phase speed of the first mode unforced baroclinic waves (15–16 cm s21). West of the date line, the correlations between thermocline displacements and local Ekman pumping are relatively small, and the ocean signals have a phase speed of ;20 cm s21, very similar to the phase speed of the first baroclinic mode in the western half of the basin (18–20 cm s21). The phase speeds of the ocean model signals have been estimated using cospectral analysis, while the WKB approximation has been used to evaluate the phase speed of the baroclinic Rossby wave modes for the given model stratification. The thermocline displacements are coherent all the way across the basin in the 108– 158N latitude band. After reaching the western boundary the signal appears to propagate along the boundary, both to the north and the south. Along the southern branch, the signal reaches the equator and propagates along the equator, contributing to low-frequency equatorial thermocline variability.