An analysis of tropical instability waves in a numerical model of the Pacific Ocean - 1. Spatial variability of the waves

Unstable oscillations confined within the mixed layer close to the equator are generated in wind-forced experiments performed in a multilevel general circulation model configured for the tropical Pacific Ocean. The experiment s indicate that the waves develop preferentially in the eastern Pacific alo ng the northern temperature front. However, there is clear evidence of a se cond unstable region along the southern temperature front in the central Pa cific. In both regions the instabilities propagate westward, but in the cen tral Pacific their phase speed is considerably smaller. The differences bet ween the wave characteristics in the eastern and central Pacific are closel y correlated to the differences in the time mean conditions of the flow. Th e eastern instabilities have a structure with two peaks in amplitude: one l ocated on the equator and the other a few degrees north of it. Their disper sion characteristics show many similarities to those of tropical instabilit y waves (TIWs) observed in the Pacific Ocean, while the instabilities which grow in the central Pacific do not have any known observed correspondents. We explore the spatial variability of the simulated waves through a wavele t analysis, which provides detailed results on how the period and wavelengt h of the instabilities change as a function of longitude, latitude, and dep th. The wavelet analysis reveals that in the eastern Pacific and close to t he surface the TIWs have a phase speed of -48 cm/s, while in the central Pa cific they have a phase speed of -11 cm/s. In particular, the change in the phase speed is due to a change in the dominant period of the TIWs: The per iod of the central Pacific instabilities is considerably longer than the pe riod of the instabilities present in the eastern Pacific.