Tropical instability waves as a resonance between equatorial Rossby waves

To understand the characteristics of sea surface height signatures of tropical instability waves (TIWs), a linearized model of the central Pacific Ocean was developed in which the vertical structures of the state variables are projected onto a set of orthogonal baroclinic eigenvectors. In lieu of in situ current measurements with adequate spatial and temporal resolution, the mean current structure used in the model was obtained from the Parallel Ocean Climate Model (POCM). The TIWs in the linear model have cross-equatorial structure and wave number-frequency content similar to the TIWs in POCM, even when the vertical structures of the state variables are projected onto only the first two orthogonal baroclinic eigenvectors. Because this model is able to reproduce TIWs with relatively simple vertical structure, it is possible to examine the mechanism for the formation of TIWs. TIWs are shown to form from a resonance between two equatorial Rossby waves as the strength of the background currents is slowly increased.