A Simple Theory for the Modulation of Tropical Instability Waves by ENSO and the Annual Cycle
Tropical instability waves (TIWs) are westward propagating high-frequency waves having the main period of about 5 weeks. Their activity is strongly modulated by the cold tongue annual cycle and El Niño–Southern Oscillation (ENSO). At the same time, TIWs transport heat meridionally to the cold tongue region and thus have a nonlinear rectification effect on ENSO in return. We developed and validated a theoretical formulation that shall be useful for evaluating and understanding the climate model's ability in simulating the TIW‐ENSO interaction.
The analytical formulations for the TIW’s nonlinear rectification effect on ENSO may serve as simple yet useful tools for the climate research community to evaluate the rectification effects of high‐frequency climate transients, such as TIWs, onto the low-frequency climate variability, which may ultimately lead to improving ENSO simulations in GCMs and prediction skills of seasonal climate forecasts.
In the two related papers on the subject, we developed a new multi-gird point-based complex index for capturing TIW’s time evolutions and a theoretical model for this index to delineate the dynamics that modulate TIW activity by the annual cycle and ENSO variability. We further derived and validated analytical formulations of nonlinear dynamic heating (NDH) induced by TIW activity as a function of the nonlinear annual cycle and ENSO’s variability.