Nonlinear shallow water tropical instability waves on the equatorial β-plane: Genesis of two distinct types of waves

Abstract

The nonlinearity of Tropical Instability Waves (TIWs) is studied using shallow-water equations with various mean states from the equatorial Pacific Ocean. In the early linear stage, unstable TIWs, centered near 5°N with a wavelength about 1000 km and a period about one month, dominate the whole domain. However neutral Yanai waves with periods about 15-22 days emerge near the equator when the unstable TIWs grow into fully nonlinear vortices and begin to rotate, which stabilizes the mean states substantially. The strength of these Yanai waves are sensitive to the instability of the initial mean flow. Meanwhile the TIWs centered near 5°N are slowed down and weakened. The external forcing terms are found to be important for them to retain their dominance from 3°N to 7°N and also be able to suppress the late emerging Yanai waves if strong enough. Copyright 2009 by the American Geophysical Union.