Low Frequency Modes of Tropical Ocean Dynamics

Basin-wide very low-frequency (VLF) free modes with zonal uniformly deepening and shoaling of the equatorial thermocline are found as solutions of linear shallow-water equations with the meridional basin boundaries under equatorial beta-plane. They can be understood as the free heat-content recharge oscillations. The interannual VLF modes are the essential part of the coupled recharge oscillator for the El Nino-Southern Oscillation (ENSO). The decadal VLF modes may also be transformed into coupled modes relevant to the Pacific decadal climate variability through the ocean-atmosphere interaction in the tropics and subtropics.