Tropical instability waves in the Pacific Ocean

The instability processes which generate Tropical Instability Waves (TIWs) in the Pacific Ocean are examined through numerical simulations of idealized cases and wind-forced experiments in a multilevel general circulation model. Both simulations show that eddies with characteristics similar to observed TIWs develop within the mixed layer close to the equator.;A local energy analysis of the wind-forced simulations shows that TIWs have a "two-mode" structure with a hybrid barotropic and baroclinic nature. Baroclinic instability is the triggering mechanism which induces growth of the waves along the northern temperature front. The eddy pressure fluxes radiate energy south of the equator where the meridional shear between the Equatorial Undercurrent and the South Equatorial Current becomes barotropically unstable.;The spatial and temporal variability of TIWs is examined via a wavelet analysis of both simulated data and in-situ observations (TOGA-TAO data). This method discloses the 3-D inhomogeneity of the waves and quantifies the spatial variability of their period and wavelength. The wavelet analysis of the TOGA-TAO data set clearly reveals the seasonal and interannual variability of the waves. The frequency signal of the waves dominates during the summer and fall when the easterly trade winds are stronger and the SEC is particularly intense. It is shown that during La Nina conditions TIWs can develop during different periods of the year at different frequencies and that their signal in the far eastern Pacific (110;The energetics of the simplified cases show that the eddies draw their energy from the eddy potential energy proving that baroclinic instability is possible close to the equator in a continuously stratified fluid. The analysis of the numerical experiments initialized with different zonally uniform background flows allows us to identify stability criteria for a continuously stratified equatorial fluid and, in particular, to emphasize the effects of stratification and mixed-layer depth on the stability of the mean flows in the ageostrophic regime.