An analysis of tropical instability waves in a numerical model of the Pacific Ocean - 2. Generation and energetics of the waves

The instability processes which generate unstable waves with characteristic s similar to observed tropical instability waves in the Pacific Ocean are e xamined through a local energy analysis based on deviations from the time m ean flow. Numerical experiments indicate that the waves develop preferentia lly in the eastern Pacific along the northern temperature front and have a westward phase speed and a structure with two peaks in amplitude: one locat ed on the equator and the other a few degrees north of it. The energy analy sis shows that the "two-peak" structure of the eastern waves is explained b y two different instability processes which occur at different latitudes. I n the time mean sense the region north of the equator is baroclinically uns table, while barotropic instability prevails at the equator. The life cycle of the waves is revealed by the time evolution of the energetics. Baroclin ic instability is the dominant triggering mechanism which induces growth of the waves along the northern temperature front. The eddy pressure fluxes r adiate energy south of the equator where the meridional shear between the E quatorial Undercurrent and the South Equatorial Current becomes barotropica lly unstable. From the numerical simulations, there is evidence of a second unstable region in the central Pacific south of the equator where the inst abilities have a lower phase speed. The energy analysis also shows that the se waves grow from both barotropic and baroclinic conversions.