The nonlinear evolution of unstable coupled equatorial ocean-atmosphere modes

P. C. F. van Der Vaart (1) , H. A. Dijkstra (1)

1 IMAU - Institute for Marine and Atmospheric Research [Utrecht]

In this paper we investigate whether observed intraseasonal variability in the equatorial Pacific can be attributed to finite amplitude waves resulting from unstable air-sea interactions. Within a Zebiak - Cane type model of the coupled equatorial ocean - atmosphere, the nonlinear equilibration of instabilities of a simple basic state is considered with periodic conditions on the ocean boundaries. Three mechanisms exist which can induce a finite amplitude equilibration on a time scale ?²t. Here t is the characteristic time scale of growth of the disturbance and ? the relative distance from the instability threshold. For each equilibration mechanism, the finite amplitude and period of the equilibrium state are computed as a function of ? and substantial amplitude can be reached for a reasonable degree of supercriticality. Thereafter the analysis is extended to include time-dependent external forcing. It is shown that interannual variability may result through the interaction of the response of a weak annual external forcing and the finite amplitude development of the intraseasonal instabilities.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Sciences de la Terre

P. C. F. van Der Vaart, H. A. Dijkstra. The nonlinear evolution of unstable coupled equatorial ocean-atmosphere modes. Nonlinear Processes in Geophysics, 1998, 5 (1), pp.39-52. ⟨hal-00301881⟩