COUPLED PROCESSES FOR EQUATORIAL PACIFIC INTERANNUAL VARIABILITY

Diagnostics for coupled processes relevant to equatorial Pacific inter annual variability are defined, using a simplified analytical model, a nd applied to general circulation model simulations. The analytical mo del is a simplification of the Neelin model (Neelin 1991). The atmosph eric component is represented by a direct zonal thermal circulation. T he oceanic dynamics take into account a shear layer and a Kelvin wave. Analytical calculations lead to an unstable slow-propagating coupled mode, referred to as the TK-mode. All the processes of this mode are e xplicitly identified, as are their contributions to the resulting tota l growth rate. These contributions are computed in forced oceanic (cov ering the 1979-93 period) and in coupled ocean/atmosphere general circ ulation model simulations. The results are sensitive to the parameters but allow relative comparisons between general circulation model simu lations. The forced oceanic simulation suggests that the total instabi lity is nearly constant all over the equatorial Pacific basin, but the central part is dominated by the zonal advective coupled process. whi le the eastern part is controlled by both zonal and vertical advective coupled processes. The thermocline coupled process appears largest in the eastern part of the basin, but its contribution is still weaker t han the other two. The coupled simulation appears globally more unstab le than the forced one. The total instability is larger in the eastern part of the Pacific basin. This result is consistent with the locatio n and the frequent occurrence of the simulated interannual signals in the eastern part of the basin. The seasonal cycle of the forced simula tion shows maximum instability between July and November. This could s uggest an approach towards the understanding of the seasonal phase loc k of the onset of warm El Nino/Southern Oscillation events. Possible i nteractions between the delayed oscillator mechanism or the westerly w ind bursts with the TK-mode processes are discussed, In order to under stand the real system, these diagnostics should be applied to the obse rvations and not just models. Nonetheless, analysis of coupled process es allows objective comparisons between different general circulation model simulations, and the results presented in this paper should be c onsidered as a first step in this direction.