The role of air-sea thermal coupling in El Nino-like warming events occurring in the tropical Pacific is investigated by analyzing the slab ocean model outputs archived in IPCC AR4. The methodology in this study involves composite computation of Nino 3.4 index, air-sea heat fluxes and surface wind stress. The radiative heat fluxes are decomposed into clouds radiation and clear-sky radiation instead of long wave and short wave radiation in order to highlight the role of clouds.

The multi-model mean heat budget shows that both the clouds radiation and the latent heat flux contribute dominantly to the development of the El Nino-like warming events in Nino 3.4 region, and the clouds radiation leads the latent heat by about 4 months. During the spinning down stage, the clouds radiation plays a dominant role, nevertheless, the latent heat forcing also contributes to the damping. The sensible heat and clear-sky radiation are negligible.

The variation of the clouds radiation and latent heat flux is attributed to the change of off-equatorial trade winds, which is part of the Subtropical Highs' variation in both hemispheres. The weakening of trade winds causes atmospheric divergence in the equatorial Pacific, inhibits deep convection, decreases cloudiness, allows more solar radiation to reach the sea surface and thus results in the warming events. Meanwhile, the positive latent heat flux anomaly also helps the warming events to develop. Similarly, stronger trade winds will cause the warming events to decay via less clouds short wave radiation and more latent heat release. The origin of the Subtropical Highs variation needs to be further studied.