On the mechanisms in a tropical ocean-global atmosphere coupled general circulation model. Part I: mean state and the seasonal cycle

The mechanisms responsible for the mean state and the seasonal and interann ual variations of the coupled tropical Pacific-global atmosphere system are investigated by analyzing a thirty year simulation, where the LMD global a tmospheric model and the LODYC tropical Pacific model are coupled using the delocalized physics method. No flux correction is needed over the tropical region. The coupled model reaches its regime state roughly after one year of integration in spite of the fact that the ocean is initialized from rest . Departures from the mean state are characterized by oscillations with dom inant periodicites at annual, biennial and quadriennial time scales. In our model, equatorial sea surface temperature and wind stress fluctuations evo lved in phase. In the Central Pacific during boreal autumn, the sea surface temperature is cold, the wind stress is strong, and the Inter Tropical Con vergence Zone (ITCZ) is shifted northwards. The northward shift of the ITCZ enhances atmospheric and oceanic subsidence between the equator and the la titude of organized convention. In turn, the stronger oceanic subsidence re inforces equatorward convergence of water masses at the thermocline depth w hich, being not balanced by equatorial upwelling, deepens the equatorial th ermocline. An equivalent view is that the deepening of the thermocline proc eeds from the weakening of the meridional draining of near-surface equatori al waters. The inverse picture prevails during spring, when the equatorial sea surface temperatures are warm. Thus temperature anomalies tend to appea r at the thermocline level, in phase opposition to the surface conditions. These subsurface temperature fluctuations propagate from the Central Pacifi c eastwards along the thermocline; when reaching the surface in the Eastern Pacific, they trigger the reversal of sea surface temperature anomalies. T he whole oscillation is synchronized by the apparent meridional motion of t he sun, through the seasonal oscillation of the ITCZ. This possible mechani sm is partly supported by the observed seasonal reversal of vorticity betwe en the equator and the ITCZ, and by observational evidence of eastward prop agating subsurface temperature anomalies at the thermocline level.