Sensitivity of the tropical Pacific to a change of orbital forcing in two versions of a coupled GCM

The changes of the variability of the tropical Pacific ocean forced by a sh ift of six months in the date of the perihelion are studied using a coupled tropical Pacific ocean/global atmosphere GCM. The sensitivity experiments are conducted with two versions of the atmospheric model, varied by two par ametrization changes. The first one concerns the interpolation scheme betwe en the atmosphere and ocean models grids near the coasts, the second one th e advection of water vapor in the presence of downstream negative temperatu re gradients, as encountered in the vicinity of mountains. In the tropical Pacific region, the parametrization differences only have a significant dir ect effect near the coasts; but coupled feedbacks lead to a 1 degreesC warm ing of the equatorial cold tongue in the modified (version 2) model, and a widening of the western Pacific large-scale convergence area. The sensitivi ty of the seasonal cycle of equatorial SST is very different between the tw o experiments. In both cases, the response to the solar flux forcing is str ongly modified by coupled interactions between the SST, wind stress respons e and ocean dynamics. In the first version, the main feedback is due to ano malous upwelling and leads to westward propagation of SST anomalies; wherea s the version 2 model is dominated by an eastward-propagating thermocline m ode. The main reason diagnosed for these different behaviors is the atmosph eric response to SST anomalies. In the warmer climate simulated by the seco nd version, the wind stress response in the western Pacific is enhanced, an d the off-equatorial curl is reduced, both effects favoring eastward propag ation through thermocline depth anomalies. The modifications of the simulat ed seasonal cycle in version 2 lead to a change in ENSO behavior. In the co ntrol climate, the interannual variability in the eastern Pacific is domina ted by warm events, whereas cold events tend to be the more extreme ones wi th a shifted perihelion.