ADJUSTMENT AND FEEDBACKS IN A GLOBAL COUPLED OCEAN-ATMOSPHERE MODEL

We report the analysis of two 20-year simulations performed with the l ow resolution version of the IPSL coupled ocean-atmosphere model, with no flux correction at the air-sea interface. The simulated climate is characterized by a global sea surface temperature warming of about 4 degrees C in 20 years, driven by a net heat gain at the top of the atm osphere. Despite this drift, the circulation is quite realistic both i n the ocean and the atmosphere. Several distinct periods are analyzed. The first corresponds to an adjustment during which the heat gain wea kens both at the top of the atmosphere and at the ocean surface, and t he tropical circulation is slightly modified. Then, the surface warmin g is enhanced by an increase of the greenhouse feedback. We show that the mechanisms involved in the model share common features with sensit ivity experiments to greenhouse gases or to SST warming. At the top of the atmosphere, most of the longwave trapping in the atmosphere is dr iven by the tropical circulation. At the surface, the reduction of lon gwave cooling is a direct response to increased temperature and moistu re content at low levels in the atmospheric model. During the last par t of the simulation, a regulation occurs from evaporation at the surfa ce and longwave cooling at TOA. Most of the model drift is attributed to a too large heating by solar radiation in middle and high latitudes . The reduction of the north-south temperature gradient, and the relat ed changes in the meridional equator-to-pole ocean heat transport lead to a warming of equatorial and subtropical regions. This is also well demonstrated by the difference between the two simulations which diff er only in the parametrization of sea-ice. When the sea-ice cover is n ot restored to climatology the model does not maintain sea-ice at high latitudes. The climate warms more rapidly and the water vapor and clo uds feedback occurs earlier.