INTERANNUAL CLIMATE SIMULATION AND PREDICTABILITY IN A COUPLED TOGA GCM

A Pacific Ocean-global atmosphere general circulation model is used ro simulate the climatic mean state and variability in the Tropics, up t o interannual timescales. For this model no long-term trend in climate occurs, but there are systematic differences between the model mean s tate and observations: in particular, the east equatorial Pacific sea surface temperature is too high by several degrees. Along the equator the seasonal variability in sea surface temperature is good although s ome features of the seasonal cycle are unrealistic: for example, the e ast Pacific convergence zone crosses the equator twice a year, residin g in the summer hemisphere. Despite some deficiencies in the simulatio n of the mean state, there is substantial interannual variability, wit h irregular oscillations dominated by a 2-yr cycle. A principal oscill ation pattern analysis shows that the interannual anomalies are typica lly generated in the west Pacific and move eastward along the equator with closely connected oceanic and atmospheric components. The pattern s are similar to those associated with observed El Nino events. Rainfa ll anomalies associated with the model El Nino events also have severa l realistic features. Idealized seasonal prediction experiments were m ade by slightly perturbing the atmospheric component: three 6-month hi ndcasts were thus made for each of several start times spread through an El Nino cycle. Predictability of central Pacific sea surface temper ature anomalies was best for hindcasts starting near a warm El Nino pe ak. Generally, hindcasts starting in September and December were more accurate, with less spread, than those starting in March and June. The behavior and predictability of seasonal rainfall in several regions w as also analyzed. For example, a warm model El Nino produces enhanced rainfall in the central equatorial Pacific and reduced rainfall in the Indian region, which is reproduced consistently in the hindcasts. The model also shows variability on shorter timescales, and an example is presented of a spontaneous westerly wind burst in the west Pacific an d its oceanic impact.