Interannual Climate Simulation and Predictability in a Coupled TOGA GCM

Abstract

A Pacific Ocean?global atmosphere general circulation model is used to simulate the climatic mean state and variability in the Tropics, up to interannual timescales. For this model no long-term trend in climate occurs, but there are systematic differences between the model mean state 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 some features of the seasonal cycle are unrealistic: for example, the east Pacific convergence zone crosses the equator twice a year, residing in the summer hemisphere. Despite some deficiencies in the simulation of the mean state, there is substantial interannual variability, with irregular oscillations dominated by a 2-yr cycle. A principal oscillation pattern analysis shows that the interannual anomalies are typically generated in the west Pacific and move eastward along the equator, with closely connected oceanic and atmospheric components. The patterns are similar to those associated with observed El Niño events. Rainfall anomalies associated with the model El Niño events also have several realistic features. Idealized seasonal prediction experiments were made by slightly perturbing the atmospheric component: three 6-month hindcasts were thus made for each of several start times spread through an El Niño cycle. Predictability of central Pacific sea surface temperature anomalies was best for hindcasts starting near a warm El Niño peak. 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 was also analyzed. For example, a warm model El Niño 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 and its oceanic impact.