CHAOTIC INFLUENCES AND THE PROBLEM OF DETERMINISTIC SEASONAL PREDICTIONS

An analysis has been performed of three simulations made by forcing a global climatic model with observed sea-surface temperature and sea-ic e distributions for the period 1979-1988. These simulations were start ed from different years of an extended control run, otherwise all cond itions were identical. The analysis concentrates on the regional and l ocal differences in the simulations caused by chaos attributable to th e differing initial conditions interacting with the non-linearities of the model climatic system. Of particular interest is whether the pres ence of sea-surface temperature variations in individual years is able to overcome chaotic influences. This interest arises because of the i ncreasing international appreciation that such temperature variations provide the basis for the development of multi-seasonal prediction sch emes. The analysis shows that between about 20 degrees N and 20 degree s S a coherent climatic reponse is obtainable owing to the boundary va lue forcing associated with the sea-surface temperature field. At high latitudes the signal-to-noise ratio rapidly deteriorates. Even at low latitudes, in a sensitive variable such as rainfall, chaos creates su bstantial differences between the three simulations, except over the P acific Ocean where a very coherent response is found. In general, over land areas, even those within recognized regions of ENSO (El Nino-Sou thern Oscillation) influence, the impact of chaos is much higher. Exam ination of Pacific North American teleconnection patterns for 1988 rev eals substantial differences between the three simulations, and the as sociated impact on rainfall and surface temperatures over the North Am erican continent. Agreement with observations ranges from rather good to poor, indicating the need for multiple runs for any given situation . Overall the results establish that sea-surface temperature variation s can overwhelm chaos in appropriate circumstances and specific region s. Importantly, this indicates that given accurate predictions of sea- surface temperature anomalies a few seasons in advance, then model-bas ed predictions will be possible over many regions of the world despite the omnipresence of chaos.