NORTH-ATLANTIC CLIMATE VARIABILITY ON DEC ADAL TIME-SCALES - IS IT PREDICTABLE

The predictability of Atlantic climate variability is an important sci entific as well as practical problem. Methods of determining predictab ility through ''identical twin'' numerical experiments with models hav e been worked out for the atmosphere. These methods have also been app lied to models of the El Nino/Southern Oscillation. Delworth, Manabe a nd Stouffer (1993) have demonstrated a 40-50 year period of climate va riability in the GFDL model. The simulated variability involves an Atl antic thermohaline conveyor belt and produces quite realistic surface temperature anomalies. Although the GFDL model does not reproduce all of the observed characteristics of the historical data for Atlantic cl imate variability, it appears to be the best available model at the pr esent time for predictability studies. Ensemble experiments have been carried out in which an initial error is inserted in the atmospheric c omponent of the GFDL model. The solutions indicate a rapid error growt h in the atmospheric component. This error then spreads at a slower ra te to the ocean component. A measure of the predictability is the time at which the variance within the ensemble becomes 50% of the climatol ogical variance. Using the maximum amplitude of the North Atlantic the rmohaline circulation as an index, experiments indicate that the 50% p oint is reached in about 2-5 years. Tests using empirical orthogonal f unctions of the meridional transport stream function and the dynamic t opography indicated greater predictability. The variance of the yearly averaged amplitudes in the ensembles reach 50% of saturation in 10-20 years for the first two empirical orthogonal functions. For compariso n, the dominant time scale of the model variability is 40-60 years. Th e results imply that the predictability is very sensitive to both spat ial and temporal smoothing and this has to be taken into account in th e design of monitoring systems.