On the interpretation of AGCMs response to prescribed time-varying SST anomalies

Recently, Bretherton and Battisti (1999) have presented an interesting inte rpretation of ensemble experiments with atmospheric general circulation mod els (AGCMs) forced by observed sea surface temperature (SST) whose mean suc cessfully simulates the decadal evolution of the observed North Atlantic Os cillation (NAO) index:. Using a linear model of atmosphere/ocean interactio n, they plausibly argue that this hind-cast skill, as measured by low-pass correlations between observed and simulated indices, is consistent with the ocean mixed layer merely integrating stochastic surface heat flux forcing governed by the natural variability of the atmosphere. They go on to sugges t, however, that predictability associated with middle-latitude SST anomali es is limited to timescales associated with the thermal inertia of the ocea nic mixed layer (perhaps a year). Here, we include ocean circulation in a s imple coupled ocean-atmosphere model and also consider hypothetical limits in which the coupled system is highly predictable at low frequencies. We fi nd that low pass correlations between observed and simulated NAO indices, o btained from ensembles of SST-forced AGCMs, are insensitive to the predicta bility of the system. Thus inferences about predictability of the atmospher e-ocean system cannot be made on the basis of this measure of the hindcast skill of atmosphere-only simulations.