Observed Decadal North Atlantic Tripole SST Variability. Part II: Diagnosis of Mechanisms

In Part I of this study, the atmospheric weather noise for 1951-2000 was inferred from an atmospheric analysis in conjunction with SST-forced AGCM simulations and used to force interactive ensemble coupled GCM simulations of the North Atlantic SST variability. Here, results from those calculations are used in conjunction with a simple stochastically forced coupled model of the decadal time scale North Atlantic tripole SST variability to examine the mechanisms associated with the tripole SST variability. The diagnosed tripole variability is found to be characterized by damped, delayed oscillator dynamics, similar to what has been found by other investigators. However, major differences here, affecting the signs of two of the crucial parameters of the simple model, are that the atmospheric heat flux feedback damps the tripole pattern and that a counter-clockwise intergyre gyre-like circulation enhances the tripole pattern. Delayed oscillator dynamics are still obtained because the sign of the dynamically important parameter, proportional to the product of these two parameters, is unchanged. Another difference with regard to the dynamical processes included in the simple model is that the major contribution to the ocean's dynamical heat flux response to the weather noise wind stress is through a delayed modulation of the mean gyres, rather than from the simultaneous intergyre gyre response. The power spectrum of a revised simple model forced by white noise has a less prominent decadal peak using the parameter values and dynamics diagnosed here than in previous investigations. Decadal time scale retrospective predictions made with this version of the simple model are no better than persistence.