Observed Decadal North Atlantic Tripole SST Variability. Part I: Weather Noise Forcing and Coupled Response

The mechanisms responsible for the decadal variability of the tripole mode of North Atlantic SST during the latter half of the twentieth century are diagnosed using a new technique. The SST and associated ocean variability are reconstructed by forcing an interactive ensemble coupled GCM by the surface fluxes resulting from weather noise. The weather noise surface fluxes are obtained from the NCEP-NCAR reanalysis by removing the simulated atmospheric feedback to the observed SST evolution. Simulations are performed to reconstruct and estimate the contributions of the local weather noise heat flux and wind stress to the observed evolution of the tripole pattern. The results indicate that the North Atlantic tripole pattern is forced primarily by the local weather noise surface heat flux. The roles of several types of ocean circulation variability, including gyres forced by the wind stress weather noise, the wind stress feedback to the SST, and the meridional overturning circulation, are also examined. Conclusions from this approach are expected to be model dependent. Further analysis, in the context of a simple model, of the mechanisms producing the tripole variability is presented in Part II.