The impacts of tropical forcing and internal atmospheric noise on decadal variations of wintertime North Pacific SST are explored using the GFDL R30 atmospheric GCM coupled to the Alexander mixed layer ocean model. Observed 1950-1999 SST is prescribed in the eastern equatorial Pacific to duplicate historic ENSO forcing and the mixed layer model is used elsewhere over the global ocean except for regions of climatological sea ice cover. The use of a one-dimensional mixed layer model enables a focus on the role of surface fluxes and local wind-driven turbulence in generating SST anomalies by excluding varying horizontal advection and ocean dynamics. Sixteen runs with identical prescribed eastern equatorial Pacific SST forcing but different atmospheric realizations were examined.

Ensemble mean North Pacific decadal SST variability resembles the observed decadal variability, but with weaker amplitude. This suggests variations in tropical forcing substantially contribute to North Pacific decadal SST variability through changes in local surface forcing. However, SST variability in individual runs can be significantly different. The fact that observed SST variability is by many measures within the ensemble envelope of the simulated SST variability suggests a substantial stochastic component exists in the observed decadal SST variations. Surface winds from the sixteen runs are used to drive an ocean GCM to explore the uncoupled dynamical response of the ocean to atmospheric internal variability and tropical forcing.