Sea surface temperature (SST) variability in the extratropical ocean has been associated with global and regional climate variability and impacts. The mechanisms that drive the SST variability from interannual to interdecadal time scales, however, has remain uncertain. Many previous studies have examined the role of atmospheric variability in driving SST variations, mainly at interannual time scales. At decadal time scale, however, the relative importance of atmospheric vs oceanic forcing on decadal SST variability has remained highly controversial. Here, we assess the strength of oceanic forcing in driving SST variability in observations and state-of-the-art climate models by analyzing the relationship between surface heat flux and SST, with the focus on decadal variability. A series of theory is developed in the framework of stochastic climate model to account for the major features of observed heat flux-SST correlation. It is shown that a change of heat flux-SST correlation from positive to negative with timescale can only be produced by a strong red noise ocean forcing of persistence time comparable with the SST, which tends to generate stronger SST variability than the white noise atmospheric forcing at decadal time scales. The application of our statistical method to global SST variability in observations and state-of-art climate models shows a largely similar pattern of decadal oceanic forcing across the global ocean basins reminiscent of wind-driven ocean gyres, characterized by dominant oceanic forcing in the mid- and high latitude regions, but by dominant atmospheric forcing in the subtropics. Climate model sensitivity experiments further confirm that this global pattern of decadal oceanic forcing is caused primarily by wind-driven oceanic circulation. In the North Atlantic, however, the heat flux-SST relation suggests that the Atlantic Multidecadal Variability (AMV) is forced by external climate forcing over the subpolar North Atlantic via the heat transport associated with the Atlantic Meridional Overturning Circulation.