The initiation of the Gulf of Alaska phytoplankton spring bloom is light limited, and therefore the onset of stratification is critical to primary productivity. A stability ratio, which represents the balance between mixing (tide and wind) and buoyancy (heat and freshwater) processes, is defined. When buoyancy dominates over mixing, stratification is possible. Use of model- and satellite-derived data allows maps of the spatial and temporal development of stratification to be constructed. Spatial variability in the dominant forcing components is also mapped. The onset of water column stability is closely linked to the timing of the phytoplankton spring bloom. Consideration of the forcing components and corresponding stability ratio provides insight into the wide range of bloom dynamics observed in the Gulf of Alaska. Interannual variability in heat and freshwater forcing corresponds to the phase of the North Pacific Index (NPI). A low NPI reflects an intense Aleutian low-pressure system and is associated with warmer, wetter winters, leading to the earlier onset of stratification and subsequently an earlier and more intense spring bloom. The results thus provide observational support for the optimal stability window hypothesis, and identify mechanisms by which basin-scale forcing can propagate, via local physical processes, to phytoplankton productivity are identified.