Subpolar phytoplankton blooms have traditionally been attributed to changes in the depth of the ocean's seasonal thermocline: as the upper ocean warms and stratifies in the spring, phytoplankton reside within increasingly shallow depths where they experience higher light levels, and, as a result, begin to bloom. Recent studies have challenged this explanation, proposing instead that bloom initiation is driven either by the onset of positive heat fluxes, decreases in wind strength, decreases in grazing pressure, or by eddy-induced stratification. We compare traditional and recent ideas of bloom initiation and present a new argument that attributes the initiation to a decrease in the dominant mixing length scales in the upper ocean. From an examination of data across the subpolar North Atlantic, we find that decreases in this length scale are a better predictor of bloom initiation than current theories, thus providing a new explanation of bloom dynamics in a one-dimensional framework. Key Points We compare and synthesize current theories of bloom initiation We develop a mixing length scale to predict the initiation of the bloom Subpolar blooms are driven by a shift from buoyancy- to wind-driven mixing © 2014. American Geophysical Union. All Rights Reserved.