In the subtropical northwestern North Pacific, nutrients brought into shallow water by winter mixing were observed to fuel local increases in phytoplankton from winter to late spring. Surprisingly, these increases in phytoplankton occurred in warm waters in winter and in cold waters in late spring. Large increases in phytoplankton require ample supplies of both light and nutrients. Our data suggest that phytoplankton increased in winter in waters that were warmed by solar heating after the cessation of vertical mixing and in late spring in waters that were cooled by the recurrence of vertical mixing. Wintertime mixed-layer depth fluctuates from year to year. Based on field observations and a model analysis, we suggest that the timing and magnitude of phytoplankton increases vary with the depth of the wintertime mixed layer, which in turn affects year-to-year variability of carbon export in this region.