Although many marine organisms have life histories adapted to seasonal events in the environment, most studies aimed at understanding biotic responses to environmental variability assume a stationary and deterministic seasonal cycle, or remove it altogether. Here we use historical sea level pressure data for the period 1948--2000 to document significant changes in the amplitude and phase of the seasonal cycle in the Aleutian Low and North Pacific High, the two dominant pressure centers over the northeast Pacific Ocean. The Aleutian Low seasonal amplitude, which is dominated by the annual component, has nearly doubled over this period, mostly due to wintertime deepening. Changes in the amplitude and phase of both the annual and semi-annual components have contributed to the time-varying structure of the North Pacific High. Observed long-term changes in the biological community structure and productivity of the North Pacific Ocean may be associated with this changing seasonality.