Volunteer observing ship expendable bathythermograph data obtained during the period 1976--1989, from 30¿N to 40¿N in the North Pacific, were used to compute the rate of change of heat storage to a fixed upper ocean temperature surface. The variability of the storage rate in the seasonal timescale, computed on a 5¿ latitude by 5¿ longitude spatial scale, is compared to the net surface heat flux computed from available surface marine weather data to the same spatial and temporal resolution. Averaged across the entire basin, the difference between the average monthly heat storage rate and the average monthly heat flux is 3.76 Wm-1. The average basin-wide absolute heat storage rate agrees to within 7.5% of the average absolute heat flux for the whole basin. An empirical orthogonal decomposition of the spatial patterns of the difference between the heat storage rate and the net flux reveals no obvious trends in the heat flux computation or possible physical processes responsible for the difference. Instead, the eddies shed by the warm boundary current, the Kuroshio, is probably responsible for the major part of the difference patterns in locations where the difference values are maximum. The most important results of this study are that the heat storage rate computed to a fixed isotherm matches the net heat flux extremely well at the chosen locations and across the whole basin; and the heat storage rate computation is sensitive to the isotherm choice and to the space scale involved. ¿ American Geophysical Union 1995