Surface heat flux components are estimated at a midshelf site over the northern California shelf using moored measurements from the 1981--1982 Coastal Ocean Dynamics Experiment (CODE) and the 1988--1989 Shelf Mixed Layer Experiment (SMILE). Time series of estimated fluxes extend from early winter through summer upwelling conditions, allowing examination of seasonal variations as well as synoptic events. On a seasonal timescale, the surface heat flux is strongly influenced net surface heat flux are the annual variation in incident shortwave solar radiation (insolation) and the atmospheric spring transition. Between mid-November 1988 and late February 1989, insolation is weak and the mean daily averaged heat flux is nearly zero (absolute value less than 10 W m-2), with a standard deviation of ~50 W m-2. Beginning in March, insolation increases markedly, and typical daily-averaged heat fluxes increase to greater than 100 W m-2 by the spring transition in April or May. In June and July, the average heat flux is near 200 W m-2, with a standard deviation of ~90 W m-2. In winter, the daily-averaged heat flux varies on periods of several days. Net heat flux losses can range up to 130 W m-2. These losses are not identified with any one type of event. For example, comparable heat flux losses can occur for very low relative humidities (RHs), moderate winds, and clear skies, and for high RHs, high winds, and cloudy skies. In summer, surface heat flux variability is strongly influenced by upwelling and relaxation events. Upwelling is characterized by clear skies and high equatorward winds, while relaxation is characterized by the presence of clouds and low or northward winds. These conditions lead to opposing changes in insolation and in longwave radiative cooling and latent heat flux. Variability in insolation dominates, and the daily-averaged heat flux into the ocean is greatest during upwelling events (up to 350 W m-2 or more) and least during relaxation events (sometimes less than 100 W m-2). ¿ 1998 American Geophysical Union