SURFACE HEAT-FLUX VARIABILITY OVER THE NORTHERN CALIFORNIA SHELF

Surface heat flux components are estimated at a midshelf site over the northern California shelf using moored measurements from the 1981-198 2 Coastal Ocean Dynamics Experiment (CODE) and the 1988-1989 Shelf Mix ed Layer Experiment (SMILE). Time series of estimated fluxes extend fr om early winter through summer upwelling conditions, allowing examinat ion of seasonal variations as well as synoptic events. On a seasonal t imescale, the surface heat flux is strongly influenced net surface hea t flux are the annual variation in incident shortwave solar radiation (insolation) and the atmospheric spring transition. Between mid-Novemb er 1988 and late February 1989, insolation is weak and the mean daily averaged heat flux is nearly zero (absolute value less than 10 M m(-2) ), with a standard deviation of similar to 50 W m(-2). Beginning in Ma rch, insolation increases markedly, and typical daily-averaged heat fl uxes 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 similar to 90 W m(-2). In winter, th e daily-averaged heat flux varies on periods of several days. Net heat flu: losses can range up to 130 W m(-2). These lasses are not identif ied with any one type of event. For example, comparable heat flux loss es can occur for very low relative humidities (RHs), moderate winds, a nd clear skies, and for high RHs, high winds, and cloudy skies. In sum mer, surface heat flux variability is strongly influenced by upwelling and relaxation events. Up-welling is characterized by clear skies and high equatorward winds, while relaxation is characterized by the pres ence of clouds and low or northward winds. These conditions lead to op posing changes in insolation and in longwave radiative cooling and lat ent heat flux. Variability in insolation dominates, and the daily-aver aged 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)).