A relaxation of the upwelling system on the northern California shelf occurs when upwelling-favorable winds decayy or reverse. Observations from the Coastal Ocean Dynamics Experiment are used to investigate this process, with emphasis on the near-surface temperature evolution. Models of relaxation warming are tested for the surface layer (0--30 m) over midshelf, with the result that the surface heat flux and poleward alongshelf advection can account for the observed temperature behavior. Onshore advection of the upwelled isopycnal field is a relatively unimportant contribution. The warming patterns that are observed and which provide the gradients for the advective process have wedgelike shapes, warmest inshore and in the south of the experimental area. These patterns appear to be related to a topographic influence of Point Reyes, possibly through the generation of gradients in the wind field. Remote advection of warm water from as far south as San Francisco occasionally occurs as well. The nature of the relaxation mechanism has general implications. A new warm, saline, nutrient-rich water mass may be formed in the surface layer. Since the warming processes are not the reverse of the cooling, an important asymmetry is present. It causes time-variable upwelling to generate conditions very different from steady upwelling: a new upwelling front is created during each event, with upwelled water seaward of it, and reversed alongshelf flow contributes to the warming and exchange mechanisms. The system cannot be described or understood without the irreversible processes during upwelling and relaxation or without a mixed layer. ¿American Geophysical Union 1987