A one-dimensional oceanic planetary boundary layer model is used to investigate the response of the upper ocean to the atmospheric conditions which are predicted to develop following a hypothetical nuclear exchange. The ocean model is driven by the surface heat and momentum fluxes predicted by an atmospheric general circulation model following a summertime injection of 1.5¿1014 g of smoke from postwar fires over Europe, Asia, and North America. Although the specific response of the upper ocean is highly dependent on the geographic location, the mid-latitude summertime mixed layer typically cools 3¿ to 5¿C and deepens 25 m during the first 30 days following the smoke injection. Moreover, a large fraction of this response is found to take place during a short 2- to 3-day period of very intense winds and falling air temperatures, which occurs during the first week or two after the smoke injection. ¿American Geophysical Union 1987