Ron Smith continues Yale's course on the atmosphere, ocean and environmental change with a lecture on how the atmosphere drives ocean circulation. After a review of the second exam, Smith explains the three ways the atmosphere forces the ocean: heat exchange, precipitation and evaporation, and wind stress. He shows how the first two alter seawater density, and how gravity acting on those density differences produces large-scale thermohaline currents. The second half turns to wind driven currents, tracing how wind stress on the ocean surface indirectly generates geostrophic currents. The lecture builds on earlier material in the course, using density and force balance concepts to explain patterns of global ocean circulation. Recorded at Yale in fall 2011 as part of the Open Yale Courses series, the talk is aimed at students already familiar with basic atmospheric dynamics.