Roll circulations approximately aligned with the surface stress vector that frequently occur in the upper boundary layer of oceans or lakes. Although similar in form to atmospheric longitudinal roll vortices, Langmuir circulations are thought to be driven by nonlinear interactions between the surface gravity wave field and the larger-scale turbulent motions within the mixed layer. They are sometimes called windrows because they form lines of surface debris or bubbles in their surface convergence zones. Their spatial scale is related to the depth of the mixed layer and their characteristic velocity is on the order of 8u*, where u* is the friction velocity in water. As a result of this scaling, Langmuir circulations generally require surface winds of at least 8 m s−1 in order to form. See coherent structures, longitudinal rolls.