Langmuir Circulations

Abstract

Langmuir circulations in the open ocean typically form in an environment of surface waves and a shear current, and a mechanism which exploits these features and leads to shear-aligned longitudinal vortices much like Langmuir circulations is Craik-Leibovich instability theory. This theory is discussed in detail. The theory is first constructed in a form which accounts for both rotational and irrotational waves in all levels of shear. This is done from the generalized Lagrangian mean equations of Andrews & McIntyre which describe an exact theory of nonlinear waves on a Lagrangian mean flow. The instability theory is then discussed at length for both weak and strong shear, as both arise in the ocean. In weak shear the instability is centrifugal and catalysed by the Stokes drift without wave modulation, while in strong shear it is calalysed by the pseudomomentum with wave modulation and is not centrifugal. Accordingly the criteria for instability in weak and strong shear are different; both criteria are given. To accentuate the differences, the instability mechanism in weak shear is denoted CL2, while in strong shear it is denoted CLg, for generalized Craik-Leibovich. Recent studies of the first bifurcation to both CL2 and CLg are outlined.