Universality of the modeled small-scale response of the upper tropical ocean to squall wind forcing

The upper ocean response to idealized surface wind forcing that is representative of conditions observed during the TOGA-COARE Intensive Observation Period is studied by numerical simulations using a second-moment closure model. A set of experiments is described with a variety of squall-like wind stress distributions and linear initial stratification in the ocean. Several physical regimes of turbulent mixing and decay during and after wind forcing are described. Differences in the structure of the upper and lower parts of the mixing layer are analyzed. The results indicate an exponential decay of turbulent kinetic energy (TKE) with time after surface forcing is removed, and TKE source terms continue to play an important role. The velocity and density structure after the squall are found to be universal, with a nearly constant Richardson number throughout the mixing layer. It is demonstrated that this implies that the mixed layer depth is determined by the initial buoyancy frequency and total momentum input from the wind stress in the same manner as in the bulk mixed layer models. It does not depend essentially on the squall duration or the time evolution of the wind stress during the squall.