A new second-order turbulence closure scheme for modeling the oceanic mixed layer

Abstract

A new second-order turbulence closure scheme is proposed for the oceanic mixed layer. The scheme is similar in complexity to a Mellor-Yamada level 2.5 scheme in that the turbulent kinetic energy is the only turbulence quantity treated prognostically with the others determined diagnostically. The main difference lies in the treatment of the turbulent fluxes. While momentum fluxes are assumed to be downgradient, the other turbulent fluxes allow for nonlocal and countergradient contributions. The model was tested against several idealized forcing experiments for wind-deepening, heating and cooling cases, and also against observational data taken from Ocean Weather Stations November and Papa. The simulations reveal good agreement with other models. The present scheme also performs reasonably well in reproducing the observed sea surface temperature and boundary layer depth for the year 1961 at stations November and Papa. Also proposed are ways of incorporating near-surface processes such as Langmuir circulation and wave breaking. Simulations have shown that wave breaking leads to negligible deepening of the mixed layer, while the inclusion of Langmuir circulations causes further deepening to occur.