Models of the surface layer of the ocean can be classified into two groups: differential models and depth–integrated bulk models. The bulk models originate with Kraus and Turner (1967), and are developed by integrating the heat and energy conservation equations over the mixed layer. These models are limited by the assumptions used in the model formulations, two notable ones being that (i) a well–mixed layer exists a priori and (ii) there is a density discontinuity at the base of the mixed layer. The differential models can be traced to the paper of Mellor and Yamada (1974) on turbulent closure models for boundary layers. These are useful but limited by substantial computational requirements.