At this point we must introduce the concept of bulk or profile models (the latter are also called diffusive or differential), depending on whether the above equations are vertically integrated through the layer, or left in their differential form (as above). Bulk models always presuppose the existence of a mixed layer and consider the changes of its momentum and heat content due to entrainment of heat and momentum at the surface and at the bottom of the mixed-layer. Examples of bulk models are the Niiler Model [47,7], and the Garwood model . The profile-type model considered here is the Mellor--Yamada model  with various degrees of turbulence closure, known as the ``Level 2, 2.5, etc.'' model. For a thorough examination and comparison of several models of each type the reader is referred to the work of Martin  and Kantha and Clayson , who used meteorological data and hydrography at several locations around the globe to assess the performance of each model.