The ocean is driven by surface boundary conditions19.1 which come from the atmosphere and the atmosphere is in turn driven by surface boundary conditions19.2 which come from the ocean. The difficulty in understanding the coupled system is that each component influences the other. For purposes of MOM, the atmosphere may be thought of as a hierarchy of models ranging from a simple idealized wind dataset fixed in time through a complicated atmospheric GCM19.3. Regardless of which is used, MOM is structured to accommodate both extremes as well as atmospheres of intermediate complexity as discussed in the following sections.