Intense mixing near the surface and bottom boundaries results in mixed layers in which temperature and density are vertically uniform. Surface and bottom mixed layers provide one characterization of the thickness of the boundary layers. Over shelves, the surface and bottom mixed layers can be 10s of meters thick, hence a substantial fraction of the entire water column (typically of order 100 m or less). Lentz [1992] found that in four different coastal upwelling regions (Peru, northwest Africa, northern California, and Oregon), variability over time scales longer than a day in the thickness of the surface mixed layer was related in a simple way to the wind stress, which tends to thicken the surface mixed layer, and the interior stratification, which tends to inhibit growth of the surface mixed layer. A scaling based on this balance between wind stress and stratification not only accounted for the temporal variability in each coastal upwelling region but also for variations between regions. This simple relationship is surprising in light of open ocean studies which suggest that the surface heat flux, which is strong in these coastal upwelling regions, should also be important in inhibiting the growth of the surface mixed layer. Lentz hypothesized that the surface heat flux is removed by an offshore heat flux associated with coastal upwelling and consequently does not influence variability in the surface mixed layer thickness. Lentz also showed that the observed offshore transport in the surface boundary layer was roughly equal to the theoretical wind-driven transport (Ekman transport) provided the variability in the surface mixed layer thickness and the tendency for the wind-driven momentum to penetrate below the surface mixed layer [ Trowbridge, 1992] are taken into account. This study was limited to coastal upwelling regions which experience a narrow range of atmospheric forcing and oceanic conditions, i.e., persistent upwelling-favorable winds and a strong surface heat flux. Further studies are clearly needed in other regions. For example, Largier [1990] used a simple model to show that a strong internal tidal current combined with the wind stress could explain the very thick surface mixed layers observed over the South African shelf.