Chapter 19 DOPPLER SONAR OBSERVATIONS OF LANGMUIR CIRCULATION

It has become increasingly clear that air-sea coupling is important to the earth's weather and climate [e.g., Grotzner et al. 1998, Xu et al. 1998]. The oceanic surface mixed layer is the link by which the air and sea are coupled. Further, the form and strength of the mixing motion are important to important concerns such as the fluxes of gases and nutrients and the growth and health of marine life. Improved understanding of all these processes depends in some measure on our understanding mechanisms and dynamics of the mixed layer. One-dimensional “slab-models” of the mixed layer have performed remarkably well [e.g., Pollard et al. 1973, Price et al. 1986, O'Brien et al. 1991, Large et al. 1994, Li et al. 1995]. Under active mixing, the oceanic density profile erodes from the surface downward, producing a uniform layer over the remaining deeper profile. This mixed layer is approximately uniform in both velocity and density, with “jumps” occurring in both at the relatively sharp thermocline at the layer’s base (like a “slab”). Horizontal gradients are assumed a priori to be of secondary importance. The erosion rate is then set to maintain a threshold value of the bulk Richardson number, depending only on the depth of the layer and the jumps in velocity and density at the base [Pollard, et al. 1973]. Some additional improvement is attained by permitting the erosion to penetrate smoothly into the thermocline over a depth proportional to the total mixed layer depth [Large, et al. 1994]. Additional deepening occurs when water at the surface is made denser by surface buoyancy fluxes; conversely, restratification occurs when heating