ENERGETICS OF A NATURALLY-OCCURRING SHEAR INSTABILITY

Using observations of an energetic shear instability (Seim and Gregg, 1994), we examine the energy budget of the mixing event by comparing m icrostructure measurements of the dissipation rates of turbulent kinet ic energy epsilon and turbulent potential energy chi(pe) with changes in fine-scale velocity and density. Two sets of observations are used. The first set sampled the shear instability early in its evolution, w hen overturns occurred in strong stratification. The second set of obs ervations found the same water vertically homogenized by turbulent mix ing. In a frame of reference moving with the billows we solve a set of time- dependent energy equations to estimate the buoyancy flux J(b), turbulent production P, and strength of nonlocal forcing in the mean k inetic and mean potential energy budgets. The turbulent energy equatio ns are approximately steady when evaluated for several buoyancy period s, simplifying to local balances. We find J(b) approximate to chi(pe)/ 2 approximate to -5.5 x 10(-7) W kg(-1) and P approximate to epsilon - J(b) approximate to 2.4 x 10(-6) W kg(-1) to within a factor of 2. Th e decrease in mean kinetic energy is approximately locally balanced by P, but unlike the kinetic energy, only 25% of the increase in mean po tential energy is explained by J(b). This implies no net radiation of energy into the surrounding stratified fluid, but the large uncertaint ies in J(b) and P make this result tenuous. We find the flux Richardso n, R(f) - J(b)/P approximate to 0.22 +/- 0.1; that is, one quarter of the turbulent energy released by the instability goes toward increasin g the mean potential energy of the water column. The billows generated an average momentum flux of 0.22 Pa for more than an hour, and peak v alues exceeded 1.5 Pa. The average valve is comparable to maximum mome ntum flux values in boundary layers over ice and under ice.