During periods with a diurnal cycle and deep thermocline, the larger temperature step definition works much better.

Precipitation becomes a dominant term in the surface buoyancy flux at 0 110W during the final stages of the 1997-98 El Nino.

The bulk Ri number is computed using hourly current meter data at 10m 25m 45m and 80m, and temperature data at 5 m intervals between 1m and 50m, and 20 m intervals to 140 m. Only values between -1 and 1 are shown, i.e. roughly in the unstable range.

MLD is defined as the depth that T is 0.5C cooler than SST. The bulk Ri=0.65 contour does a fairly good job at predicting the MLD (except for during Oct-Nov). The Obukhov depth (2*usr^3/B0) is shown only for stable buoyancy fluxes.

In a steady state system subject to wind stirring and a mean stabilizing buoyancy flux, you might expect the MLD to be equivalent to 2 * mean(usr^3) / mean(B0), which in our case is 16 m. The mean MLD for this period though is 51 m. Obviously more is going on here.

What's going on in Oct-Nov? Both the surface forcing estimate (OB) and the shear instability estimate (Zri=0.65) predict a much shallower MLD than observed. Perhaps our MLD is not a good estimate of the true mixed layer depth?

For periods where the Z(ri=0.65) is defined, its mean depth is 35 m and the mean MLD is 45 m. Taking out OCT-NOV, the mean Z(ri=0.65) is 38 m and the mean MLD is 40 m.

Z20 and critical Ri number depths appear to be fairly well correlated with MLD. (still need to compute correlation coef's). Note: The Ri number depth comparison does not show cause of shear instability, e.g. from surface forcing or variability in background thermocline structure.

Meghan F. Cronin Pacific Marine Environmental Laboratory 7600 Sand Point Way NE Seattle, WA 98115 USA |
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