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Velocity microstructure measurements in the western and central equatorial Pacific Moum, James Norman


Measurements of velocity microstructure were made in two quite different oceanic regimes using the free-falling profiler, Camel III. In conjunction with the Pacific Equatorial Ocean Dynamics (PEQUOD) expedition, profiles were made at or near the equator between 138°W and 153°W. Estimates of the rate of dissipation of turbulent kinetic energy, e, made from the velocity microstructure measurements are surprisingly small in magnitude. Averaged values at 70, 90 and 110 meters (in the region of large mean shear just above the core of the equatorial undercurrent) are more than ten times smaller than those previously reported. The dissipation integrated from the level of no zonal velocity (≈ 70 meters) to the undercurrent core is less than 10% of an estimate made of the work done by the zonal pressure gradient. It is possible that the proposed balance between the work done by the zonal pressure gradient and the turbulent friction does not hold at all places at all times for the equatorial undercurrent. A second set of measurements was made along 152°E between 27°N and 42°N, south of the Kuroshio Extension current. A strong main thermocline between 500 and 800 meters depth and south of 34°N manifested itself as a secondary subsurface maximum in buoyancy frequency, N, which concurred with a subsurface maximum in averaged dissipation. A plot of e vs N indicates that e scales with N rather than depth. A simple model was developed to explain the relatively greater occurrence of turbulent patches in the main thermocline which assumes that the turbulence is generated by internal waves. The prediction of the probability of occurrence of small Richardson number is proportional to e[sup -1/N] which predicts the shape of the distribution of the turbulence relatively successfully.

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