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Mixing in the equatorial surface layer and thermocline

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  • Twelve days of microstructure measurements at the equator (140°W) in November 1984 showed a surprisingly strong effect of both the daily cycle of solar heating and wind on mixing in the upper ocean. Because of limited variations in atmospheric forcing and currents during the experiment, processes in the daily mixing cycle were similar from day to day. Only the intensity of mixing varied. The lower boundary of the diurnal surface layer separated two distinct mixing regimes, the diurnal surface layer and the thermocline. Within the diurnal surface layer (which extended to 10- to 35-m depth), turbulent kinetic energy dissipation rates ε varied relatively little. Although variations in surface layer depth coincided with the daily change in direction of air-sea surface buoyancy production of turbulent kinetic energy (or simply, the surface buoyancy flux), ε was significantly greater relative to the buoyancy flux than was expected for a simple convective layer. In the thermocline below the diurnal surface layer, ε was highly intermittent; the day-night cycle was stronger, and variability was enhanced by turbulent "bursts" of 2-3 hours duration, which may be related to internal wave breaking events. The turbulent heat flux crossing 20-m depth was almost equal to the surface heat flux less the irradiance penetrating below 20 m. Seventy percent of the surface heat flux was transported vertically to the water below 30 m by turbulent mixing. Only a negligible amount penetrated to the core of the Equatorial Undercurrent. The gradient Richardson number Ri distinguishes between statistically different mixing environments. However, ε cannot be predicted from the value of Ri, since the intensity of mixing depends on the intensity of forcing in a way not specified by the value of Ri alone.
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  • Moum, J. N., Caldwell, D. R., and Paulson, C. A. ( 1989), Mixing in the equatorial surface layer and thermocline, J. Geophys. Res., 94( C2), 2005– 2022, doi:10.1029/JC094iC02p02005.
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  • 94
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  • C2
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  • We are grateful to the master and crew of the R/V Wecoma for their efforts in making the experimental effort successful, to Melora Park and Rick Baumann for computing support, to Ayal Anis for doing many of the statistical computations, and to David Halpern for allowing us to use some of the T44 mooring data.Numerous conversations with Teri Chereskin and Tom Dillon over the course of this work are acknowledged. We have enjoyed a fruitful working relationship with Hartmut Peters and Mike Gregg of the Applied Physics Laboratory, University of Washington. This work was conducted under grants OCE-8214639 and OCE-8608256 from the National Science Foundation.
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