Evolution of Turbulence, Heat Content, and Freshwater Lenses in the Diurnal Warm Layer Public Deposited



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  • Thorough understanding of the mechanisms controlling the temperature structure in the surface mixed layer of the ocean and, in particular, accurate values of sea surface temperature are critical for properly parameterizing air-sea heat exchange and quantifying the amount of heat redistributed below the surface. It is however difficult to obtain routine in-situ measurements of the sea surface temperature from oceanographic moorings or research vessels, and even more difficult to measure the detailed evolution of the temperature structure. Oceanographers instead rely on parameterizations of a diurnal warm layer forced by temperature profiles or time series to estimate the time-varying surface temperature structure. For the first time, the time-varying near-surface temperature structure, turbulence and surface heat fluxes were measured at the same time in the Indian Ocean during the DYNAMO field experiment. These measurements showed the abrupt termination of nighttime mixing at sunrise and subsequent decay during approximately one hour, they showed a rapid growth of turbulence thereafter as a balance of shear and buoyancy production and turbulent kinetic energy dissipation, and they showed an equilibrium state in the afternoon. Elevated turbulence were attributed to shear instabilities from the observation of temperature ramps in low-moderate wind conditions, but could not be distinguished from Langmuir circulations in higher winds. Distinct relationships of the vertical temperature gradient, wind speed and turbulence dissipation emerged when classifying data by presence of temperature ramps. These measurements also permitted a re-assessment of the vertical structure and physics of the diurnal warm layer with implications for heat budget assessment, therefore helping to identify weaknesses in current parameterizations. The shape of temperature profiles results from the ability of turbulence to export downward the heat deposited near the surface by exponentially attenuated subsurface solar radiation. When stratification was weak in the early morning surface heat was distributed over the top eight meters resulting in heat in excess of local solar radiation divergence. After complete restratification, surface heat was trapped above the mixed layer depth where it was both input from local divergence of the absorbed solar radiation and from the downwelling of surface heat through mixing. Below the mixed layer, the divergence of attenuated solar radiations was the only heat source. Shear instabilities at the base of the mixed layer entrain cooler fluid from below thereby deepening the mixed layer depth and distributing heat and momentum over a thicker layer. In late afternoon when net surface cooling exceeded the net heating from the divergence of penetrating solar radiation, the temperature structure was destabilized from above, mixing heat downward. The heat accumulated over the previous hours and stored in the mixed layer was then eroded both from above through convection and from below through shear instabilities. Our observations also permitted a detailed look at freshwater lenses deposited by strong localized precipitation, which can affect the heat content directly from the addition of cooler rainwater, but also indirectly by modifying the stratification of the upper ocean. Twenty-six lenses were identified, ten of which propagated at the internal wave speed and featured buoyant gravity current characteristics. The temperature and salinity anomalies of lenses were related to their age and rain volume precipitated, and they were either cooler or warmer than ambient water. This propensity to retain heat created a patchy temperature environment both at the surface and in the near-surface as pockets of warm and cool water were observed within lenses. Thermohaline anomalies were estimated to dissipate in three days on average, but up to 25 days, if the turbulent mixing of ambient water was the only source of heat and salt.
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