Enhanced Turbulence in the Upper Mixed Layer Under Light Winds and Heating: Implications for Gas Fluxes
Sally MacIntyre, J. H. Amaral, John M. Mélack
Abstract
Abstract Measurements of turbulence, as rate of dissipation of turbulent kinetic energy ( ε ), adjacent to the air‐water interface are rare but essential for understanding of gas transfer velocities ( k ) used to compute fluxes of greenhouse gases. Variability in ε is expected over diel cycles of stratification and mixing. Monin‐Obukhov similarity theory (MOST) predicts an enhancement in ε during heating (buoyancy flux, β + ) relative to that for shear ( u * w 3 / κz where u * w is water friction velocity, κ is von Karman constant, z is depth). To verify and expand predictions, we quantified ε in the upper 0.25 m and below from profiles of temperature‐gradient microstructure in combination with time series meteorology and temperature in a tropical reservoir for winds <4 m s −1 . Maximum likelihood estimates of near‐surface ε during heating were independent of wind speed and high, ∼5 × 10 −6 m 2 s −3 , up to three orders of magnitude higher than predictions from u * w 3 / κz , increased with heating, and were ∼10 times higher than during cooling. k, estimated using near‐surface ε , was ∼10 cm hr −1 , validated with k obtained from chamber measurements, and 2–5 times higher than computed from wind‐based models. The flux Richardson number ( R f ) varied from ∼0.4 to ∼0.001 with a median value of 0.04 in the upper 0.25 m, less than the critical value of 0.2. We extend MOST by incorporating the variability in R f when scaling the influence of β + relative to u * w 3 / κz in estimates of ε , and by extension, k , obtained from time series meteorological and temperature data.