A novel method for estimating vertical eddy diffusivities using diurnal signals with application to western Long Island Sound

We present an approach that allows the estimation of vertical eddy diffusivity coefficients from buoy measurements made at two or more depths. By measuring the attenuation and phase lag of a scalar signal generated periodically at the surface as it propagates downwards, the vertical eddy diffusivity coefficients can be calculated as K(v) = omega Delta z(2)/2ln(2)(alpha(2)/alpha(1)), where alpha(2)/alpha(1) is the ratio of the real amplitudes at frequency omega at the two depths separated by Delta z = z(2) - z(1); as K(v) = omega Delta z(2)/2 phi(2), where phi is the phase lag at the frequency omega; or as K(v) = i omega Delta z(2)/ln(2)(U(2)/U(1)), where U(2)/U(1) is the ratio of the complex signal amplitudes at the two depths. The method requires that horizontal fluxes be small at the omega frequency and that the signal-to-noise ratios at the two depths allow the determination of the amplitude and phase of omega. Application of this method to summertime 2004 western Long Island Sound oxygen and temperature buoy measurements at two depths provides a time-series of two-day average vertical eddy diffusivity estimates. Using these eddy diffusivities in conjunction with measured vertical concentration gradients, we obtain a time-series of vertical transport rates for oxygen heat and estimate mean downward fluxes for June and July as 150-260 mMol m(-2) day(-1) and 100-400 W m(-2) respectively. These estimates are of a similar magnitude to sub-pycnocline O(2) and heat demands of 240 +/- 200 mMol m(-2) day(-1) and 180 +/- 60 W m(-2) that we infer from simple budgets, implying that vertical transport is significant to both budgets. The eddy coefficients obtained from the independent O(2) and temperature measurements have a 68% correlation, and the O(2) flux estimates show a correlation of 41% to measured rates of change in bottom dissolved oxygen levels. Our results indicate that extended time-series of eddy diffusivity coefficients can be obtained from in situ buoy measurements and the method shows promise as a way to constrain the vertical transport variability in budgets of dissolved materials in estuaries. (C) 2008 Elsevier B.V. All rights reserved.