Estimates of eddy diffusivity in bottom boundary layer of the Bornholm Deep

Abstract

Reliable estimate of eddy diffusivity in the bottom boundary layer (BBL) of the Bornholm Deep is the issue of practical importance because it would control dispersion of the chemical warfare agents dumped in the Deep after WW II [Helcom, 1994]. The most common way to arrive at an estimate of the eddy diffusivity, K is to apply a well-known parameterization of mixing in the Baltic halocline due to internal wave instability suggested by [A. Stigebrandt, 1987] K = min(K_max, alpha/N), where K_max is the maximum value of K, N is the buoyancy frequency, and alpha =1times10^-7 m² s^-2 [H.E.M. Meier, 2007] or alpha = 0.87times10^-7 m² s^-2 [H.-U. Lass, 2003] is the empirical constant. Taking N_BBL =0.02 s^-1 for near-bottom layer and N_halocline =0.03 s^-1 for the halocline in the Bornholm Deep [J. Elken, 1996] one obtains K_BBL= 0.5times10^-5 m² s^-1 and K_halocline = 0.33times10^-5 m² s^-1. However, the above estimate of K_BBL does not take into account the possibility of enhancement of eddy diffusivity in BBL due shear instability of the near-bottom current and a variety of mechanisms of boundary mixing [J.H. Reissmann, 2007]. Numerical simulations based on the Princeton Ocean Model with a second and a half moment turbulence closure by Mellor and Yamada [1982] embedded brought for eddy diffusivity in the Bornholm Deep BBL and the BBL thickness the values of K_BBL1= 2.5times10^-4 m² s^-1 and h_BBL1 = 2 m respectively. Eddy diffusivity in the Bornholm Deep BBL can be also estimated using data of tow-yo CTD profiling in a near-bottom layer performed aboard R/V Shelf in Summer 2006.