Mixing in the surface waters of the western Bay of Bengal using 228Ra and 226Ra

Ra-228 and Ra-226 have been measured in the surface waters of the western Bay of Bengal during five cruises conducted between 1988 and 1999. The ranges and mean (given in brackets) concentrations for Ra-228 and Ra-226 are 6.8-42.1 (17.8 +/- 7.9) dpm/100 kg and 6.0-16.7 (9.2 +/- 2.2) dpm/ 100 kg, respectively. (Ra-228/Ra-226) Activity Ratio (henceforth denoted as [228/226]) ranges from 0.8 to 3.4 with a mean of 1.9 +/- 0.5. Both Ra-228 and Ra-226 show inverse correlation with salinity, the former much stronger. A surface 2-D diffusion-advection model is used with a new approach, A simple bivariate function, C-(x,C-y) = C(0)e(-Ax .) e(-By) where C-0, A and B are constants, is fitted to the whole Ra-228 and Ra-226 data C(X,,). Substituting C(x,,) in the two-dimensional steady-state diffusion equation of Ra, the estimated values of the constants A and B can be related to eddy diffusivities and advection velocities in the zonal (x) and meridional (y) directions. From this relationship, the horizontal eddy diffusivities in the zonal and meridional directions are inferred to be 1.3 X 10(7) and 2.1 X 10(8) cm(2) s(-1), respectively in the absence of advection terms. Similarly, neglecting the influence of diffusion, one can estimate the advection velocities, w(x) and w(y), in the zonal and meridional directions, as 0.2 and 1.1 cm s(-1), respectively. The model-fit values C-(x,C-y) of Ra-228 concentrations are in good agreement with the measured values except in regions showing exceptionally high and low values. Incorporating both the advection rates and eddy diffusivities into the equation, it is found that increasing advection velocities depending on the direction can decrease or increase the eddy diffusivities and that such changes are more effective in the meridional direction compared to zonal direction in the region of study. On the whole, 228Ra appears a good tracer to derive rates of mixing between low salinity waters in the north and their high salinity southern counterparts of the western Bay of Bengal. The eddy diffusivities, K-x and K-y (without advection) derived for the Bay of Bengal are higher by about an order of magnitude than the ones similarly obtained for the Arabian Sea. This is not unexpected due to the turbulent conditions prevailing in the Bay of Bengal for most of the year.