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

Abstract

²²⁸Ra and ²²⁶Ra 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 ²²⁸Ra and ²²⁶Ra 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. (²²⁸Ra/²²⁶Ra) Activity Ratio (henceforth denoted as [228/226]) ranges from 0.8 to 3.4 with a mean of 1.9±0.5. Both ²²⁸Ra and ²²⁶Ra 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,y)=C₀e^−Ax.e^−By where C₀, A and B are constants, is fitted to the whole ²²⁸Ra and ²²⁶Ra data C_(x,y). Substituting C_(x,y) 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 1.3×10⁷ and 2.1×10⁸ cm²s⁻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,y) of ²²⁸Ra 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.