Modelling vertical mixing in the surface boundary layer using artificial age tracers

The temporal evolution of biogeochemical state variables in shallow coastal seas and estuaries is tightly coupled to the turbulent mixing in the water column. In addition to the general regulation of nutrients for living organisms, vertical mixing in the surface boundary layer also strengthens the coupling between the pelagic and benthic communities in areas where the surface boundary layer reaches the bottom boundary layer. Here we analyse vertical mixing rates in the surface mixed layer with a one-dimensional model simulating the conditions in a shallow fjord during a 5-year period. The mixing rates are quantified by the distribution of an artificial water age tracer, where the age is related to the time elapsed since the water left the surface. A method is developed and applied taking into account the diffusive mixing of old water from deeper water, and the influence of upward diffusive mixing on the water age throughout the mixed layer is quantified. The resulting age of the water at different depth levels ranges from typically about 6 h at 2 m depth to 1-4 days at 8 m depth. Large interannual variability of the age tracer in the mixed layer is seen during the late spring and summer period, and this can be related to the variability of wind generated turbulence. The distribution of the water age in the mixed layer is relevant for analysing the coupling between the pelagic and benthic biomasses in shallow coastal systems on length and timescales corresponding to the mixed layer depth and about 1 week, respectively.