Numerical study on the planetary Rossby modes in the Black Sea

Abstract

We address in this paper the planetary basin modes and their possible impact on the circulation of Black Sea. These oscillations could become important in semi-enclosed seas where the horizontal scales are smaller than the barotropic radius of deformation and the baroclinic radius of deformation is comparable with the basin scales. The physical balances studied here are based on theoretical results for the Rossby modes in rectangular basins and on model simulations carried out with Bryan–Cox ocean model with eddy resolution. Eddies with horizontal scales of 100–200 km spontaneously form and propagate to the west with phase velocity of few kilometers per day. External mode oscillations dominate the model solution and show close similarity with estimates based on simplistic theoretical models. In the baroclinic oceans these oscillations trigger active baroclinic variability and western propagation, but with strongly modified wave characteristics in comparison to the barotropic case. The comparisons between simulations and data from surveys and altimeters demonstrates that the planetary modes are a dominating form of wave motion in this sea and can explain the westward propagation of signals in its deep part. It is suggested that the excitation of rotational modes can be also important for the dispersion of tracers.