Effect of density stratification on vortex merger

We study the effect of density stratification in the plane on the merging of two equal vortices. Direct numerical simulations are performed for a wide range of parameters. Boussinesq and non-Boussinesq effects are considered separately. With the Boussinesq approximation, moderate to high Prandtl number and Froude number close to unity, there is a monotonic drifting away of the vortices from each other, and merger is completely prevented. Among non-Boussinesq effects, the inertial effects of density stratification are highlighted. These give rise to a breaking of symmetry, and consequently, the vorticity centroid is found to drift significantly from its initial position. Using an idealized model, we explore the role of baroclinic vorticity in determining these features of the merger process. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4773445]