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The merger of geophysical vortices at finite Rossby and Froude number

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Abstract

We investigate the merger of two co-rotating geophysical vortices at finite Rossby and Froude number.  The initial conditions consist of two uniform potential vorticity vortices in near equilibrium and in a near 'balanced' state (i.e. with negligible emission of inertia-gravity wave radiation).  We determine the critical merger distance between the two vortices.  This distance is found to increase with the magnitude of the Rossby number: intense cyclones or intense anticyclones.  Note that the Froude number is proportional to the Rossby number for the near equilibrium initial conditions considered.  The critical merging distance also depends on the sign of the potential vorticity anomaly, which is positive for 'cyclones' and negative for 'anticyclones'.  We show ageostrophic motions occurring at finite Rossby number tend to draw cyclones together but draw anticyclones apart.  On the other hand, we show that anticyclones tend to deform more, in particular when subject to vertical shear (as when the vortices are vertically offset).  These two effects compete.  Overall, nearly aligned cyclones tend to merge from further apart than their anticyclonic counterparts, while vertically offset anticyclones merger from further apart than cyclones.
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Original languageEnglish
Pages (from-to)388-410
JournalJournal of Fluid Mechanics
Volume848
Early online date5 Jun 2018
DOIs
Publication statusPublished - 10 Aug 2018

    Research areas

  • Rotating flows, Stratified flows, Vortex dynamics

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