The barotropic vorticity equation (with free surface)
Barotropic Rossby waves (rigid lid)
Rossby waves
The 2D vorticity equation ( f plane, no free-surface effects )
In the absence of dissipation and forcing, 2D barotropic flows conserve two quadratic invariants: energy and enstrophy As a result, one has a direct enstrophy cascade and an inverse energy cascade
Two-dimensional turbulence: the transfer mechanism As a result, one has a direct enstrophy cascade and an inverse energy cascade
Two-dimensional turbulence: inertial ranges As a result, one has a direct enstrophy cascade and an inverse energy cascade
Two-dimensional turbulence: inertial ranges As a result, one has a direct enstrophy cascade and an inverse energy cascade
Two-dimensional turbulence: inertial ranges As a result, one has a direct enstrophy cascade and an inverse energy cascade log k log E(k) k -3 k -5/3 EZ
Is this all ?
Vortices form, interact, and dominate the dynamics Vortices are localized, long-lived concentrations of energy and enstrophy: Coherent structures
Vortex studies: Properties of individual vortices (and their effect on tracer transport) Processes of vortex formation Vortex motion and interactions, evolution of the vortex population Transport in vortex-dominated flows
Coherent vortices in 2D turbulence
Qualitative structure of a coherent vortex (u 2 +v 2 )/2 Q=(s 2 - 2 )/2
The Okubo-Weiss parameter u 2 +v 2 Q=s 2 - 2
The Okubo-Weiss field in 2D turbulence u 2 +v 2 Q=s 2 - 2
The Okubo-Weiss field in 2D turbulence u 2 +v 2 Q=s 2 - 2
Coherent vortices trap fluid particles for long times (contrary to what happens with linear waves)
Motion of Lagrangian particles in 2D turbulence Formally, a non-autonomous Hamiltonian system with one degree of freedom
The Lagrangian view
Effect of individual vortices: Strong impermeability of the vortex edges to inward and outward particle exchanges
Example: the stratospheric polar vortex
Vortex formation: Instability of vorticity filaments Dressing of vorticity peaks But: why are vortices coherent ? Q=s 2 - 2
Instability of vorticity filaments Q=s 2 - 2
Existing vortices stabilize vorticity filaments: Effects of strain and adverse shear Q=s 2 - 2
Processes of vortex formation and evolution in freely-decaying turbulence: Vortex formation period Inhibition of vortex formation by existing vortices
Vortex interactions: Mutual advection (elastic interactions) Opposite-sign dipole formation (mostly elastic) Same-sign vortex merging, stripping, etc (strongly inelastic) 2 to 1, 2 to 1 plus another, ….
A model for vortex dynamics: The (punctuated) point-vortex model
Q=s 2 - 2 Beyond 2D: Free-surface effects Dynamics on the -plane Role of stratification
The discarded effects: free surface
The discarded effects: dynamics on the -plane
Filtering fast modes: The quasigeostrophic approximation in stratified fluids
The stratified QG potential vorticity equation
Vortex merging and filamentation in 2D turbulence
Vortex merging and filamentation in QG turbulence: role of the Green function