Publication Date:
2017-04-04
Description:
We study the quasi-geostrophic merging dynamics of axisymmetric baroclinic vortices to understand how
baroclinicity affects merging rates and the development of the nonlinear cascade of enstrophy. The initial
vortices are taken to simulate closely the horizontal' and vertical structure of Gulf Stream rings. A quasigeostrophic
model is set with a horizontal resolution of 9 km and 6 vertical levels to resolve the mean stratification of the
Gulf Stream region.
The results show that the baroclinic merging is slower than the purely barotropic process, The merging is
shown to occur in two phases: the tirst, which produces clove-shaped vortices and diffusive mixing of vorticity
contours; and the second, which consists of the sliding of the remaining vorticity cores with a second diffusive
mixing of the intemal vorticity field. Comparison among Nof, Cushman-Roisin, Polvani et al, and Dewar and
Killworth merging events indicates a substantial agreement in the kinematics of the DYOCRSS.
Parameter sensitivity experiments show that the decrease of the baroclinicity parameter of the system, Γ^2,
[defined as Γ^2 = (D^2 fo^2)/ (No^2 H^2)], increases the speed of merging while its increase slows down the merging.
However, the halting elfect of baroclinicity (large Γ^2 or small Rossby radii of deformation) reaches a saturation
level where the merging becomes insensitive to larger F2 values. Furthermore, we show that a regime of small
Γ^2 exists at which the merged baroclinic vortex is unstable (metastable) and breaks again into two new vortices,
Thus, in the baroelinic case the range of Γ^2 detemines the stability of the merged vortex.
We analyze these results by local energy and vorticity balances, showing that the horizontal divergence of
pressure work term [∇ *(pv)] and the relative-vorticity advection term (v * ∇ (∇ ^2 φ) trigger the merging during
the first phase. Due to this horizontal redistribution process, a net kinetic to gravitational energy conversion
occurs via buoyancy work in the region external to the cores of the vortices. The second phase of merging is
dominated by a direct baroclinic conversion of available gravitational energy into kinetic energy, which in tum
triggers a horizontal energy redistribution producing the final fusion of the vortex centers. This energy and
vorticity analysis supports the hypothesis that merging is an internal mixing process triggered by a horizontal
redistribution of kinetic energy.
Description:
The work has been financed by a grant from the Progetto Finalizzato "Calcolo Parallelo"
Description:
Published
Description:
1618/1637
Description:
4A. Clima e Oceani
Description:
JCR Journal
Description:
restricted
Keywords:
Ocean modeling
;
Vortex dynamics
;
Baroclinicity
;
Eddies
;
03. Hydrosphere::03.01. General::03.01.01. Analytical and numerical modeling
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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