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Article Dans Une Revue Journal of the Atmospheric Sciences Année : 2009

Nonlinear Baroclinic Dynamics of Surface Cyclones Crossing a Zonal Jet

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Mechanisms leading a synoptic surface cyclone to cross an upper-level zonal jet and its subsequent deepening are investigated using a two-layer model on a β plane. The baroclinic interaction of a low-level circular cyclonic perturbation with an upper-level one is first studied in vertical and horizontal cyclonic or anticyclonic uniform shears. A first nonlinear effect acting on the shape and energetics of the perturbations is analyzed. If the background shear is anticyclonic, the perturbations are stretched horizontally; they lose energy barotropically but gain it baroclinically by a well-maintained westward tilt with height. Conversely, if the shear is cyclonic, perturbations remain quite isotropic, but they do not keep a favorable vertical tilt with time and the baroclinic interaction is thus only transient. The latitudinal motion of the perturbations also results from a nonlinear effect. It is found to depend strongly on the background potential vorticity (PV) gradient. This effect is a baroclinic equivalent of the so-called nonlinear barotropic "β drift" and combines the nonlinear advection and vertical stretching terms.These results are confirmed when the anomalies are initially located south of a confined westerly jet. The poleward shift of the lower cyclonic anomaly occurs faster when the vertically averaged PV gradient is strongly positive, which happens when the jet has a large barotropic component. The lower anomaly crosses the jet from the warm to the cold side and deepens afterward. After a detailed description of this regeneration process with the help of an energy budget, it is shown that linear dynamics are not able to reproduce such behavior.
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hal-00961374 , version 1 (03-09-2021)

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Jean-Baptiste Gilet, Matthieu Plu, Gwendal Rivière. Nonlinear Baroclinic Dynamics of Surface Cyclones Crossing a Zonal Jet. Journal of the Atmospheric Sciences, 2009, 66 (10), pp.3021--3041. ⟨10.1175/2009JAS3086.1⟩. ⟨hal-00961374⟩
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