On the Poleward Motion of Midlatitude Cyclones in a Baroclinic Meandering Jet

Abstract

International audienceThe motion of surface depressions evolving in a background meandering baroclinic jet is investigated using a two-layer quasigeostrophic model on a beta plane. Synoptic-scale finite-amplitude cyclones are initialized in the lower and upper layer to the south of the jet in a configuration favorable to their baroclinic interaction. The lower-layer cyclone is shown to move across the jet axis from its warm-air to cold-air side. It is the presence of a poleward-oriented barotropic potential vorticity (PV) gradient that makes possible the cross-jet motion through the beta-drift mechanism generalized to a baroclinic atmospheric context. The potential vorticity gradient associated with the jet is responsible for the dispersion of Rossby waves by the cyclones and the development of an anticyclonic anomaly in the upper layer. This anticyclone forms a PV dipole with the upper-layer cyclone that nonlinearly advects the lower-layer cyclone across the jet. In addition, the background deformation is shown to modulate the cross-jet advection. Cyclones evolving in a deformation-dominated environment (south of troughs) are strongly stretched while those evolving in a rotation-dominated environment (south of ridges) remain quasi isotropic. It is shown that the more stretched cyclones trigger a more efficient dispersion of energy, create a stronger upper-layer anticyclone, and move perpendicularly to the jet faster than the less stretched ones. Both the intensity and location of the upper-layer anticyclone explain the distinct cross-jet speeds. A statistical study consisting in initializing cyclones at different locations south of the jet core confirms that the cross-jet motion is faster for the more meridionally elongated cyclones evolving in areas of strongest barotropic PV gradient

Corrigendum

“…The result above implies the opposite. The above is in fact consistent with the recent baroclinic studies of Riviére et al (2012) and Oruba et al (2013), who found that a stronger barotropic PV gradient was linked to more rapid poleward translation. As we noted in the paper, our numerical results were inconclusive with regards to changes in the PV gradient.…”

The poleward shift of storm tracks under global warming: A Lagrangian perspective

“…A decomposition of this term to its different components (not shown) shows that it is the zonal flow that is responsible for the eastward advection of the perturbation PV, while the nonlinear advection terms are those responsible for the poleward tendency [Tamarin and Kaspi, 2016]. This poleward advection is associated with lower level winds induced by PV at upper levels [Gilet et al, 2009;Oruba et al, 2013;Coronel et al, 2015;Tamarin and Kaspi, 2016]. In the PV perspective, cyclogenesis is described as the mutual amplification of a positive low-level PV anomaly, associated with the cyclone, and a positive upper level PV anomaly to its west [Bretherton, 1966;Hoskins et al, 1985].…”

Meridional and Zonal Wavenumber Dependence in Potential Vorticity Flux in Rossby Waves

“…The case c = 0 consists of when the jet disappears; there are then square cells of alternating vorticity (the Taylor-Green flow [52]). Here, the generic situation of when −1 < c < 0 is the focus; Figure 1 has been produced with c = −0.1868 (associated with realistic parameters for a typical atmospheric midlatitude cyclone as reported by Oruba et al [9], to be discussed in detail later). The dimensional wavenumbers (k 0 , l 0 ) are associated with the zonal and meridional reciprocal lengthscales of the eddy.…”

“…The parameters A and β do not independently affect the flux. For numerical work, parameters associated with a midlatitude cyclone evolving in a background meandering atmospheric jet as studied by Oruba et al [9] will be used. Their Table 2 and Figure 5 indicate the values [9] f 0 = 10…”