The Poleward Motion of Extratropical Cyclones from a Potential Vorticity Tendency Analysis

Abstract

The poleward propagation of midlatitude storms is studied using a potential vorticity (PV) tendency analysis of cyclone-tracking composites, in an idealized zonally symmetric moist GCM. A detailed PV budget reveals the important role of the upper-level PV and diabatic heating associated with latent heat release. During the growth stage, the classic picture of baroclinic instability emerges, with an upper-level PV to the west of a low-level PV associated with the cyclone. This configuration not only promotes intensification, but also a poleward tendency that results from the nonlinear advection of the low-level anomaly by the upper-level PV. The separate contributions of the upper- and lower-level PV as well as the surface temperature anomaly are analyzed using a piecewise PV inversion, which shows the importance of the upper-level PV anomaly in advecting the cyclone poleward. The PV analysis also emphasizes the crucial role played by latent heat release in the poleward motion of the cyclone. The latent heat release tends to maximize on the northeastern side of cyclones, where the warm and moist air ascends. A positive PV tendency results at lower levels, propagating the anomaly eastward and poleward. It is also shown here that stronger cyclones have stronger latent heat release and poleward advection, hence, larger poleward propagation. Time development of the cyclone composites shows that the poleward propagation increases during the growth stage of the cyclone, as both processes intensify. However, during the decay stage, the vertical alignment of the upper and lower PV anomalies implies that these processes no longer contribute to a poleward tendency.

Storm-Track Response to SST Fronts in the Northwestern Pacific Region in an AGCM

“…13c). The negative anomaly displays adiabatic cooling associated with the strong updraft induced by latent heat release, and it enhances the baroclinic structure in CNTL, which is consistent with Tamarin and Kaspi (2016). These results reveal that latent heat release associated with large-scale condensation contributes to the development in CNTL, whereas dynamical forcing in the upperlevel troposphere contributes more to the development of the surface cyclone in SMTHK without concentrated precipitation.…”

The Role of Warm Conveyor Belts for the Intensification of Extratropical Cyclones in Northern Hemisphere Winter

“…They explained the eastward motion by the diabatically produced low-level PV anomaly as in the case of DRWs and the northward motion by nonlinear advection by the upper-level positive and diabatically enhanced negative PV anomalies forming up-and downstream, respectively. The importance of the upper-level PV and diabatic heating for the cyclone's poleward propagation was confirmed by Tamarin and Kaspi (2016), who performed a detailed PV tendency analysis for cyclones in an idealized GCM.…”

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

“…Thus, a poleward wind is induced at low levels, advecting the low‐level PV poleward. This was demonstrated in Tamarin and Kaspi [] using PV inversion, which showed explicitly that the poleward advection of the low‐level cyclones is a result of wind induced by the upper levels.…”

“…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].…”

“…From its definition (equation (2)), the PV therefore increases at low levels and decreases at upper levels. The positive diabatically produced PV tendency at low levels contributes to the northeastward propagation of the low-level cyclone [Tamarin and Kaspi, 2016]. The processes associated with latent heating may be especially important for future projections of climate change [Hawcroft et al, 2016a], particularly given the observed biases in precipitation intensity produced by some models [Hawcroft et al, 2016b].…”