Abstract
Abstract An analysis of the potential vorticity gradient and the refractive index in quasigeostrophic (QG) flows on the sphere reveals that the absolute vorticity and the stretching parts have two contradictory effects on the horizontal shape of the baroclinic waves when the full variations of the Coriolis parameter are taken into account in each term. The absolute vorticity effect favors the anticyclonic (southwest–northeast) tilt and anticyclonic wave breaking (AWB) and is stronger in the upper troposphere. In contrast, the stretching effect promotes the cyclonic (northwest–southeast) tilt and cyclonic wave breaking (CWB) and is more efficient at lower levels. A positive eddy feedback acting on the latitudinal variations of the zonal winds is deduced. Because the absolute vorticity and the stretching effects are respectively more and less efficient with increasing latitude, a more northward (southward) jet renders AWB more (less) probable and CWB less (more) probable; the jet is pushed or maintained more northward (southward) by the eddy feedback. Idealized numerical experiments using two aquaplanet models on the sphere, a three-level QG model, and a 10-level primitive equation (PE) model, confirm our analysis. Two strategies are employed: first, a normal-mode approach for jets centered at different latitudes; second, an analysis of long-term integrations of the models where the temperature is relaxed toward zonally as well as nonzonally uniform restoration-temperature profiles located at different latitudes. The positive eddy feedback is much less visible in the QG model and CWB is very rare because it does not contain the stretching effect (because of the constant Coriolis parameter in the stretching term).
Highlights
Storm tracks and large-scale flow variabilities are closely linked in midlatitudes and their mutual interaction has been the focus of intense research during the last decades
In the low-latitude case, a weak cyclonic wave breaking (CWB) signature is visible, especially at 500 hPa (Fig. 14g), whereas it does not appear in the high-latitude case, which is more characterized by pure anticyclonic wave breaking (AWB) events (Fig. 14h)
An analysis of the potential vorticity (PV) gradient in quasigeostrophic flows on the sphere has revealed that the absolute vorticity and the stretching terms have two contradictory effects on the horizontal shape of the baroclinic waves
Summary
Storm tracks and large-scale flow variabilities are closely linked in midlatitudes and their mutual interaction has been the focus of intense research during the last decades. Synoptic weather systems that form the so-called storm tracks take their origin in regions of strong thermal contrasts on the western side of oceanic basins via baroclinic interaction (Hoskins and Valdes 1990) As they reach their maximum amplitude on the eastern side, they feed back onto the large-scale circulation and modify the large-scale winds through the divergence of their momentum fluxes (Hoskins et al 1983). All the large-scale flow variabilities are usually related to variations in position and amplitude of the uppertropospheric jets (Vallis and Gerber 2008). It can be explained why baroclinic waves are able to move. The latter aspect will be investigated in the present paper
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