The Asymmetry of Vertical Velocity in Current and Future Climate

Abstract

A fundamental aspect of vertical velocities in the atmosphere is their asymmetric nature, with stronger upward than downward motions. Here we study this asymmetry from a synoptic‐scale perspective, by employing a storm‐tracking algorithm in observations and climate change simulations. We show that midlatitude cyclones and anticyclones are both skewed toward stronger upward motions, and that this asymmetry intensifies in a warmer climate. Downward motions are generally influenced by changes in the dry static stability, which increases in a warmer climate, and therefore weaken. However, upward motions are influenced by a reduced static stability, which takes into account the influence of latent heating on saturated ascent. The latter decreases locally in regions of upward motions, especially for strong cyclones, leading to an intensification of the upward velocity. The projected increase in the upward velocity of strong storms has potential implications for extreme midlatitude precipitation events.

The life cycle of upper-level troughs and ridges: a novel detection method, climatologies and Lagrangian characteristics

“…The anticyclonic orientation increases toward lower latitudes. This meridional dependence of the mean trough orientation is in agreement with the conventional interpretation of cyclonic and anticyclonic wave life cycles and the associated wave breaking, which occurs poleward (cyclonic) or equatorward (anticyclonic) of the mean jet position (Thorncroft et al, 1993). Over northern Africa, the climatological trough orientation is strongly anticyclonic.…”

“…A trough or a ridge tilts cyclonically if it forms under cyclonic shear, while it acquires an anticyclonic orientation if it forms under anticyclonic shear (Davies et al, 1991;Thorncroft et al, 1993). In the case of a pronounced equatorward (poleward) excursion of a trough (ridge), it may wrap up and undergo irreversible deformation in a wave-breaking event (McIntyre and Palmer, 1983;Thorncroft et al, 1993). Under anticyclonic shear, the trough deforms into a narrow band, called a streamer, which crosses the jet and extends equatorward of the mean jet position and cut-off formation may occur.…”

“…During the associated wave-breaking event, the jet is pushed poleward. With cyclonic shear, wave breaking occurs poleward of the mean jet position, thereby pushing the jet equatorward (Thorncroft et al, 1993;Lee and Feldstein, 1996;Orlanski, 2003). The notion of cyclonically and anticyclonically oriented wave development is schematically summarized in Fig.…”

“…The notion of cyclonically and anticyclonically oriented wave development is schematically summarized in Fig. 12 in Thorncroft et al (1993). This process, through which cyclonically or anticyclonically breaking troughs and ridges displace the jet, is even able to excite positive or negative North Atlantic Oscillation events (Benedict et al, 2004;Franzke et al, 2004;Rivière and Orlanski, 2007).…”

The Future of Midlatitude Cyclones

“…Finally, other work (using idealized baroclinic life cycle simulations) has shown that higher moisture content will lead to an increased asymmetry between strong, narrow ascent, and broad, slow descent in the midlatitudes [45]. This signal has also been found in GCMs [46]. Corresponding structural changes to the precipitation distribution have also been identified using cyclone-centric compositing techniques, with the heaviest precipitation concentrated near the center of the ETCs in a warmer climate [41,47].…”