Sensitivity of idealised baroclinic waves to mean atmospheric temperature and meridional temperature gradient changes

Abstract

The sensitivity of idealised baroclinic waves to different atmospheric temperature changes is studied. The temperature changes are based on those which are expected to occur in the Northern Hemisphere with climate change: 1) uniform temperature increase, 2) decrease of the lower level meridional temperature gradient, and 3) increase of the upper level temperature gradient. Three sets of experiments are performed, first without atmospheric moisture, thus seeking to identify the underlying adiabatic mechanisms which drive the response of extra-tropical storms to changes in the environmental temperature. Then, similar experiments are performed in a more realistic, moist environment, using fixed initial relative humidity distribution. Warming the atmosphere uniformly tends to decrease the kinetic energy of the cyclone, which is linked both to a weaker capability of the storm to exploit the available potential energy of the zonal mean flow, and less efficient production of eddy kinetic energy in the wave. Unsurprisingly, the decrease of the lower level temperature gradient weakens the resulting cyclone regardless of the presence of moisture. The increase of the temperature gradient in the upper troposphere has a more complicated influence on the storm dynamics: in the dry atmosphere the maximum eddy kinetic energy decreases, whereas in the moist case it increases. Our analysis suggests that the slightly unexpected decrease of eddy kinetic energy in the dry case with an increased upper tropospheric temperature gradient originates from the weakening of the meridional heat flux by the eddy. However, in the more realistic moist case, the diabatic heating enhances the interaction between upper-and low-level potential vorticity

Potential Vorticity Diagnostics to Quantify Effects of Latent Heating in Extratropical Cyclones. Part II: Application to Idealized Climate Change Simulations

“…They argued that the warm-frontal LH maximum in these warmer simulations shifts more eastward and thus degrades the phasing between drybaroclinically and diabatically induced vertical motion, which even dampens cyclone intensification. Rantanen et al (2019) found a similar reduction of eddy kinetic energy (EKE) in warmer compared to cooler moist idealized simulations as Kirshbaum et al (2018), but, in contrast to Kirshbaum et al (2018), obtained a lower sea level pressure (SLP) minimum. In addition, Rantanen et al (2019) showed that enhanced LH due to higher environmental temperatures does not increase the intensity of an idealized cyclone when lower-tropospheric baroclinicity is simultaneously reduced, but amplifies cyclone intensification when upper-tropospheric baroclinicity is increased.…”

The Future of Midlatitude Cyclones

“…These analyses revealed that the ETC response to increasing moisture changes depending on the initial temperature of the background state. Furthermore, the EKE does not increase at higher temperatures due to shifts in the location of the upper level ridge and the diabatic heating, which reduces the positive interaction between upper and lower levels [42,43]. Increasing upperlevel baroclinicity can increase the EKE, which is also related to the role of moisture in the interaction between upper-level and near-surface instabilities [42].…”

“…For example, baroclinic life cycle experiments have been undertaken in which the atmospheric moisture content is increased (either directly or by increasing temperature and keeping relative humidity constant). Such experiments indicate that changes in ETC intensity depend on the background state and the complex interactions between LH and dry baroclinic processes [40][41][42].…”

“…Furthermore, the EKE does not increase at higher temperatures due to shifts in the location of the upper level ridge and the diabatic heating, which reduces the positive interaction between upper and lower levels [42,43]. Increasing upperlevel baroclinicity can increase the EKE, which is also related to the role of moisture in the interaction between upper-level and near-surface instabilities [42]. In an idealized GCM simulation, it was found that ETC strength is more sensitive to changes in upper-tropospheric baroclinicity than to changes in the lower troposphere [44].…”

The characteristics and structure of extra-tropical cyclones in a warmer climate

“…However, when temperatures and moisture content are increased to values higher than in the current climate, baroclinic life cycle experiments show divergent results. For example, Rantanen et al (2019) found that uniform warming acts to decrease both the eddy kinetic energy and the minimum surface pressure of the cyclone whereas Kirshbaum et al (2018) showed that for large temperature increases with constant relative humidity the eddy kinetic energy decreases whereas the minimum surface pressure increases. Furthermore, Tierney et al (2018) documented non-monotonic behavior of the cyclone intensity in terms of both maximum eddy kinetic energy and minimum mean surface pressure with increasing temperature.…”