Warm conveyor belts in present-day and future climate simulations. Part I: Climatology and impacts

Abstract

Abstract. This study investigates how warm conveyor belts (WCB) will change in a future climate. WCBs are strongly ascending airstreams in extratropical cyclones which are responsible for most of their precipitation. In conjunction with the strong cloud formation, latent heat is released which has an impact on the potential vorticity distribution and therefore on the atmospheric circulation in the mid- and upper-troposphere. Because of these and other impacts of WCBs, it is of great importance to investigate their changes in a warmer climate. To this aim, future climate simulations (RCP8.5 scenario; 2091–2100) are performed with the Community Earth System Model version 1 (CESM1) and compared to a present-day climate (1991–2000). WCB trajectories are calculated based on the six-hourly 3D wind fields. WCBs are represented reasonably well in terms of location and occurrence frequency compared to WCBs in the ERA-Interim data set. In a future climate, WCB inflow regions in the North Pacific are systematically shifted northward in winter, which is in agreement with the northward shift of the storm track in this region. In the North Atlantic, increased frequencies are discernible in the southwest and a decrease to the south of Iceland. Finally, in the Southern Hemisphere, WCB frequencies increase in the South Atlantic, whereas they decrease near Madagascar. Part of these changes are consistent with corresponding changes in the occurrence frequencies of extratropical cyclones, i.e., the driving weather systems of WCBs. Changes are also found in the WCB characteristics, e.g., in specific humidity of the WCB inflow, the WCB-related precipitation, the cross-isentropic ascent and the isentropic level reached by the WCB outflow. This has implications for WCB impacts in a future climate. For instance, the strong increase in inflow moisture leads to: (i) a strong increase in WCB-related precipitation, especially in the upper percentiles, thus extreme precipitation related to WCBs might increase; (ii) a strong increase in diabatic heating in the mid-troposphere; and (iii) a higher outflow level which favours WCBs to more strongly interact with the upper-level Rossby waveguide. In summary, by investigating a distinct weather system, the WCB, and how it changes in its occurrence frequency and characteristics in a future climate, this study provides new insights into the dynamics and impacts of climate change in the mid-latitudes.

Warm conveyor belts in present-day and future climate simulations – Part 2: Role of potential vorticity production for cyclone intensification

“…The identification of WCB trajectories is the same in ERA-Interim and the CESM simulations and is described in detail in Madonna et al (2014) and Joos et al (2022), respectively. Using the Lagrangian analysis tool (LAGRANTO; Wernli and Davies, 1997;Sprenger and Wernli, 2015), 48 h forward trajectories are started every 6 h from a horizontally equidis-tant grid with x = 80 km resolution and from vertically equidistant ( p = 20 hPa) levels in the lower troposphere (1050-790 hPa).…”

“…Using the Lagrangian analysis tool (LAGRANTO; Wernli and Davies, 1997;Sprenger and Wernli, 2015), 48 h forward trajectories are started every 6 h from a horizontally equidis-tant grid with x = 80 km resolution and from vertically equidistant ( p = 20 hPa) levels in the lower troposphere (1050-790 hPa). To be selected as WCB air parcels, the trajectories must undergo an ascent of at least 600 hPa within 48 h. In addition, their horizontal position must coincide with the surface field of an extratropical cyclone at least once during the 48 h. Note that we decided to use the same threshold for ascent to identify WCB trajectories in both climate periods, despite the fact that the extratropical tropopause rises slightly in the warmer climate (Joos et al, 2022).…”

“…To have confidence in CESM-LE's projections of extratropical cyclones and their associated WCBs in a warming climate, the model needs to be able to correctly represent them in the current climate. Joos et al (2022) compared the WCB climatology in CESM-HIST with ERA-Interim. They showed that in most regions CESM-HIST is able to realistically simulate WCBs in terms of their geographical distribution, frequency of occurrence, and ascent behaviour.…”

“…As the strongest diabatic heating in extratropical cyclones occurs in WCBs, the question arises of how WCB-related diabatic PV production changes with global warming. In an accompanying study by Joos et al (2022), a climatology of WCB trajectories has been calculated for the first time in climate model simulations with the Community Earth System Model Large Ensemble (CESM-LE; Kay et al, 2015), and their geographical distribution, seasonal frequency, and characteristics have been investigated in the present-day and end-of-century climate. It was shown that the present-day simulations are able to realistically capture the geographical distribution and frequency of occurrence of the WCBs in many regions.…”

“…The projected overall increase in the number of WCB trajectories and in the diabatic heating rate along the trajectories in a warmer climate potentially influences the development of the associated cyclones. In this study, we use the same WCB climatology as in Joos et al (2022) and combine it with a cyclone climatology to evaluate how WCBs and their associated diabatic PV production affect cyclone intensification in present-day and future CESM-LE simulations during NH and SH winter. In addition to a climatological analysis of all cyclones, we investigate potential future changes in the characteristics of a subset of cyclones with particularly strong WCBs and explosive deepening, referred to as C1 cyclones (see Binder et al, 2016).…”