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

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.