Warm conveyor belts in present-day and future climate simulations – Climatology, impacts and role for cyclone intensification 

This study investigates how warm conveyor belts (WCBs) and their associated extratropical cyclones will change in a future climate. WCBs are strongly ascending airstreams in extratropical cyclones that are responsible for most of their precipitation. In conjunction with the cloud formation, latent heat is released, which has an impact on the potential vorticity (PV) distribution and therefore on the intensification of the associated cyclone and the upper tropospheric wave guide. Because of these and other impacts of WCBs, it is of great importance to investigate changes in their frequencies, regions of occurrence, physical characteristics and impacts on the associated cyclones in a warmer climate. To this aim, future climate simulations (Representative Concentration Pathway 8.5 – RCP8.5 – scenario;2091–2100) are performed with the Community Earth System Model version 1 (CESM1) and compared to present-day climate (1991–1999). Trajectories are calculated based on 6-hourly 3D wind fields, and WCBs are identified as trajectories that ascend at least 600 hPa in 2 d. 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 there is a decrease to the south of Iceland. In the Southern Hemisphere, WCB frequencies increase in the South Atlantic in both seasons and to the east of South Africa and the Indian Ocean in June–July–August (JJA). These changes are partly 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, which have implications for WCB impacts in a future climate. The increase in inflow moisture in the different regions and seasons leads to (i) an increase in WCB-related precipitation, especially in the upper percentiles and thus a possible increase in extreme precipitation related to WCBs, (ii) a strong increase in diabatic heating in the mid-troposphere, and (iii) a higher outflow level, which favours WCBs more strongly interacting with the upper-level Rossby waveguide. In terms of the associated cyclones, we find that the number of explosive cyclones with strong WCBs, referred to as C1 cyclones, is projected to increase in both hemispheres, while the number of explosive cyclones with weak WCBs (C3 cyclones) is projected to decrease. A composite analysis reveals that in the future climate C1 cyclones will be associated with even stronger WCBs, more WCB-related diabatic PV production, the formation of a more intense PV tower, and an increase in precipitation. They will become warmer, moister, and slightly more intense. The findings indicate that (i) latent heating associated with WCBs (as identified with our method) will increase, (ii) WCB-related PV production will be even more important for explosive cyclone intensification than in the present-day climate, and (iii) the interplay between dry and moist dynamics is crucial to understand how climate change affects cyclone intensification.