Northern Hemispheric extratropical cyclones during glacial times: impact of orbital forcing and ice sheet height

Extratropical cyclones are a major source of natural hazards in the mid latitudes as wind and precipitation extremes are associated to this weather phenomenon. Still the response of extratropical cyclones and their characteristics to strong external forcing changes is not yet fully understood. In particular, the impact of the orbital forcing as well as variations of the major ice sheets during glacial times on extratropical cyclones have not been investigated so far.  

Thus, the aim of this study is to fill this gap and to assess the impact of orbital forcing and northern hemispheric ice sheet height variations on extratropical cyclones and their characteristics during winter and summer. The main research tool is the Community Earth System Model CESM1.2. We performed a set of time slice sensitivity simulations under preindustrial (PI) conditions and for the following different glacial periods: Last Glacial Maximum (LGM), Marine Isotopic stage 4 (MIS4), MIS6, and MIS8. Additionally, we vary the northern hemispheric ice sheet height for all the different glacial periods by 33%, 66%, 100% and 125% of the ice sheet reconstructed for the LGM. For each of the simulations the extratropical cyclones are identified with a Lagrangian cyclone detection and tracking algorithm, which delivers a set of different cyclone characteristics, such as, cyclone frequency maps, cyclone area, central pressure, cyclone depth, precipitation associated to the extratropical cyclones as well as extremes in cyclone depth and extratropical cyclone-related precipitation. These cyclone characteristics are investigated for the winter and the summer season separately.

Preliminary results show that the extratropical cyclone tracks are shifted southwards on the Northern Hemisphere during the winter season. This shift has rather strong implication for the Mediterranean, with an increase of winter precipitation during glacial times over the western Mediterranean. The increase is modulated when changing the ice sheet height as extratropical cyclone tracks shift further south with increasing northern hemispheric ice sheet height. The orbital forcing shows a higher impact during summer, where mean precipitation is further reduced over Europe when comparing MIS4 and MIS8 with LGM. The changes in the cyclone tracks and related precipitation changes in the Mediterranean for the summer season need to be assessed. Additionally, the effect of the orbital forcing on changes in cyclone tracks and associated precipitation changes in the North Pacific must be evaluated for both seasons.