Observed and Projected Decrease in Extratropical Cyclone Activity in Northern Hemisphere Summer and Its Impacts on Extreme Weather

Extratropical cyclones give rise to much of the high impact weather over the mid-latitudes, including heavy precipitation, strong winds, and coastal storm surges. These storms develop due to the equator-to-pole temperature difference, and are thus stronger in winter than during summer. Hence it is not surprising that much effort has been put in examining the characteristic, variability, and projected change in winter extratropical cyclone activity. Nevertheless, much of the heavy and extreme precipitation events in summer over the mid-latitudes, including those over most parts of the United States, have been shown to be associated with the passage of extratropical cyclones and their frontal systems. There are also indications that the severity of heat waves may also be linked to extratropical cyclone activity. Thus it is important to also examine how summer extratropical cyclone activity has changed, how it is projected to change in the future, as well as how such variability and change may impact the frequency of occurrence of heavy precipitation events and heat waves.

Here a significant decrease in summer extratropical cyclone activity over the past three decades that has occurred over much of the Northern Hemisphere is reported. The largest decrease in cyclone activity is found near North America. This can be related to rapid warming in the high latitudes reducing the pole-to-equator temperature gradient. Climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) ensemble project such a decrease under increasing greenhouse gas forcing, but the observed decrease is near the extreme end of model projections. Using observed surface air temperature and precipitation data, it is shown that storm track decrease gives rise to significant impacts on the occurrences of heat waves and heavy precipitation events. Physical mechanisms behind the storm track impacts on extreme weather have been examined and will be discussed in the presentation.