Impacts of Decreasing Extratropical Cyclone Activity in Summer on Extreme Heat Events

Increase in greenhouse gases has given rise to increasing temperature and increased frequency of extreme heat events in summer, and such increase is expected to continue into the future. In this study, we demonstrate that part of that increase can be related to changes in extratropical cyclone activity. Changes in cyclone activity will lead to changes in cloud cover, giving rise to changes in maximum temperature and frequency of extreme heat events. These relationships between cyclone activity, cloud cover, and maximum temperature can be clearly identified from reanalysis data, station data, and satellite observations. However, our analyses of climate model simulations show that climate models have difficulties simulating the relationship between cyclone activity and cloud cover, systematically underestimating the impact of cyclone variability on cloud cover and maximum temperature.

With increasing greenhouse gases, enhanced high-latitude warming will lead to weaker cyclone activity. In fact, we show that between 1979 and 2014, the number of strong cyclones in Northern Hemisphere in summer has decreased at a rate of 4% per decade, with even larger decrease found near northeastern North America. Climate models project a decrease in summer cyclone activity, but the observed decreasing rate is near the fastest projected. We have developed a statistical model relating cyclone activity to cloud cover and maximum temperature. Based on this model, the projected decrease in summer cyclone activity can give rise to 0.5 K or more increase in maximum temperature over Northern Hemisphere continental regions including Europe, North America, and the northern part of Asia towards the end of the century. Since climate models display biases in underestimating the impacts of cyclone changes on cloud cover and temperature changes, we hypothesize that these impacts may be underestimated in current climate model projections.