Summer Season Lengthening and Extreme Heat Wave Expansions over the Northern Hemisphere Assessed Using Multi-AGCM Large-ensemble Simulations

A significant increase in summer season length was observed over Northern Hemisphere (NH) for recent 60 years, which has been attributed to anthropogenic greenhouse gas increases (Park et al. 2018). This study evaluates future changes in summer season length and its impacts on extreme heat waves using multi-AGCM large-ensemble simulations available from the Half a degree Additional warming, Prognosis and Projected Impacts (HAPPI) project. Each of five AGCMs provides more than 100 runs of 10-year length for three experiments: All-Hist (current decade: 2006-2015), Plus15 (1.5 degree warmer), and Plus20 (2.0 degree warmer than pre-industrial condition). Results indicate that summer season lengthening in NH will continue at 1.5 and 2.0 degree warmer conditions. Regional analysis reveals that the summer season lengthening will be generally larger in mid latitudes than high latitudes with the largest change in East Asia and the smallest change in North Asia. We further analyze spatio-temporal changing patterns of the frequency of extreme hot days (e.g., one-in-10-year event) associated with the summer season lengthening. Results show robust increases of the areas experiencing extreme hot days, centered around summer onset and withdrawal periods. The extreme heat wave expansion occurs more strongly in lower latitudes, and largest expansion is observed over East Asia and Mediterranean region, where summer season expansion is also largest. Our results demonstrate that global warming mitigation by a half degree would significantly reduce summer season lengthening and associated expansion of extreme heat waves.