The changes of high-temperature extremes and their links with atmospheric circulation over the Northern Hemisphere

The present study investigated the changes of high-temperature extremes and their links with atmospheric circulation over the Northern Hemisphere during 1979-2012 based on daily records of maximum temperature and geopotential height fields. We mainly used the 90th percentile of daily maximum temperatures as a threshold to identify hot day. The number of hot days (NHD) shows significant increasing trends over the East Asia (EA), Mediterranean (TM), and United States of America (USA) from 1979 to 2012, suggesting that these regions may suffer from increasing high-temperature extremes The regional mean linear trend over EA is 0.18 days/year and showed an increase in 1996, with the maximum trends in Mongolian Plateau and Loess Plateau. In the TM region, NHD increased with a rate of 0.35 days/year and showed an increase in 1997, and most significant trend was found in the Arabian Peninsula. In the USA, the NHD had a significant inter-annual variability and increased with a rate of 0.1 days/year. Moreover, high-temperature extremes over most parts of the three regions are associated with barotropical anticyclonic anomalies and subsidence, which may enhance solar radiation to surface.