Changes in hot days and heat waves in China during 1961-2007

Based on the daily maximum temperature (DMT) records at 512 stations during 1961-2007, the geographical patterns and temporal variations of hot days (HDs) and heat waves (HWs, including those persisting for 3-5 days and longer) over mainland China were studied. The HD (and hence HW) was defined in two ways, one by an absolute criterion, DMT >35 degrees C, as applied in the nationwide meteorological agencies and another in a relative sense, DMT > the 90th percentile threshold of a local daily temperature distribution around the day. Two centers of high frequencies (over 5 days per year) of the absolute HDs during June September were found in the regions of Xinjiang and the mid-lower reaches of the Yangtze River. The highest frequencies of the absolute HWs were about 1.5 times per year in the Xinjiang region and to the south of the mid-lower reaches of the Yangtze River. The frequencies of the relative HWs were about 1-1.5 times per year in most of China. The HDs and HWs increased significantly during the studied period in most of China, especially over the southeastern coast and northern China (by over 4 days per decade for relative HDs and 0.4 times per decade for relative HWs), but decreased significantly at some stations in the lower reaches of the Yellow River. Over most of China except northwestern China, the frequency of HDs was high during the 1960s-1970s, low in the 1980s, and high afterwards, with strong interannual variations. A remarkable increasing trend of HDs occurred after the 1990s in all regions. The changes in HDs and HWs were closely related to those in rain days and atmospheric circulation patterns at the interannual and interdecadal scales. Copyright (C) 2009 Royal Meteorological Society