Trends of summer dry spells in China during the late twentieth century

In the present study the trends in frequency and duration of dry spells in six sub-regions of China were analyzed for the summer-half-year season (April-September) in period 1956-2000. A dry spell was defined as a number of consecutive days without measurable precipitation. For the frequency of short dry spells (length <10 days), significant changes are observed in the North, Northeast and Southwest China. For the frequency of long dry spells (length >= 10 days) there are significant trends in North and Northeast China; while no remarkable trends in frequency are found in other regions. There are also significant lengthening trends in dry spell duration in North and Northeast China, resulting mainly from the long-term changes in short dry spells. No significant change is observed for the maximum length in all regions. It is found that the temporal distribution of precipitation within the rainy season would notably impact the features of dry spells. An increase in the precipitation amount does not necessarily mean a synchronous reduction in dry spell frequency and/or duration. Seasonal mean anomalies of 500 hPa heights in association with the long dry spells show similar spatial patterns over the middle to high latitudes for five of the six sub-regions (with exception of the case of Southwest China), resembling a west-east direction dipole in latitudes about 30 degrees N northwards. For the case of Southwest China the dominant feature in 500 hPa heights is the negative anomalies over most middle to high latitude Asia. Among these cases there are recognizable differences, particularly, in the tropical regions in western Pacific. That would provide useful information of circulation background for understanding the climate extremes.