Interannual Variation of Tropical Night Frequency in Beijing and Associated Large-Scale Circulation Background

This study examined the variability in frequency of tropical night occurrence (i.e., minimum air temperature >25oC) in Beijing, using a homogenized daily temperature dataset during the period 1960--2008. Our results show that tropical nights occur most frequently in late July and early August, which is consistent with relatively high air humidity associated with the rainy season in Beijing. In addition, year-to-year variation of tropical night occurrence indicates that the tropical nights have appeared much more frequently since 1994, which can be illustrated by the yearly days of tropical nights averaged over two periods: 9.2 days of tropical nights per year during 1994--2008 versus 3.15 days during 1960--1993. These features of tropical night variations suggest a distinction between tropical nights and extreme heat in Beijing. We further investigated the large-scale circulations associated with the year-to-year variation of tropical night occurrence in July and August, when tropical nights appear most frequently and occupy 95% of the annual sum. After comparing the results in the two reanalysis datasets (NCEP/NCAR and ERA-40) and considering the possible effects of decadal change in the frequency of tropical nights that occurred around 1993/94, we conclude that on the interannual time scale, the cyclonic anomaly with a barotropic structure centered over Beijing is responsible for less frequent tropical nights, and the anticyclonic anomaly is responsible for more frequent occurrence of tropical nights over Beijing.