Large-scale circulation anomalies associated with 'tropical night' weather in Beijing, China

On the basis of the homogenized daily temperature data of China and daily mean National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data during July and August 1979-2008, we analysed the large-scale circulation anomalies associated with tropical night (TN; minimum temperature >= 25 degrees C) in Beijing, and compared these anomalies with those associated with extreme heat (EH; maximum temperature >= 35 degrees C). Composite analyses revealed that, for TN days, there is an upper-tropospheric anticyclonic anomaly to the northwest of Beijing and a lower-tropospheric anticyclonic anomaly to the southeast of Beijing. Associated with the lower-tropospheric anticyclonic anomaly, a significant southerly anomaly occurs to the south of Beijing and leads to stronger northward water vapour flux, overlapping the climatological southerlies. Correspondingly, there is a belt of higher humidity over North China, Northeast Asia and northern Japan, which reduces longwave radiation cooling around Beijing and favours the occurrence of TN. The EH-related anomalies, however, are distinct from the TN-related ones. There is a significant cyclonic anomaly located to the northeast of North China in both the upper and lower troposphere. Associated with this cyclonic anomaly, an obvious northerly anomaly occurs over Beijing and reduces the northward water vapour flux. Hence, there is significant lower humidity over Beijing, which is also due to the significant descending anomaly over Beijing, favouring the occurrence of EH through enhancing solar radiation at the surface.