On the atmospheric background for the occurrence of three heat wave types in East China

Compared with daytime (occurring only in daytime) and nighttime (occurring only in nighttime) heat waves (HWs), daytime-nighttime compound HWs (occurring simultaneously in daytime and nighttime) are highlighted to exert much severer impacts especially on human health. However, the physical mechanisms underlying compound HWs are poorly understood. Based on the observed maximum and minimum temperatures and NCEP/ NCAR reanalysis data, this article addressed the physical processes for the occurrence of compound HWs in East China, where compound HWs occur most frequently across China. Comparisons with those related to daytime or nighttime HWs were also performed. The results indicate that the occurrences of three HW types are all associated with anticyclonic circulation anomalies from the upper troposphere to the lower troposphere, whereas their locations and intensities determine the configuration of atmospheric conditions for different categories of HWs. The resultant less (more) cloud cover and humidity as well as increased downward shortwave (longwave) radiation at the surface favor the warming of daytime (nighttime), conducive to the occurrence of daytime (nighttime) HWs. The combination of above conditions associated with daytime and nighttime HWs, which helps the persistence of high temperatures from daytime to nighttime, benefits the occurrence of compound HWs. In addition, nighttime and compound HWs occur with the northwestward extension of the western Pacific subtropical high (WPSH), while it stays in the climatological location for the occurrence of daytime HWs. Further investigation suggests that daytime (nighttime) HWs are accompanied with an upper-tropospheric meridional (zonal) wave train propagating downstream from western Siberia (the east to the Caspian Sea). In comparison, the wave train related to compound HWs shares the mixed features of daytime and nighttime HWs, characterized by a meridional wave propagation from the Scandinavian Peninsula to East China and then a zonal propagation toward the western Pacific.