Obvious difference of dominant circulation patterns between dry-type and humid-type heatwaves in North China

Based on the observed maximum temperature (T_max), relative humidity (RH) and NCEP/NCAR reanalysis data during 1981–2021, basic temporal features and dominant atmospheric circulation patterns of dry-type and humid-type heatwaves in North China are investigated and compared. Statistical results indicate the dry heatwaves occur mainly in early summer (from early June to early July), that is, before the rainy season of North China, while the humid heatwaves have a high frequency in mid-July to mid-August. During the research period, the increasing trend of dry heatwaves is 0.67 days·decade⁻¹, while the humid heatwaves increase at a greatly higher rate of 1.85 days·decade⁻¹. For the dry heatwave, a high ridge in the subtropical westerlies plays the main role, and the northerly wind in the east of the ridge reduces the air moisture convergence over the region. However, for the humid heatwave, the westward and northward propagations of the western Pacific subtropical high (WPSH) may make the major contribution, and the southerly wind anomalies in the west of the WPSH enlarge the water vapour to the region. The adiabatic heating in subsiding air at all levels and horizontal temperature advection at lower troposphere are stronger for dry heatwaves than for humid heatwaves, which cause a higher T_max for the former type. These results highlight the diversity of the heatwaves in North China, which suggests that multiple local and large-scale subseasonal circulations should be considered to improve the subseasonal to seasonal forecast skills for heat extremes.