Characteristics and circulation patterns for wet and dry compound day-night heat waves in mid-eastern China

Compound day-night heat waves (CohotEs) have more adverse effects on human health and ecosystem, and the consequences vary with humidity. This study classifies the CohotEs over Chinese mainland into wet and dry CohotEs, and compares their characteristics and associated circulation anomalies. Here, a populated region of mid-eastern China (MEC) with higher occurrence of both types of CohotEs is mainly focused. Results show that, during past six decades, wet CohotEs have higher frequency, longer duration and broader spatial scale, while dry CohotEs are less, shorter and more regional. Both types of CohotEs are associated with anticyclonic anomalies accompanied by the strengthening and northwestern expansion of western Pacific subtropical high (WPSH), leading to adiabatic heating with anomalous subsidence. The anomalies in wet CohotEs are influenced by the eastward expansion of East Asian westerly jet stream and northeastward expansion of the south Asia high. Anomalous southwesterlies transport additional water vapor from the tropics, leading to above-normal humidity that could reduce the amplitudes of anomalous subsidence but favor heat maintenance. By contrast, dry CohotEs are related to a more stable barotropic anticyclonic structure, and the anomalous signals are stronger in the middle-lower troposphere. Meanwhile, tropical cyclones appear over the South China Sea below 500 hPa, promoting the intensification of northwestward shifts of WPSH and anomalous northeasterlies over South China. So, lower humidity results from weakened northward water vapor transport could lead to faster heat dissipation and thus shorter duration for dry CohotEs. Our results emphasize the diversity of CohotEs, indicating that different types of CohotEs should be analyzed separately and multiple local and large-scale circulations need to be considered for better forecasts and predictions of CohotEs.