Mechanism and predictability of consecutive extreme heat waves and heavy rainfall events in North China in 2023

Global warming has triggered increases in heat waves and heavy rainfall events, and their combined occurrences can lead to more severe impacts. Therefore, it is imperative to study the superposition of the two types of extreme event. Using temperature and precipitation data collected at meteorological stations and ERA5 and NCEP/NCAR reanalysis data as well as data released by the National Climate Centre, synoptic evolution characteristics and causes of Consecutive Heat Waves and Heavy Rainfall (CHWHR) events in North China in 2023 are analyzed. Based on multiple similar events in history, key atmospheric circulation characteristics for the occurrence of CHWHR events are discussed from a predictability perspective. We have found that this event consisted of two heatwaves and two heavy rainfall events, and lasted for 41 d. It is characterized by two heatwaves, followed by a rapid heat-humidity transition and a period of heavy rainfall with two heavy rainfall events occurring within 20 d. The duration of the two heat waves was long, and temperature exceeded the threshold in 13 out of 20 d. The heat waves are caused by the strong continental high pressure. The northward shift of the Western Pacific subtropical high and the activities of the short-wave trough in the westerly belt during the late stage of the heat wave are important circulation characteristics for the conversion from heat wave to heavy rainfall. The heat wave event triggered drought in North China, while the heavy rainfall event caused severe floods, resulting in direct economic losses of nearly 100 billion Chinese yuan. To identify the early signals of such disaster events, we further compare historical cases with and without heavy rainfall events within 7 d after heat waves in North China from 1991 to 2022. It is found that the reason for the heavy rainfall in the later stage of the CHWHR event is attributed to the presence and maintenance of a northward Western Pacific subtropical high during the heat wave with the summer monsoon advancing to North China and the mid-latitude westerly trough moving eastward in the later stage of the heat wave. In addition, whether the average Convective Available Potential Energy (CAPE) value in North China significantly decreases after a heat wave is an effective forecasting indicator. Results of the present study have important implications for the prevention and mitigation of disasters caused by extreme weather events.