Abstract
With global warming, the probability of summer compound hot and dry extreme (CHDE) days, which are higher risk compared with single-factor extreme events, increases in some regions. However, there have been few studies on the winter precursor signals of such events. In this study, we found that summer CHDEs have generally increased in the last 20 years, with the increases in the middle and lower reaches of the Yangtze River region and Southwest China being more than double those in other regions of China. The dominant mode of summer CHDEs in China is characterized by more hot–dry days in the Yangtze–Huaihe River Basin (YHRB). Importantly, we found that there is an obvious cross-seasonal relationship between the first mode of winter snow cover in the Northern Hemisphere (NH) and summer CHDEs in China. When the mode of winter snow cover in the NH is in a positive phase with a negative-phase Arctic Oscillation (AO), i.e., more snow cover in Europe, Northeast China, and the northern United States, and less snow cover in central Asia and the midlatitudes in winter, more CHDEs in China in the following summer. Compared with the signals from the AO, these signals from winter snow can be better stored and transmitted into summer through the snow, soil and ocean, inducing a northward shift of the upper-level westerly jet and strengthening of South Asia high. Through the strong dynamic forcing of negative vorticity advection with the change of westerly jet, the subsidence movement in the western Pacific subtropical high (WPSH) region is strengthened, resulting in the stable maintenance of the WPSH in the YHRB. Under the synergy of a remote mid- and high-latitude wave train in summer, which also relates closely to winter snow cover, more CHDEs ultimately occur in the YHRB of China.
Highlights
Compound hot and dry extremes (CHDEs) in summer are becoming more frequent under enhanced greenhouse gas forcing at the global scale, and this trend will continue with higher global warming (IPCC, 2021)
We further studied contribution of higher temperature and lower humidity to more summer CHDEs in the Yangtze–Huaihe River Basin (YHRB), respectively, in the last 20 years Figures 3E,F show that correlation maps between the PC1 of CHDEs and temperature and humidity fields and Figure 3G is contribution ratios of temperature and humidity to the CHDEs in the YHRB
Some studies have shown that the enhancement of surface heating in the mid–high latitudes associated with the Arctic amplification can significantly affect the midlatitude summer circulation and promote anomalous high pressure in the troposphere through weakening of midlatitude storm tracks, northward movement of jets, and amplification of synoptic-scale quasi-stationary waves (Coumou et al, 2018; Ting et al, 2018)
Summary
According to the sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2021), the global average temperature in the past 10 years was about 1.1°C higher than that in 1850–1900, and this will result in a general increase in heat waves and an extension of the warm season. It has been reported that drought will be more severe in some areas in the future, such as East Asia, Western North America, Western and Central Europe and so on (Leonard et al, 2014; Chen et al, 2018; Sippel et al, 2018; Ribeiro et al, 2020). This means that the probability of multivariate compound extreme events is likely to increase. The increasingly frequent occurrence of CHDEs brings great pressure to transportation, urban water supply, and power supply, and affects people’s daily lives and the safety of property. It is of scientific significance to study the changes in CHDEs and their early signals, but it helps to provide a reference for meteorological departments to improve the accuracy of predictions
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