Speeding extreme cold events under global warming

Regional anthropogenic warming caused stronger and shorter cold events in the winter (December–February) of 2020–21, with the strongest cooling of −10 °C covering an area of 1.63 × 107 km2 over East Asia. In contrast to previous cold events, the extreme cold events in 2020–21 were a result of meridional circulation change due to stronger regional anthropogenic warming. Our results show a multi-aspect anthropogenic effect in the process of cold events, and illustrate that anthropogenic effect played a role not only in the thermodynamic process but also in the dynamic process. The exchange of equilibrium from low to high index does not take fewer cold events anymore; new principles on equilibrium have appeared and will soon play an effect in more fields of climate change.


Introduction
Previous studies have shown that global warming increases the background mean temperature and seems to decrease the frequency of cold events (e.g. Christiansen et al 2018). However, the winter of 2020-21 was a year with high-frequency cold events, which raises the question of the effect of anthropogenic warming on changes in cold events. The circulation configuration of 2020-21 was closely related to the persistent and extensive anticyclone centered around the Ural Mountains, which was induced by the negative phase of the Arctic Oscillation, and by La Niña condition over the Pacific Ocean (WMO 2020). During late December of 2020, heavy snow occurred in North China, with the maximum decrease in temperature exceeding 8 • C. Nine counties in Jilin and Liaoning experienced their maximum decrease in temperature on record . During early January of 2021, cold-wave processes affected East Asia. The maximum decrease in temperature was more than 12 • C in Eastern China. Low-temperature freezing and snow disasters influenced agriculture and animal husbandry, caused road icing and was harmful for transportation and power supply (Li et al 2022).
Previous studies have shown that the occurrence of cold events is closely associated with the equilibrium of a low-index flow, and fewer cold events in the equilibrium of a high-index flow (Charney andDeVore 1979, Huang et al 2017). The change in equilibrium is closely associated with regional warming, such as the obvious zonal temperature difference between ocean and land, and the meridional thermal difference between mid and high latitudes of the Northern Hemisphere (Li et al 2020b). The new multiple equilibrium theory (Li et al 2020a) proposed that the equilibrium of a low index with stronger meridional circulation leads to a probability of cold events. Cold events in 2020-21 are examples of a low index of equilibrium with a stronger meridional thermal index. Therefore, it is worth conducting further investigation of these cold events. In this study, we analyzed the characteristics of cold events in the winter of 2020-21 and qualified the roles of anthropogenic effects in these cold events.

Data and methods
The study area covers (100 • -140 • E, 20 • -60 • N). The daily mean surface air temperature (SAT) and geopotential height fields at 1000 and 500 hPa were obtained from the NCEP I Reanalysis (Kalnay et al 1996). Multi-model ensemble simulations for the period 1951-2100 were obtained from the Coupled Model Intercomparison Project 6 (CMIP6) archive.    To explore the future change of cold events, 26 CMIP6 climate models were considered in our study (table 1) The dynamic adjustment method was proposed by Wallace et al (2012) to separate dynamic and adjusted SATs. Dynamic SAT is associated with atmospheric circulations, and is called dynamically induced SAT. After removing the dynamically induced SAT variability, the residual is associated with radiative forcing factors, such as the buildup of greenhouse gases (GHGs), stratospheric ozone depletion, volcanic eruptions, and aerosol emissions, with a dominant role played by anthropogenic GHGs (Zhang et al 2017). This component is called adjusted SAT or radiatively forced SAT variability.

Results
The cold events in 2020-21 caused high-frequency and stronger cooling ( figure 1(a)). Cold events were reported on 12-14, 27, and 29-30 December in 2020, and on 1-5 and 14-16 January in 2021 (China Meteorological Administration 2020, 2021), and reached the historic lowest record during the last 67 years in some areas (Xu and Singh 2021). As seen from the associated circulation pattern, the geopotential height at 1000 hPa exhibits a larger change around the Ural Mountains ( figure 1(b)), and East Asia was dominated by a sustained anticyclone centered over the Ural Mountains at 500 hPa during this period ( figure 1(c)). Meanwhile, there were unusual lowpressure centers in both upstream regions (North Africa and western Europe) and downstream regions (East Asia).
Influenced by this circulation pattern, Northeast China was behind the deepened East Asian trough and ahead of the enhanced Siberian cold high; therefore, a descending movement and upper-level northwesterly prevailed, which were favorable for cold events (Zheng et al 2021). The resulting northerly cold air provided favorable condition for severe cold winters in East Asia. When the polar vortex weakened or even split, it allowed frigid air to escape and push southward toward East Asia and cooled East Asia, the cold events that occurred in the winter of 2020-21. The areas affected by the cold events showed clear zonal differences (figure 1(d)). The most widespread area was located near 50 • N with the largest area of 5.71 × 10 7 km 2 , where the decrease was even over 10 • C, followed by the area near 40 • N; however, the largest cooling area was concentrated at −3 • C ( figure 1(d), table 2).
According to reports on the cold events in 2020-21, the cold event frequency in the winter of 2020-21 was five times (China Meteorological Administration 2020, 2021). Traditionally, cold events in China are defined as temperatures lower than the 10th percentile of the average temperature, persisting for 5 consecutive days (Zhang and Qian 2011). According to this definition, the frequency of the latest cold events was two times, which is not consistent with the reports of the CMA (figure 2(a)). However, cold events with the definition of 5% and 3 d occurred five times, which is consistent with the reports of the CMA.
As is shown in figures 3(a)-(c), both raw and thermodynamic temperature anomalies were mainly in the negative phase at 20 • -60 • N before 1980, and in the positive phase after 1980. The raw and thermodynamic temperatures increased sharply after 1980, especially north of 40 • N. From the time series of temperature in figure 3(d), the temperatures were maintained at a low level before 1980. The obvious increasing of raw temperature since the 1990s was mainly induced by the enhanced warming of thermodynamic temperature. The thermodynamic temperature was much higher than the dynamic temperature in 2020-21 (The light red background area in figure 3(d)). When the thermodynamic temperature was in an upward (warming) phase in 2020-21, it balanced or reduced the dynamic cooling and resulted in a relatively high value of raw temperature. Therefore, the 2020-21 was a good period for studying the effect of regional warming on cold events.
The occurrence of cold events in the winter of 2020-21 decreased temperatures over the mid-tohigh latitudes of the Northern Hemisphere, reduced the temperature difference between the high and mid latitudes, and led to a weak meridional circulation ( figure 2(a)). Yao et al (2022) suggested that the weak meridional circulation favors the decrease of westerly wind and results in more cold air from the Arctic. Meanwhile, the temperature adjusted by anthropogenic factors (figure 3) was at a high level in 2020-21, which quickly offset the cooling from the Arctic (Guan et al 2015), the meridional circulation turn positive (histogram in figure 2(a)), terminating the cold events, and shortening the duration of each cold event. Such a mechanism in the equilibrium of a low index resulted in stronger and shorter cold events in the past winter.

Conclusions
During the winter of 2020-21, cold events swept over Northeast Asia, which were characterized by the lowest historical temperatures. The enhanced Arctic warming contributed to the stronger explosion of cold air, and the equilibrium state of the low index by thermodynamic temperature decreased the duration of each cold event and led to stronger and shorter cold events. From 2000 to 2010, the natural oscillation was stronger than the anthropogenic meridional thermal temperature, and showed a high index of equilibrium, resulting in typical high-frequency cold events (Huang et al 2017). In the winter of 2020-21, although the natural oscillation was abnormally strong, the thermodynamic meridional thermal index was also powerful and dominated equilibrium in the low index. Therefore, the cold events in 2020-21 were typical of stronger and shorter characteristics due to anthropogenic warming.
The anthropogenic effect is also illustrated by the CMIP6 models' outputs. The positive contribution by GHGs is illustrated in frequency of different cold events in SSP 1-2.6 (figures 4(a), (c), (e) and (g)) and SSP 5-8.5 (figures 4(b), (d), (f) and (h)) from 2021 to 2070 by CMCC-CM2-SR5 model (it has the largest correlation coefficient of 0.32, passing the 95% confidence level with the reanalysis dataset on the extreme cold events of 5th percentile of the average temperature and persisting 3 consecutive days; figure 5(a)). Figure 4 illustrates that the high emission scenario has a higher-level frequency of extreme cold events, even the temperatures lower than the 5th percentile of the average temperature and persisting 5 consecutive days (figure 4(h)); it is also a result of enhanced decreasing of meridional temperature (figure 5(b)) difference in SSP 5-8.5 and contributes to more extreme cold events.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).