Widespread observed and projected increases in warm extremes, along with decreases in cold extremes, have been confirmed as being consistent with global and regional warming. However, frequent severe winters, including persistent snow cover and cold extremes, occurred over Europe and Asia in recent years, including 2005/2006, 2009/10, 2010/11, and most recently in 2020/21, have received major public attention due to their serious impacts on living creatures and ecosystems, as well as on human society. An open question that is critically important for scientists and policy-makers is whether any such increase in weather extremes is natural or anthropogenic in origin.

Based on the reanalysis datasets and CMIP6 model simulations, this study discloses that the decadal variations in the frequency of the surface air temperature (SAT) extremes over Siberia in winter are primarily dominated by the Atlantic Meridional Overturning Circulation (AMOC), which even masks the global warming signal. The stronger AMOC induced more warm and cold extremes through increasing the variance of winter SAT over Siberia while the direct effect of external forcings, especially anthropogenic greenhouse gases, had little impact on the summation of warm and cold extremes due to equivalent effects on the increases in warm extremes and decreases in cold extremes.

The mechanisms of the AMOC effect on the SAT extremes found by previous can be summarized in thermal and dynamic ways. In terms of the thermal effects, the upper branch of the AMOC can transport warm water and excess ocean heat from tropical regions to the mid- to high latitudes of the North Atlantic. This process is like the "Achilles heel" of the global climate system. Once it weakens or even collapses, less heat is transferred to the high latitudes. The cooling first occurs in Western Europe and North America, which are closest to the North Atlantic, and then occurs throughout the entire Northern Hemisphere. The AMOC suddenly weakened occurred approximately 12,800 years ago and halted global warming, with the global average temperature dropping by approximately 6 °C. The whole event lasted approximately 1,200 years and is known as the Younger Dryas event.

A frozen night of New York, due to the collapse of AMOC. Image from the movie "the day after tomorrow"

On the other hand, AMOC modulates the downstream climate in a dynamic way. The warm water brought by the North Atlantic Current penetrates into the Arctic region to strengthen the AA. The AMOC variation is significantly anticorrelated with the Arctic sea ice extent anomalies and correlated with the Arctic SAT anomalies on decadal time scales in the Atlantic sector of the Arctic. Then, cold air masses such as the Arctic vortex that originally swirled over the Arctic region are driven towards Siberia and finally aggravate the occurrence of the SAT extremes there.

We put forward another possible mechanism, which is, the stronger AMOC stimulates the propagation of the wave train originating in the North Atlantic Ocean, across mid- to high latitudes, thereby increases the variabilities in the circulations over the Ural blocking region and Siberia, which are critical to the SAT extremes there. In addition, a strengthened AMOC enhances the Arctic amplification (AA) effect and thereby weakens the upper-level jet stream and enhances the Ural blockings. Thus, more cold air from the Arctic is transported into the relatively low latitudes and entails an increasing probability of cold extremes over Siberia. In conjunction with the increases in warm extremes due to global warming, more SAT extremes have occurred over Siberia since 2000.

The possible mechanism of the AMOC effect on the SAT extremes over Siberia

This paper provides fundamental advances in our understanding of the role for natural climate variability in temperature extremes. The new insight also refreshes our knowledge in the impact of the anthropogenically forcing on the temperature extremes. The Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) has concluded that AMOC will very likely decline over the 21st century for all SSP scenarios. It means that both warm and cold extremes in winter will decrease over mid- to high latitudes in Eurasia according to our study. This is good news for the people living there. However, if AMOC weakens to the point of collapse, it will be a different story. Further studies concerning the AMOC effect on regional to hemisphere SAT and extremes are needed in the future.