In the summer of 2022, scientists predicted that a "triple-dip" La Nina—three years in a row of La Nina conditions—is very likely to occur during the winter of 2022. During La Nina winters, Eurasia often experiences colder conditions. So a team of researchers set out to study what the Eurasian climate will look like in the upcoming 2022 winter.
To better understand the current La Nina, the team studied a variety of climate models. "Almost all the climate models we examined predict an overall warm condition over most of Eurasia. It somewhat contradicts the knowledge that La Nina facilitates cold Eurasian winters, especially when the Arctic sea ice is still at its low level compared with the historical average," said WANG Lin, from the Institute of Atmospheric Physics, Chinese Academy of Sciences (CAS).
The team suggests these results may arise partly from global warming or the models' biases. "Nevertheless, some post-processes to the model outputs indicate a possibility of below-normal temperature over mid-latitude Eurasia. Hence, there is still a chance of severe winter or some cold spells over Eurasia in the upcoming winter," said WANG.
La Nina is a climate pattern in the Pacific Ocean that can impact weather worldwide. During La Nina, trade winds blowing along the equator are stronger than usual and push more warm water toward Asia. These shifts push the jet stream northward. During La Nina winters, the northern hemisphere often experiences frequent cold air outbreaks and heavy snowfalls.
Anticipating the triple-dip La Nina, scientists wondered how it would impact weather on a large scale. The climate in Eurasia is of great concern because of the dense population there. "How the 2022-2023 winter weather and climate over the Eurasian continent respond to such a significant global climate signal and whether the Eurasian continent will experience a cold winter or not is an urgent question for seasonal climate prediction," said ZHENG Fei, from the Institute of Atmospheric Physics, CAS.
The team used an ensemble approach, based on four different state-of-the-art coupled general circulation models developed at the Institute of Atmospheric Physics to study the weather in the coming winter. Seasonal predictions for surface air temperature and precipitation anomalies were carried out using this ensemble approach. To increase the accuracy of their study, the team calculated the prediction anomalies relative to the average of hindcasts for the years 2002 to 2021.
In a typical La Nina winter, the meridional temperature gradient in Eurasia's middle and high latitudes favors the formation of an abnormal ridge near the Ural Mountains. This circulation system is a key contributor to the East Asian cold waves. With this blocking high system in place around the Ural Mountains, cold air accumulates over Central Siberia. Then when the blocking high collapses, cold air spreads over China, leading to cold waves. This blocking, along with the East Asian winter monsoon and an atmospheric phenomenon known as North Atlantic Oscillation, all have an effect on the winter climate in Eurasia.
How the future Eurasian climate will evolve in winter is still subject to some uncertainty, mostly according to the unpredictable internal atmospheric variability. "Persistently improving the seasonal forecast skills of climate models and further understanding the influencing factors and physical processes on controlling the winter Eurasian surface air temperature, especially under a global warming background, should be our ultimate goal," said ZHENG.
Looking ahead to future research, the team sees the opportunities for improved predictions. The researchers know that the winter climate in the Eurasian continent is closely associated with the North Atlantic Oscillation. But the predictive skill for the North Atlantic Oscillation is still very low. "We need to solve this problem by improving our models in North Atlantic Oscillation simulations and developing a more advanced coupled assimilation scheme," said WU Bo, from the Institute of Atmospheric Physics, CAS.
The team wants to continue to improve the capabilities of short-term climate prediction. "In particular, in the context of global warming, the multi-system synergistic effect on climate has become increasingly significant. How to transform the synergistic effect into an effective predictor in the model is an important scientific problem to be solved, and also a great challenge," said YAO Yao, from the Institute of Atmospheric Physics, CAS.
The research team includes scientists from the Institute of Atmospheric Physics, CAS, the CMA Earth System Modeling and Prediction Centers, and the Department of Atmospheric and Oceanic Sciences & Institute of Atmospheric Sciences at Fudan University.
Citation: Zheng, F., and Coauthors, 2023: Can Eurasia experience a cold winter under a third-year La Ni？a in 2022/23? Adv. Atmos. Sci., https://doi.org/10.1007/s00376-022-2331-8.
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