Under global warming, extreme weather occurs frequently, bringing a lot of inconvenience to people's lives. A prolonged sustained heat wave that swept through Europe in July and early August 2018, setting new records for maximum temperatures in several locations. In the same year, maximum temperatures in the Arctic Circle exceeded 30°C, and record highs were reached at the Badufoss, Norway and Kevo, Finland stations with temperatures of 33.5°C and 33.4°C, respectively.

Whereas similar extreme events are supported by local meteorological conditions, anomalous atmospheric large-scale circulation creates favorable background conditions for the excitation of extreme weather processes. Many studies have focused on the influence of Rossby waves and blocking circulation on European heat waves. However, the collaborative effects of the North Atlantic Oscillation (NAO) and blocking on heat waves have not been revealed.

Recently, researchers from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences revealed the importance of the collaborative effect of NAO+ (NAO positive phase) and blocking on European heat waves. With the collaborative effect of NAO+, the upstream westerly winds are stronger, which facilitates the energy dispersion to the downstream and maintains the blocking persistence.

The study was published in Environmental Research Letter on Oct. 14.

It is worth noting that the main body of the European blocking with NAO+ collaboration has a northeast-southwest tilting, which is favorable for warm advection to western and northern Europe, while the southern side of the European blocking without NAO+ collaboration is a trough, which is favorable for cold air to invade southern Europe.

Further analysis showed that the peak of NAO+ was ahead of the peak day of European blocking by about four days, and the weakening of European high temperature lagged behind the peak day of European blocking by about one day. All these signals can be potential forecast signals for a persistent European heat wave.

"If there is an embryonic formation of European blocking during the NAO+ process, then a European heat wave is likely to occur, especially in the southwestern part of Europe, and vigilance should be raised," said Dr. YAO Yao, the corresponding author of the study. In addition, the synergistic character of NAO and blocking is verified by the CMIP6 model historical simulation data, indicating that this collaboration is not an accidental phenomenon in the observations.