Winter Arctic warming and its linkage with midlatitude atmospheric circulation and associated cold extremes: The key role of meridional potential vorticity gradient

The surface air temperature over the Eurasian continent has exhibited a significant cooling trend in recent decades(1990–2013), which has occurred simultaneously with Arctic warming and Arctic sea ice loss. While many studies demonstrated that midlatitude cold extremes are linked to Arctic warming and Arctic sea ice loss, some studies suggest that they are unrelated.The causal relationship between midlatitude cold extremes and Arctic change is uncertain, and it is thus an unsolved and difficult issue. It has been widely recognized that the severity and location of midlatitude cold extremes are closely related to the persistence, location and movement of blocking systems. It might be possible that the Arctic sea ice decline or the Arctic’s warming influences midlatitude cold extremes by changing the blocking system. This paper reviews the recent research advances on the linkages between the blocking system and Arctic warming. The nonlinear multiscale interaction model of Luo et al.revealed that the magnitude of the meridional gradient(PVy) of the background potential vorticity(PV) is a key parameter that reflects changes in the dispersion and nonlinearity of the blocking system. It was found that Arctic warming played a role in reducing the dispersion of the blocking system and enhancing its nonlinearity by reducing the magnitude of PVy. A small PVyis a favorable background condition for increasing the duration of blocking events and producing midlatitude cold extremes.However, because the magnitude of PVyreflects the difference between the background PVof the Arctic high latitudes and the midlatitude continent, the occurrence of midlatitude cold extremes not only depends on an anomalous background PVover Arctic high latitudes but also on its value over the midlatitudes. Thus, Arctic warming or sea ice decline is not necessary for the occurrence of midlatitude cold extremes.