Weakened potential vorticity barrier linked to recent winter Arctic Sea ice loss and midlatitude cold extremes

A winter Eurasian cooling trend and a large decline of winter sea ice concentration (SIC) in the Barents- Kara Seas (BKS) are striking features of recent climate changes. The question arises as to what extent these phenomena are related. A mechanism is presented that establishes a link between recent winter SIC decline and midlatitude cold extremes. Such potential weather linkages are mediated by whether there is a weak north-south gradient of background tropospheric potential vorticity (PV).Astrong background PV gradient, which usually occurs in North Atlantic and Pacific Ocean midlatitudes, acts as a barrier that inhibits atmospheric blocking and southward cold air intrusion. Conversely, atmospheric blocking is more persistent in weakened PV gradient regions over Eurasia, Greenland, and northwestern North America because of weakened energy dispersion and intensified nonlinearity. The small climatological PV gradients over mid- to high-latitude Eurasia have become weaker in recent decades as BKS air temperatures show positive trends due to SIC loss, and this has led to more persistent high-latitude Ural-region blocking. These factors contribute to increased cold winter trend in East Asia. It is found, however, that in years when the winter PV gradient is small the East Asian cold extremes can even occur in the absence of large negative SIC anomalies. Thus, the magnitude of background PV gradient is an important controller of Arctic-midlatitude weather linkages, but it plays no role if Ural blocking is not present. Thus, the ''PV barrier'' concept presents a critical insight into the mechanism producing cold Eurasian extremes and is hypothesized to set up such Arctic- midlatitude linkages in other locations.