Regime Transition of the North Atlantic Oscillation and the Extreme Cold Event over Europe in January–February 2012

Abstract

In this paper, large-scale aspects for the onset of the extreme cold European weather event in JanuaryFebruary 2012 are investigated. It is shown that the outbreak of this extreme cold weather event may be attributed to the transition from a positive North Atlantic Oscillation (NAO 1 ) event to a long-lasting blocking event over the eastern Atlantic and western Europe (hereafter ENAO 2 ). A persistent decline of the surface air temperature (SAT) is seen over all of Europe during the long-lived ENAO 2 event, while the main region of enhanced precipitation is located over southern Europe and part of central Europe, in association with the presence of a persistent double storm track: one over the Norwegian and Barents Seas and the other over southern Europe.The NAO 1 to NAO 2 transition events are divided into NAO 1 to ENAO 2 and NAO 1 to WNAO 2 transition events [ENAO 2 (WNAO 2 ) events correspond to eastward-(westward-) displaced NAO 2 events whose positive center is defined to be located to the east (west) of 108W], and a statistical analysis of the NAO 1 to ENAO 2 transition events during 1978-2012 is performed. It is found that there has been a marked increase in the frequency of the NAO 1 to ENAO 2 transition events during the period 2005-12. Composites of SAT anomalies indicate that the marked decline of the SAT observed over much of Europe is primarily associated with NAO 1 to ENAO 2 transition events. Thus, NAO 1 to ENAO 2 transition events may be more favorable for the extreme cold events over Europe observed in recent winters than other types of NAO 2 events.

Major cause of unprecedented Arctic warming in January 2016: Critical role of an Atlantic windstorm

“…3a). The occurrence of the European blocking (or Scandinavian blocking) after the breaking of an Atlantic windstorm is not a rare event but has been discussed in previous studies2728. In our case, the northeastward intrusion of low potential vorticity at 350 K was observed over western Europe while Storm Frank was traversing the ocean between Greenland and Europe during 29–31 December 2015 (Supplementary Fig.…”

“…In addition to these thermodynamically driven processes, the sustaining mechanism also includes the generation of dynamically sustained blocking flows in the Eurasia and Greenland regions. In the blocking area, the warm advection and thickened geopotential height help to maintain warm temperatures at the surface3334. Therefore, both thermodynamic and dynamic processes worked to maintain the Arctic warming.…”

Climatological features of blocking highs from the perspective of air mass and mass transport

“…Obviously, the climatological features of the air mass and its transport during blocking events demonstrated in this study could be used to investigate these associations and to provide further understanding of the dynamical mechanisms behind. For example, the known associations between blocking and North Atlantic Oscillation investigated through traditional methods (Woollings et al ., ; Cohen et al ., ; He et al ., ; Luo et al ., ; Hanna et al ., ; ; Overland et al ., ) may also be understood from the perspective of air mass and mass transport.…”

“…These blocking systems are closely associated with weather and climate anomalies in the local and surrounding regions, especially outbreaks of cold air and extreme weather over the North American and Eurasian continents (Sillmann and Croci‐Maspoli, ; Sillmann et al ., ; Luo et al ., ; ; Chen and Luo, ). For example, cold air outbreaks over Eurasia and North America are closely related with the location of the GBH (Chen and Luo, ), as well as the thermodynamic features of the NPBH (Masato et al ., ); winter extreme weather events in Europe are mainly related to AEBH (Sillmann and Croci‐Maspoli, ; Buehler et al ., ; Sillmann et al ., ); strong cold air outbreaks over the East Asia and China are mostly associated with UBH (Cheung et al ., ); in addition, evidence shows that influences from the North Atlantic Oscillation (Woollings et al ., ; Luo et al ., ; Hanna et al ., , ), Arctic warming (Cohen et al ., ; He et al ., ; Woollings et al ., ; Overland et al ., ) and the extent of Arctic sea ice (Cohen et al ., ; Overland et al ., ; Chen and Luo, ) on regional weather anomalies may also involve changes in blocking highs. Therefore, increasing knowledge of the onset, maintenance and decay processes of blocking highs is of great importance for our understanding of the global and regional weather and climate regimes.…”

Large‐scale atmospheric circulation control of summer extreme hot events over China

“…For the NWC and NC types, the wave train propagates from the North Atlantic to northwestern China and northern China, respectively. The NWC type is related to an NAO− pattern (Figure b), and the NC type is related to a western NAO+ pattern (Luo et al ., ) (Figure c). Their EHE anticyclone is also in phase with the hot region, and the upstream blocking is relatively strong with a large spatial scale, situated over the Scandinavian Peninsula.…”