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Dominant trajectories and influenced regions of the near-surface cold air in the Arctic during positive and negative AO/NAM events

LIANG SuJie;ZHAO Nan;DING YiHui;Tianjin Climate Center;Chinese Academy of Meteorological Sciences;National Climate Center;  
The Arctic Oscillation(AO)/Northern Hemisphere Annular Mode(NAM)has a strong influence on weather and climate on different time scales,of which the intrinsic time scale is about 10~20 days.The synoptic time-scale variable of the AO/NAM determines its intrinsic and direct influence on weather.And usually extreme weather events occur corresponding to the abnormal AO/NAM indices.This is because the AO/NAM can modulate the movement of nearsurface cold air in the Arctic that can affect the atmospheric circulation in the North Hemisphere and is the direct cause of cold surges and cold air outbreaks.Therefore,it is of great importanceto investigate the motions of the near-surface cold air over the Polar under the modulation of the AO/NAM.Based on previous studies,we track trajectories of the near-surface air parcels pole ward of 60°N during AO/NAM events from the Lagrangian viewpoint.The dominant trajectories during the AO/NAM events are direct reflections of the AO/NAM′s modulation to surface air temperature.During positive AO/NAM events,the near-surface atmospheric masses are restricted in the regions pole ward of 45°N.The air parcels converge in the regions of Aleutian islands and Norwegian Sea consistent with low pressure.The prominent feature of the cold air parcels during positive AO/NAM events is that latitudinal circulation prevails at middle and high latitudes,which limits the cold surge outbreak.During negative AO/NAM events,the air parcels flow out anticyclonically from the polar to the northern Pacific and Atlantic,respectively,especially in the valley of the AO/NAM index.Of particular importance,there are two dominant trajectories strongly influencing the Eurasian continent during the middle and later phases of the negative AO/NAM index decreasing stage.The trajectories of the near-surface air parcels over the polar match the correlation map of wintertime surface air temperature with the AO/NAM index very well and the results give a visualized interpretation of the AO/NAM′s influence on weather.Also,near-surface air parcels during the positive and negative AO/NAM events exert different effects on northern and southern China.During positive AO/NAM events,the south-branch trough strengthens,which favors the transfer of cold air from mid-and high-latitude areas of Europe as well as water vapor from the Bay of Bengal to southern China,which results in conditions that favor the formation of cold waves and even frozen rain and snow in southern China.In comparison,during negative AO/NAM events,near-surface cold air in the eastern New Island and northern Iceland affect most of Europe and the easterly movement of these cold air masses directly affect northeastern China and key cold-wave regions.With the Siberian High and the East Asian Major Trough,the latter can cause temperature decreases,e.g.,cold waves and strong winds,and further influence temperature in other regions of China.
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