The Intraseasonal Feature Of Aleutian Low And Its Impact On Pan-north Pacific Climate In Boreal Winter

Based on the daily ERA-Interim reanalysis dataset and GPCP(Global Precipitation Climatology Project)precipitation data,the period of 30-60 days of intraseasonal events of the Aleutian Low(AL)are selected.This study investigates the intraseasonal variation features of Aleutian low oscillation in winter and their impacts on air temperature and precipitation over pan-North Pacific by applying the composite method.The characteristics and evolution of AL's intraseasonal variation in winter are discussed first.Secondly,atmospheric water balance formula and Atmospheric River(AR)are concerned to investigated how AL's intraseasonal variation impacts precipitation and extreme precipitation over North Pacific.Finally,according to the relationship between AL's intraseasonal variation and the 850h Pa temperature,the extreme low temperature events in the Gulf of Alaska region are driven by the temperature advection term.This study selects a total of 40 complete AL intraseasonal oscillation events from1979 to 2018 which firstly strengthening and then weakening evolution in the life cycle of the eight phases.Exact dates of each phase are recorded to conduct composite analysis.With the strengthening of AL(phase three),the distribution of the intraseasonal cyclone/anticyclone at 850h Pa over the North Pacific Basin experience a process of intensifying and its location is westward compared to AL.At the same time,the geopotential height at 850h Pa is decreasing,companied with the strengthen of850h Pa zonal wind.The negative lobe in specific humidity at 850h Pa is near Southeast China,while the positive one occupies the west coast of North America.There is a time lag between distribution of the 850h Pa temperature anomaly and the eighth position of the Aleutian Low.It shows a positive-negative-positive wave train pattern from the east coast of the Eurasian continent through the Bering Sea to the northern part of North America,which has a certain hysteresis compared with the intraseasonal changes of the Aleutian Low.The seventh phase(the weakening of AL)is the opposite.With the strengthening(weakening)of AL,the distribution of intraseasonal precipitation anomalies presents a“east positive-west negative”(“east negative-west positive”)pattern over the North Pacific basin.Meanwhile,a positive anomaly is observed over the subtropical western Pacific.The intraseasonal changes of evaporation and the divergence of integrated water vapor transport contributes to the above pattern in precipitation.As the North Pacific basin appears as a cyclonic moisture transport belt,the convergence of intraseasonal water vapor over eastern(oceanic)and the divergence over western(oceanic)are the reasons for the intraseasonal distribution of precipitation to the north of 35°N during the AL's strengthening stage.At the same time,the humidity and temperature contrast between sea surface and near-surface have caused moisture supply through evaporation over continental East Asia,which is conducive to intraseasonal intensive precipitation over the subtropical western Pacific.The frequency of Atmospheric Rivers increases from the central North Pacific to the North American coastal area when AL gets stronger.As a result,the strong eastward and northeastward moisture transport at 35°N and the large moisture convergence over the eastern and northeastern North Pacific from the center basin.Thus,more extreme precipitation tends to occur over the west coast of North America.The phase change of the intraseasonal temperature lag behind the intensity of AL.In phase 4,from Northeast Asia to northern North America through the Bering Strait,the temperature anomaly field presents a positive-negative-positive distribution of intraseasonal temperature anomaly at 850h Pa.The temperature advection term-v'(?)is the dominant factor resulting to the intraseasonal 850h Pa temperature change in the Gulf of Alaska.During the AL's weakening stage,meridional temperature advection anomaly appears negative in the Gulf of Alaska.The cold and dry air transports north to northwestern North America,where the occurrence frequency of the extreme cold events remarkably decreases.