Low-frequency Variability Of The Aleutian Low

This study investigates the characteristics and evolution of the Aleutian Low on low-frequency scale during wintertime,by extracting durable events that represent the low-frequency of the Aleutian Low and applying the composite method through the NCEP/NCAR reanalysis daily dataset from 1979 to 2013.Then the production and maintenance mechanisms of the low-frequency variability of the Aleutian Low are discussed in two aspects:the synoptic transient eddy forcing and the low-frequency air-sea interaction.Finally,the characteristics of the low-frequency anomalous climate in East Asian along with the low-frequency variability of the Aleutian Low are analyzed.This study finds that the low-frequency variability of the Aleutian Low during wintertime exhibits the evolution of a low-frequency cyclone/anticyclone in the east-central of North Pacific at 850hPa.The specific situation is that the anomalous cyclone/anticyclone experiences the forming,strengthening,weakening and dissipating period from the lead 14 days to the lag 14 days,the corresponding relative vorticity positive/negative anomalies and geopotential height negative/positive anomalies experience the weak-strong-weak process.These reveal the changing process of the Aleutian Low on the low-frequency scale:it strengthens/weakens first then gradually restores to the normal state.The areas of 2m air temperature anomalous field mainly concentrate on the North Pacific coast,with positive and negative anomalies in different regions strengthening and weakening successively,the evolution of 2m specific humidity anomalies have a similar situation,but the areas of the anomalies mostly concentrate on the ocean.By analyzing the anomalous change of synoptic-scale transient eddy activities and transient eddy forcing,this study comes to a conclusion that the transient eddy dynamic forcing anomalies show the negative/positive values during the forming and strengthening phase of the low-frequency cyclone/anticyclone in North Pacific,making the tendency of the time-mean geopotential height negative/positive,providing a beneficial environment for the production and maintenance of the space form of the Aleutian Low on the low-frequency scale.The transient eddy thermal forcing anomalies are generally not beneficial for the production and maintenance of the associated low-level air temperature field with the low-frequency variability of the Aleutian Low.By analyzing the lead-lag relationship between the evolution characteristics of the North Pacific sea surface temperature(SST)anomalies and the atmospheric circulation anomalies,the phase difference is revealed between SST anomalies and low-level atmospheric anomalies,the atmospheric anomalies take the lead phase.On the low-frequency scale,the atmosphere changes the latent heat fluxes which play a dominant role in the air-sea interaction through the anomalies of the low-level air temperature and horizontal wind speed,thus making the ocean lose more heat to the atmosphere and resulting in the SST anomalies.SST anomalies exist negative feedback process to the atmospheric circulation field,it weakens the primary atmospheric anomalies and turns it to the opposite phase.Such negative feedback doesn't allow the low-frequency anomalies of the Aleutian Low continuously keeping on the positive phase(low-frequency cyclone)or negative phase(low-frequency anticyclone)without limit,thus conducive to the maintenance of air-sea coupling on the low-frequency scale.Accompanied by the low-frequency variability of the Aleutian Low,the climate in east Asia changes anomalously.When the Aleutian Low is during the anomalous strengthening period,the Siberian High anomalies invade southward into China,accompanied with the increasing northerly wind in the west side of the Aleutian Low,it brings the northern cold air into the south,most parts of China experience the cooling process from north to south.When the anomalous Aleutian Low is gradually restoring to the normal state,the northerly wind in most parts of China weakens,the primary cold air disappears and the land surface begins to get warm.The precipitation anomalies in east and southeast Asia appear uneven distribution.The precipitation anomalously reduces in the middle and lower sections of Yangtze River and Indonesian islands,and anomalously increases in Taiwan and the Philippines.The following reasons cause the anomalous reduced precipitation in the middle and lower sections of Yangtze River and south Japan:One,the anomalous weakening of water vapor transport leads to the decrease of the moisture content in the middle and low troposphere,creating the unfavorable moisture condition for precipitation.Another,the anomalous divergence in low-level,the anomalous convergence in high-level and the anomalous reduced upward movement make an unfavorable circulation configuration for precipitation.