Anomalous Migration Of Atmospheric Mass Over Regions Between Asia/North America And North Pacific In Boreal Summer And Its Associations With Climate Variations

Using both reanalysis and observational data, we have investigated the anomalous migration of atmospheric mass over regions between Asia/North America and North Pacific in boreal summer and its associations with climate variations by employing some statistical methods. Our results have demonstrated that:An index of Migration of Atmospheric Mass over Regions between Lands and Oceans (MAMLO) is defined. It is found that an anti-phase variation of anomalous atmospheric mass exists between Asia mid-high latitude and North Pacific mid-low latitude in the northern hemisphere. The summer MAMLO index varies with a slowly decreasing trend in the study period, the periodicity of 3 yrs, and the distinct interdecadal variations. The atmospheric mass fluxes anomalously diverge in lower and converge in upper troposphere in Siberia, and in North Pacific the opposite situations are observed, forming the anomalous vertical circulation characterized by anomalous downdrafts in Asia and updrafts in Pacific. The other possible causes of MAMLO mainly attribute to the contrast of anomalous heating between lands and oceans and energy propagation of Rossby wave. In summer, MAMLO is highly correlated with climate anomalies in Eurasia and Pacific, especially over China. When the MAMLO index is positive, the precipitation increases and the surface temperature decreases significantly over northern China, as opposed to the decrease of precipitation and increase of surface temperature over the south of China.The anti-phase distribution of anomalous atmospheric mass are also observed between North America mid-high latitude and North Pacific mid-low latitude during boreal summer. The summer MAMLO index varies mainly with the periodicity of 2 yrs. The winds in lower troposphere anomalously diverge in the American Great Plains and converge in North Pacific, and the opposite situations are observed in upper troposphere. Anomalous vertical circulation bridges North America and Pacific with downdrafts and updrafts between these two regions. Also, the anomaly of atmospheric diabatic heating, the energy dispersion of quasi-stationary Rossby wave and the difference of water vapor content anomalies between lands and oceans are in favour of promoting the exchange of atmospheric mass between land and sea. MAMLO is closely related to the summer precipitation and temperature variations. When the index is positive, the precipitation decreases significantly over the Canadian plains, the south of Great Bear Lake and the northeast of Yukon Plateaus, as opposed to the increase over the Hawaiian Islands and the south of Gulf of Alaska. At the same time, the temperature increases obviously over the northwest of American Great Plains and the central and eastern Pacific, along with the anomalous decrease in the mid-eastern North America and Pacific Northwest.With different phase combinations of migration of atmospheric mass between Asia and North Pacific (MAMLO) and convective activity in MC domain (CAMC), the climate anomalies in Eurasia are diverse. It is found that the MAMLO and CAMC are almost independent of each other in statistical sense. Four types of phase combinations of MAMLO and CAMC have been configured. The mean climatological surface air pressure distributions in boreal summer can be modified by these four types of anomalous surface air pressure patterns. Different types of MAMLO and CAMC phase combination have different impacts on precipitation and surface air temperature anomalies.All the results above are helpful for us to better understand the mechanisms of circulation changes during boreal summer and to seek for clues to better prediction of the summer climate anomalies.