Using the NCEP/NCAR monthly mean reanalysis data for the period of 1979—2014,CMAP(CPC Merged Analysis of Precipitation) precipitation data,in situ observational data at 160 stations in China,SST(Sea Surface Temperature) data provided by British Hadley Centre as well as CRU high-resolution surface temperature data by the University of East Anglia,this paper has investigated the migration of atmospheric mass over regions between Asia and North Pacific in boreal summer and its associations with climate anomalies by employing some statistical methods.
An index based on surface air pressure anomaly is defined to describe the Migration of Atmospheric Mass over regions between Lands and Oceans(MAMLO) on interannual time-scale.It is found that an anti-phase variation of anomalous atmospheric mass exists between mid-high latitudes in Asia and mid-low latitudes in North Pacific.This anti-phase relation can also be referred to Asia-North Pacific teleconnection pattern(ANPT).The summer MAMLO index varies with a dominant period of three years,a slowly decreasing trend,and distinct interdecadal variations in the study period.The ANPT is formed due to three factors.Firstly,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 downward motion in Asia and upward motion in Pacific,which links surface pressure variations along with horizontal circulation changes between Asia and Pacific.Secondly,MAMLO is mainly dominated by the contrast of anomalous heating between lands and oceans.Water vapor content is higher over the subtropical Pacific than that over eastern Asia,facilitating the atmosphere to absorb surface long wave radiation and solar short wave radiation over oceans more than over Asia continent.The striking contrast of the anomalous atmospheric diabatic heating between Asia and North Pacific is in favor of driving atmospheric mass to exchange between them.Thirdly,the energy dispersion of quasi-stationary Rossby wave also plays an important role in the anomalous loss and accumulation of atmospheric mass.
MAMLO is closely related to the summer climate anomalies in Eurasia and Pacific.When the MAMLO index is positive,the precipitation decreases significantly in Siberia,the southern area of Europe and the east coast of China,as opposed to the increase over northern China,the Hawaiian Islands and the south of Gulf of Alaska.At the same time,the temperature increases obviously over the West Siberian Plain and the central and eastern Pacific,along with the anomalous decrease in much of Eurasia,northern China and Northwest Pacific.Further researches by use of observational data over Chinese 160 stations conclusively indicate that correlations of MAMLO with variations of anomalous precipitation and temperature are particularly significant in China.When the MAMLO index is positive(negative),precipitation increases(decreases) and surface temperature decreases(increases) prominently over northern China.Instead,there is a decrease(an increase) in precipitation and an increase(a decrease) in surface temperature over southern China.
All the results above are helpful for us to better understand the mechanisms behind circulation changes and the causes of climate anomalies in Eurasia and Pacific during boreal summer and to provide useful clues to prediction of the summer climate anomalies.