Analysis Of Atmospheric Teleconnection Wave Trains In The Upper Troposphere During Boreal Summer

In this study,a 66-year NCEP/NCAR reanalysis dataset is used to analyze the relationship between Circumglobal Teleconnection(CGT)and Silk Road Pattern(SRP).The results indicate that there are no evident diversities between them over Eurasia in both temporal and spatial variations.After removing the linear influence of SRP,CGT disappears over Eurasian continent;while deleting that of CGT,SRP maintains.Therefore,both CGT and SRP represent a common mode over mid-latitude Eurasia during boreal summer.However,SRP is the internal intrinsic mode of atmosphere,whereas CGT mostly depends on SRP,which can be considered as a part of SRP.The South Asian High(SAH)is a strong and stable anticyclone system in the upper-level atmosphere during boreal summer.The annual variations of SAH include southeast-northwest(SE-NW)movement,east-west(EW)movement and north-south(NS)movement.The results indicate that these three kinds of movement patterns of SAH have different influences on atmospheric circulations,and incur different wave train patterns.The EW movement triggers a highly similar wave train pattern with CGT,especially on the geopotential and the meridional wind fields.The NS shift leads to a north-south dipole wave train,mainly streching over the Asian continent,especially on the geopotential and the zonal wind fields.The SE-NW variation,combining both the meridional and zonal movements of SAH,has closer relationship with CGT,which manifests clearer wave train structure.The wave train pattern associated with SE-NW movement has similar spatial distributions with CGT,but with reverse phases.In the temporal variations,the two teleconnection indices are strongly negative correlated.Using the partial correlation method,it is revealed that the SE-NW shift of SAH plays a significant role on the eastward propagation of CGT,especially to the east of 90oE.Considering that SAH is a linkage between Indian and East Asian summer monsoon rainfall,removing the influences of SE-NW movement means removing the collaborative influences of both Indian and East Asian summer monsoon rainfall.The numerical experiments indicate that the latent heat release from East Asian summer monsoon rainfall stimulates an upper-tropospheric teleconnction,which shows the same spatial structure with the negative CGT.This study demonstrates that the generation of CGT is not only associated with the Indian summer monsoon rainfall,but also closely with the East Asian summer monsoon rainfall.The CGT is maintained by the latent heat released from the rainfall of both monsoons.In order to analyze the contribution of global sea surface temperature and sea ice to the wave trains,this study utilizes the ECHAM5 numerical model to conduct a set of experiments.One run,called RT,is driven by the annual varied historical(1958–1999)monthly global sea surface temperature and sea ice data from AMIP2 bountry condition datasets.And another run,called CT,is driven by the climatological global sea surface temperature and sea ice data from the same datasets.In the CT run,we find that CGT,SRP and the SAH-associated wave train show clear wave train patterns over Eurasian continent,while the downstream wave trains over East Asia,North Pacific and North America are extremely weak.However,in the RT run,where the sea surface temperature and sea ice with annual variation are used to drive the model,the wave trains show great circumglobal characteristics,and propogate further to East Asian,North Pacific and even to the North America.This may be triggered by an increase in water vapor divergence over the key area at northwestern Pacific,which results in a reduction in precipitation and a negative anomaly in the release of condensation latent heating.This mechanism strengthens the CGT pattern propagating to the downstream.The results of the numerical experiments also indicate that the external forcing of global sea surface temperature and sea ice has few influences on the relationship between CGT and SRP over Eurasian continent.Their relationship is stable and agrees on the results in the reanalysis data.