Interannual Variation of Summer Precipitation in Xinjiang and Asian Subtropical Westerly Jet Stream

The relationships between Asian subtropical westerly jet stream (ASWJS) and summer precipitation in Xinjiang are examined using the NCEP/NCAR reanalysis dataset and 75 stations monthly precipitation data in Xinjiang from 1960 to 2003. Summer precipitation in Xinjiang is closely related to meridional displacement of west Asian jet stream and quasi-stationary wave activity along ASWJS. It is confirmed by Eliasson-Palm flux diagnoses that stationary waves propagation over Scandinavian Peninsulan-middle Europe has important impacts on stationary waves activity along ASWJS. It is found that anomalous summer more (less) rainfall in Xinjiang is related to the West Asian Westerly Jet axis moving to Southern (Northern) rather than normal, and subtropical westerly front also migrates southward (northward) over the whole troposphere. As a result, anomalous convergence and divergence occur over Xinjiang. The composite plots show the troposphere atmospheric steady wave weakens or strengthens over West Asia and strengthens or weakens over Xinjiang which are associated with more or less rainfall in Xinjiang. It is investigated that the horizontal wave vector propagation is obviously different between more and less rainfall years. During more (less) rainfall years, it is found that horizontal wave activity flux divergence center over Scandinavian Peninsula-middle Europe is favorable for moving eastward (westward and southward), and horizontal wave activity propagation strengthens (weakens) toward eastward leading to turning to southeastward near Ural mountain and entering ASWJS at Aral Sea-Xinjiang and the wave propagation strengthens (weakens). In the meantime, the direct wave propagation toward southeastward enters ASWJS in the east Mediterranean and Black Sea and weakens (strengthens) because of which quasi-stationary wave activity along West Asian westerly jet stream weakens (strengthens). Xinjiang summer rainfall is closely related to wave activity and propagation over Scandinavian Peninsula-middle Europe-ASWJS. Furthermore, the meridional wave train along 60°E connecting the two hemispheres is revealed, and this wave train which propagates meridionally upward from lower-troposphere in polar region to low-latitude tropopause and continues turning downward to mid-latitude in northern hemisphere is associated with west Asian jet variation. This wave train EP flux divergence performs opposite the variation near the south and north along 40°N, leading to different west Asian westerly jet stream intense variation near the south and north along 40°N for drought/flood years. EP flux divergence to the south (north) of 40°N and convergence to the north (south) of 40°N enhances (weakens) in flood (drought) years leading to the enhanced (weakened) upper level westerly to the south of 40°N and weakens (enhances) upper level westerly to the north of 40°N.