Decadal Variability of the Subtropical Westerly Jet and Its Association with Circulation and Rainfall over Central Asia

Using reanalysis data from the National Center for Environmental Prediction/National Center for Atmospheric Research and the monthly precipitation data from the Global Precipitation Climatology Center for the period 1961–2016, this study investigates the decadal relationships between the subtropical westerly jet and summer rainfall over central Asia. Results indicate that the subtropical westerly jet experienced significant decadal change points in 1997/1998. Before 1997, when the jet shifted southward, an anomalous cyclone dominated central Asia, while the anomalous anticyclone over the Indian Peninsula and anomalous cyclone over central Asia transported water vapor from the tropical Indian Ocean into central Asia, causing increased summer rainfall. After 1997, the relationship between the subtropical westerly jet and summer rainfall weakened. Furthermore, when the jet shifted southward, the anomalous cyclone over central Asia weakened and shifted westward, while the anomalous anticyclone over the Indian Peninsula strengthened and shifted westward. This resulted in weakened meridional water vapor transport from the tropical Indian Ocean into central Asia, thus, no longer reaching the eastern and northern parts of the region. The East Atlantic/West Russia (EA-WR) pattern exhibited different decadal relationships with the meridional location of the Central Asian subtropical westerly jet, acting as an indirect influence. Before 1997, a negative EA-WR pattern corresponded to an anomalous anticyclone over the Ural Mountains region, with the northerly flow on the anticyclone’s eastern flank transporting cold air from high latitudes southward. This led to a cooling of the upper troposphere and the formation of an anomalous cyclone over Central Asia, corresponding to the jet shifting southward. After 1997, the anomalous anticyclone over the former Ural Mountains region shifted eastward, no longer causing upper troposphere cooling over central Asia, and the relationships between the subtropical westerly jet location and EA-WR decreased. Before and after the decadal change in subtropical westerly jet location, a positive Pacific–North America pattern could cause the anomalous cyclone to determine central Asia, corresponding to the jet shifting southward.