Abstract:

On the basis of the ERA-Interim monthly reanalysis data provided by the European Center for Medium-Range Weather Forecasts from 1979 to 2019 and Climatic Research Unit monthly precipitation data, the relationships between the North Africa subtropical high and the summer precipitation over Central Asia are analyzed. The results show that the variations of the ridge line index and eastern extension ridge index of the North Africa subtropical high are closely related to those of the summer precipitation over Central Asia. The effects of single and concurrent variations of the North Africa subtropical high on summer precipitation and circulations over Central Asia are significantly different. The ridge line of the subtropical high mainly affects the precipitation anomalies in the south and north parts of Central Asia, whereas the eastern extension ridge of the subtropical high plays an important role in modulating summer precipitation in the central and southern regions of Central Asia. Furthermore, we analyze the anomalous distribution of summer precipitation and atmospheric circulations under the concurrent variations of two indices. When the subtropical high shifts southeastward, there is an anomalous cyclone over the Caspian Sea and Aral Sea, corresponding to more summer precipitation over most areas of Kazakhstan. Meanwhile, Xinjiang is controlled by an anomalous anticyclone over the Mongolian Plateau, corresponding to less summer precipitation. When the subtropical high shifts southwestward, Central Asia is mainly controlled by an anomalous anticyclone, but there is an anomalous cyclonic shear in its northeast parts. Thus, more rainfall occurs in the northeast of Central Asia and less precipitation in other regions. When the subtropical high shifts northwestward, Central Asia is controlled by an anomalous anticyclone, corresponding to less summer precipitation. When the subtropical high shifts southeastward, Central Asia is controlled by an anomalous cyclone, and tropical Indian Ocean water vapor can enter Central Asia via two-step transportation, forming favorable dynamic and water vapor conditions; thus, Central Asia receives more summer precipitation.

Key words: North Africa subtropical high, ridge line, eastern extension ridge, concurrent variation, summer rainfall over Central Asia