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Quan, X.-W., H. F. Diaz, C.-B. Fu, 2003: Interdecadal change in the Asia-Africa summer monsoon and its associated changes in global atmospheric circulation. Global and Planetary Change, 37, 171-188.


ABSTRACT

Previous studies have noted that summer precipitation in the sub-Saharan region of North Africa showed a large decrease around the year 1968, and this drought has persisted for much of the period since then. A trend toward decreased summer precipitation is also observed in northeast China. A decrease in precipitation over northeastern China also began around 1968. In this study, we examine the changes in atmospheric circulation that are associated with the interdecadal changes in summer precipitation over Asia and Africa. Data from the reanalysis project by the National Center for Environmental Prediction and National Center for Atmospheric Research (NCEP reanalysis) are used. Precipitation based on the NCEP reanalysis are compared to precipitation observed at stations in China and the sub-Saharan region. We found that the NCEP reanalysis precipitation well represents both the sign and timing of the observed (in situ) change in the summer precipitation over North Africa, as well as the spatial pattern of the observed precipitation changes over eastern China. We have also examined corresponding changes in 850 and 200 mb divergent and rotational circulation features in the NCEP reanalysis. The persistent drought over the sub-Saharan North Africa (the Sahel region) is accompanied by pronounced intensification of divergence (convergence) in the lower (upper) troposphere above the region. The persistent drought in the Sahel is related to changes in the Asia summer monsoon in two ways: (1) In the tropics, the intensification of the lower level divergence is accompanied by stronger convergence over Southeast Asia where summer precipitation increased. The changes in the divergent circulation implies an interdecadal change in the zonal circulation in the tropics, with stronger convergence over Southeast Asia leading to an intensified Hadley cell over East Asia, and associated stronger descending motion and lower level divergence over the region of northeast China. (2) The persistent drought over North Africa became a source of planetary wave activity that helped in the development of stronger anticyclones over northeast China and the south Indian Ocean.