An Analysis of Interdecadal Variations of the Asian-African Summer Monsoon

The response of the Asian-African summer monsoon (AASM) to the fast global warming in the 1980s is studied based on several datasets, which span a long time period of nearly 100 yr, with two special periods 19801985 and 19901995 being focused on. Wavelet analyses are employed to explore the interdecadal variations of the AASM.It is found that after the mid-1980s, the global annual mean surface temperature rises more signi cantly and extensively over most parts of the African Continent, north of the Indian Ocean, and the Eurasian Continent excluding the Tibetan Plateau. Correspondingly, the global precipitation pattern alters with in creased rainfall seen over the Sahel and North China in 1990-1995, though it is not recovered to the level of the rainy period before the mid-1960s.Changes of monsoonal circulations between the pre- and post-1980s periods display that, after the fast global warming of the 1980s, the African summer monsoon intensi es distinctly, the Indian summer monsoon weakens a little bit, and the East Asian summer monsoon remains almost unchanged. The summer precipitation over the Asian-African Monsoon Belt (AAMB) does not change in phase coherently with the variations of the monsoonal circulations.Wavelet analyses of the land-sea thermal contrast and precipitation over North China and the Sahel indicate that interdecadal signals are dominant and in positive phases in the 1960s, leading to an overall enhanced interdecadal variation of the AASM, although the 1960s witnesses a global cooling. In the 1980s, however, in the context of a fast global warming, interdecadal signals are in opposite phases, and they counteract with each other, leading to a weakened interdecadal variation of the AASM.After the mid-1960s, the AASM weakened remarkably, whereas after the mid-1980s, the AASM as a whole did not strengthen uniformly and synchronously, because it is found that the interannual variations of the AASM in the 1980s are stronger than those in the 1960s, and they superimposed on the counteracting interdecadal signals, causing di erent regions of the AAMB behaving di erently. Therefore, the response of the AASM to the accelerated global warming post the mid-1980s is not simply out-of-phase with that after the mid-1960s; it may involve more complicated multiscale physical elements.