Climatic Features And Inter-decadal Variability Of The Afro-Asian Summer Monsoon System

Afro-Asian monsoon is a belt-like planetary system,extending from North Africa via South Asia to East Asia.Its intensity and position anomalies could trigger widespread droughts and floods simultaneously.The investigation on inter-decadal variability of Afro-Asian monsoon is of great scientific importance and high practical value.By employing multiple reanalysis data and output from CMIP5 models under RCP4.5 scenario,this study reveals horizontal and vertical structure of the Afro-Asian monsoon system.Major attention has been devoted to portray decadal to inter-decadal variations of Afro-Asian monsoon and further unearth physical mechanism.The projection of monsoonal precipitation changes under RCP4.5 scenario aims to deepen the understanding of future evolution of Afro-Asian monsoon system and responsible drivers.Main conclusions are summarized as follows:(1)Afro-Asian monsoon is characterized by coherent widespread rainbelt.The precipitation in the Sahel,South Asia and East Asia(Huang-Huai to North China)exhibited in-phase variations.Multiple representative matrixes(zones of maximum precipitation,low-level winds and zones of minimum OLR)consistently portray belt-like distribution of Afro-Asian monsoon system.The newly defined intensity index and position index could exactly describe coherent variations of Afro-Asian monsoon system,i.e.systematic enhancement and northward migration during late 1990s.(2)The leading decadal to inter-decadal mode of Afro-Asian monsoon system during the last century.The Afro-Asian monsoonal precipitation experienced three regime shifts:the first one occurred around 1920,featured by increased monsoonal precipitation in the Sahel,South Asia and eastern China concurrently.The second shift appeared in late 1960s,evidenced by concomitant decreased precipitation in above three regions,the period usually referred to as the "decaying period of monsoon".Late 1990s registered another reversal,indicative of intensification and northward migration of monsoonal precipitation in the Sahel and eastern China.(3)The reason for decadal to inder-decadal changes in Afro-Asian monsoonal precipitation.The inter-decadal variability of Afro-Asian monsoonal was mainly modulated by the Atlantic Multi-decadal Oscillation(AMO).During 1920s-1950s,in the context of positive AMO phase,above-normal monsoonal precipitation was observed in Afro-Asian region.During 1960s to 1980s,the negative phase of AMO corresponds to reduced monsoonal precipitation.In particular,the concomitant modulation by AMO and PDO play a key role in this relatively dry regime.The configuration of negative AMO phase and positive PDO phase jointly excite a teleconnection wavetrain,with its two negative centers located around the Mediterranean areas and North China-Bo Hai area,sandwiched by a positive center around central Europe.Thus,the coupling between upper-level cyclone and lower-level anticyclone favored the prevalence of descending motion and further suppressed local precipitation in North Africa.Similar circulation patterns also dominated in East Asia,dampening precipitation around Huai-Huai region.(4)During the past 30 years,in the context of AMO transition toward its warm phase,the third regime shift of Afro-Asian monsoonal precipitation during mid-late 1990s and its mechanism.Since late 1990s,the major rainband of monsoonal precipitation over eastern China advanced northward,arriving at Huai-Huai and Jiang-Huai with increased intensity.Meanwhile,wetting tendency was recorded in the Sahel region.The synchronous variations between these two remote areas could be attributed to similar responses of temperature,pressure and wind fields to the warm phase of AMO,which triggered a eastward propagating wavetrain.The coupling between upper-level anticyclones and low-level thermal lows enhanced ascending motion in the Sahel and East Asia respectively.Accordingly,monsoonal precipitation intensified simultaneously in these two regions.(5)The structure of low-level westerlies and upper-level easterlies within Afro-Asian monsoonal precipitation;and modulation by warm phase of the AMO.At the lower level,three convergence zones form via convergences between three branches of cold air from north and westerlies from Gulf of Guinea,cross-equator flows via Somali and southwesterlies from the Bay of Bangle.At the upper level,the tropical easterly jet enhanced and marched northward during AMO warm phase.The warming of upper-level air temperature determines above intensification and northward displacement of the tropical easterly jet,particularly in the entrance region(east Asian branch)and exit region(North African branch).Thus,jet-induced secondary circulation cells were excited and further intensified in the northern side of jet(Huang-Huai)and southern flange of exit region(Sahel),leading to simultaneous increases of precipitation.(6)Multi-model mean projected a general increase in Afro-Asian monsoonal precipitation and northward migration of major rainband.In later period of the 21st century(2070s-2099s),the entire Afro-Asian monsoonal region will see a wet regime.The future changes in Afro-Asian summer monsoonal precipitation are jointly contributed by natural variability and anthropogenic warming.But during the second half of 21st century,anthropogenic influence would be more dominant.