Interdecadal Variability of the East Asian Summer Monsoon and Associated Atmospheric Circulations

Based on the National Centers for Environmental Prediction and National Center for Atmospheric Re- search(NCEP/NCAR)reanalysis data from 1950-1999,interdecadal variability of the East Asian Summer Monsoon(EASM)and its associated atmospheric circulations are investigated.The EASM exhibits a distinct interdecadal variation,with stronger(weaker)summer monsoon maintained from 1950-1964(1976-1997). In the former case,there is an enhanced Walker cell in the eastern Pacific and an anti-Walker cell in the western Pacific.The associated ascending motion resides in the central Pacific,which flows eastward and westward in the upper troposphere,descending in the eastern and western ends of the Pacific basin.At the same time,an anomalous East Asian Hadley Cell(EAHC)is found to connect the low-latitude and mid-latitude systems in East Asia,which strengthens the EASM.The descending branch of the EAHC lies in the west part of the anti-Walker cell,flowing northward in the lower troposphere and then ascending at the south of Lake Baikal(40°-50°N,95°-115°E)before returning to low latitudes in the upper troposphere, thus strengthening the EASM. The relationship between the EASM and SST in the eastern tropical Pacific is also discussed.A possible mechanism is proposed to link interdecadal variation of the EASM with the eastern tropical Pacific SST.A warmer sea surface temperature anomaly(SSTA)therein induces anomalous ascending motion in the eastern Pacific,resulting in a weaker Walker cell,and at the same time inducing an anomalous Walker cell in the western Pacific and an enhanced EAHC,leading to a weaker EASM.Furthermore,the interdecadal variation of summer precipitation over North China is found to be strongly regulated by the velocity potential over the south of Lake Baikal through enhancing and reducing the regional vertical motions.