A mid-latitude Asian circulation anomaly pattern in boreal summer and its connection with the Indian and East Asian summer monsoons

Using the NCEP-NCAR reanalysis for 1948-98. this study identifies a dominant pattern for the interannual variation of upper-level winds over mid-latitude Asia in boreal summer. This pattern, called the mid-latitude Asian summer (MAS) pattern, features two anomalous anticyclones: one centred at 37.5degreesN 65degreesE and the other at 42.5degreesN, 130degreesE. The MAS pattern significantly influences East Asian summer monsoon variability. In the positive phase of the MAS pattern, contrasting meridional wind anomalies between eastern China and Japan lead to above- and below-normal summer rainfall in north China and south Japan respectively. The ear-to-year change of the MAS pattern is related to that of the Indian summer rainfall, especially in central and northern India. Thus, the MAS pattern plays a role in connecting anomalous Indian heating with the East Asian summer monsoon variability. The East Asian anomalous anticyclone displays a southeast ward shift after the late 1970s. This results in a similar change of anomalous summer rainfall re-ions in East Asia. The West Asian anomalous anticyclone moves northeastward after the late 1970s. The relation of the MAS pattern with the Indian summer rainfall experienced an obvious weakening in the late 1970s. As a result. the statistical relation between the Indian and north China summer rainfall becomes weak after the late 1970s. Copyright (C) 2002 Royal Meteorological Society.