The Antarctic Oscillation-East Asian Summer Monsoon Connections in NCEP-1 and ERA-40

Connections between the spring Antarctic Oscillation (AAO) and the East Asian summer monsoon (EASM) in two reanalysis datasets---NCEP-1 (NCEP/NCAR Reanalysis 1) and ERA-40 (ECMWF 40-year Reanalysis)---are investigated in this study. Both show significant correlation between AAO and EASM rainfall over the Yangtze River valley, especially after about 1985. Though ERA-40 shows weaker anomalous signals connecting AAO and EASM over southern high latitudes than NCEP-1, both datasets reveal similar connecting patterns between them. A wave-train-like pattern appears in the upper levels, from southern high latitudes through east of Australia and from the Maritime Continent to East Asia. In positive AAO years, the cross equatorial southeasterly flow over the Maritime Continent in the lower levels is strengthened, the specific humidity of the whole atmosphere over East Asia increases, and convective activity is enhanced; thus the summer rainfall over East Asia increases. The spring AAO-EASM connection may be better represented in ERA-40.