Interannual Variability of the Asian Summer Monsoon
- Critical Roles of the Monsoon and Monsoon Ocean Interaction -
Bin Wang
IPRC and Department of Meteorology, University of Hawaii
Hawaii, USA
Analysis of 50 years of NCEP/NCAR reanalysis data reveals that interannual variations of the two Asian monsoon subsystems, the Indian summer monsoon (ISM), and the western North Pacific/East Asian (WNP/EA) summer monsoon, are remarkably different in terms of their spatial structure, dominant time scale, their relationships with ENSO, and their associated teleconnection patterns. Since the late 1970s, variability of the WNP/EA monsoon has increased considerably, but its relationship with El Nino-Southern Oscillation (ENSO) remained steady. In contrast, the relationship between ISM and ENSO has dramatically weakened in association with decreasing ISM variability. The WNPSM is generally weak (strong) during decay of an El Nino (La Nina), whereas the ISM tends to be weak (strong) when an El Nino (La Nina) develops. The physical processes by which ENSO affects Asian monsoon, particularly, the East Asian monsoon, will be addressed. The critical roles of the monsoon and monsoon ocean interaction in generation of the monsoon anomalies are elaborated. We show that ENSO affects East Asian monsoon through establishing an anomalous lower tropospheric anticyclone located over the Philippine Sea. The anticyclone develops rapidly in late fall due to remote forcing from central Pacific and tropical-extratropical interaction over the East Asia. It persists until the next spring/early summer, provides a prolonged ENSO impacts, causing excessive Meiyu/Baiu in the decay phase of ENSO. Giving the chaotic nature of the atmospheric motion, how can the anticyclone maintain for 2-3 seasons even when ENSO decays? We proposed a theory that attributes the persistence of the teleconnection to a positive thermodynamic feedback between the anticyclone and sea surface cooling in the presence of mean northeasterly trades.
