Distinct quasi-biweekly features of the subtropical East Asian monsoon during early and late summers

Distinct quasi-biweekly features of the subtropical East Asian monsoon during early and late summers
Jing Yang1, Qing Bao2, Bin Wang3, Dao-Yi Gong1, Haozhe He1, Miao-Ni Gao1
1 State Key Laboratory of Earth Surface Processes and Resource Ecology (ESPRE), Beijing Normal University, Beijing 100875, China;
2 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), IAP/CAS, Beijing, China;
3 Department of Meteorology, and International Pacific Research Center, University of Hawaii at Manoa, Honolulu, HI, USA.
Abstract: Using Global Precipitation Climatology Project daily rainfall and ERA interim reanalysis data, we investigate the distinct characteristic of quasi-biweekly variation (QBV: 12–20 days) over East Asia (EA) during early (June 10–July 20) and late (July 21–August 31) summer. The QBV maximum variance is found over the core region of EA (30°–40°N, 110°–130°E), which includes eastern China (lower reaches of the Yellow, Huaihe, and Yangtze rivers) and the Korean Peninsula. At both its peak wet and dry phases, QBV over the core region has a baroclinic structure, but with different spatial distributions, different lower-level prevalent wind anomalies, and different upper-level major circulation anomalies in the two subseasons. Meanwhile, the two subseasons have different propagating tracks prior to reaching the peak phase, and different precursors associated with the local genesis of QBV. Furthermore, during the transition from the peak dry to peak wet phase of QBV, the major monsoon circulations have different behaviors that tropical monsoon trough extends eastward in early summer but retreats westward in late summer and the South Asia high (SAH) and western North Pacific (WNP) subtropical high move toward (away from) each other in early (late) summer. The abrupt change of mean state in mid to late July, which includes the northward migration of westerly jet, SAH and WNP, and the weakening and broken of westerly jet, is considered the root cause of the change in behavior of QBV. Finally, we indicate that the tropical monsoon trough and midlatitude westerly jet are possible sources of QBV over subtropical EA in both subseasons and provide useful guidance for 2–3 week predictions over EA.
Keywords: Quasi-biweekly; East Asian summer monsoon.
Published in Climate Dynamics. 2013, DOI 10.1007/s00382-013-1728-6.