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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 upperlevel 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.

The Yangtze–Huaihe River basin (YHRB) is the core region of sultry heat wave occurrence over China during peak summer [July and August (JA)]. The extremely hot and muggy weather is locally controlled by a descending high pressure anomaly connected to the western Pacific subtropical high. During 1961–2015, the heat wave days (HWDs) in JA over the YHRB exhibit large year-to-year and decadal variations. Prediction of the total number of HWDs in JA is of great societal and scientific importance. The summer HWDs are preceded by a zonal dipole SST tendency pattern in the tropical Pacific and a meridional tripole SST anomaly pattern over the North Atlantic. The former signifies a rapid transition from a decaying central Pacific El Niño in early spring to a developing eastern Pacific La Niña in summer, which enhances the western Pacific subtropical high and increases pressure over the YHRB by altering the Walker circulation. The North Atlantic tripole SST anomalies persist from the preceding winter to JA and excite a circumglobal teleconnection pattern placing a high pressure anomaly over the YHRB. To predict the JA HWDs, a 1-month lead prediction model is established with the above two predictors. The forward-rolling hindcast achieves a significant correlation skill of 0.66 for 1981–2015, and the independent forecast skill made for 1996–2015 reaches 0.73. These results indicate the source of predictability of summer HWDs and provide an estimate for the potential predictability, suggesting about 55% of the total variance may be potentially predictable. This study also reveals greater possibilities for dynamical models to improve their prediction skills.

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