Causes of Atypical Meiyu in the Lower Yangtze River Basin in 2022: Subseasonal Perspective and Predictions

Meiyu is the main rainy season in the middle and lower reaches of the Yangtze River basin. However, the mechanism and predictability of atypical Meiyu are not clear. In 2022, an atypical “hot-and-dry” Meiyu occurred in the lower Yangtze River basin (LYRB). By taking the 2022 dry Meiyu as an example, causes of the atypical Meiyu are diagnosed from the perspective of subseasonal evolution, and its predictability is further evaluated with ECMWF and CFSv2 forecast models. The 2022 atypical Meiyu featured an early onset and a three-stage back-and-forth swing through LYRB, with two sudden meridional rainband shifts: a southward withdrawal in early June and a northward leap in late June respectively. Influenced by the active phases of the Boreal Summer Intraseasonal Oscillation (BSISO) and the “Cold Vortex-Heat Dome” (CVHD) pattern in northern East Asia, the Meiyu rainband was suppressed and retreated southward to South China in early June. A northwestward expansion of WPSH and an abrupt northward leap of the rainband terminated Meiyu near early July due to a sudden northward displacement of the East Asian summer westerly jet and the BSISO2-modulated anomalous cyclone in the subtropics. Two eastward-propagating intraseasonal wave trains along the polar front and subtropical westerly jets played crucial roles in triggering the CVHD pattern and forming an enormous upper-tropospheric anomalous anticyclone over western East Asia, responsible for the jet displacement. Both ECMWF and CFSv2 models exhibited lower prediction skills on extratropical ISOs than tropical ISOs. The models well predicted the rainband position in the first and third stages, but overestimated the LYRB rainfall in the second stage, which was caused by poor representation of the two intraseasonal wave trains and the CVHD pattern. These results highlight the cooperative effects of tropical and extratropical intraseasonal variabilities on atypical Meiyu events.