Anomalous Meiyu onset averaged over the Yangtze River Valley

A range of Meiyu onset dates (MODs) have been presented but they are highly controversial. The problem is, however, crucial to local activities and meteorological service. The 1957-2001 MODs determined at several typical stations in the mid- and lower reaches of the Yangtze River valley (MLYRV) are employed to construct a regionally averaged index for quantifying the Meiyu onset time in the valley, and the ocean-atmospheric features in the extremely early and late Meiyu onset years are investigated, statistically. Results show that, typically, the Meiyu season starts in June at a regional mean level, and the MOD is of good relationship with the Meiyu duration that dominates the total precipitation during the Meiyu period, i.e., the earlier the Meiyu establishes, the longer it may persist, and thus the more the total rainfall might be. In June, at the lower troposphere the meridional temperature gradient is much larger over the MLYRV with the Meiyu front more northward in early than late onset years. Additionally, the early onset years are often accompanied in the upper troposphere by the extended South Asia High, western Pacific subtropical high more north-westward, East Asian summer monsoon and Walker circulation stronger in comparison to the late onset years. And viewed from sea surface temperature (SST) field for the early onset years, SST is lower in the central and eastern Pacific near the equator, maybe in association with the cool phase of ENSO, and considerably higher in the subtropical western North Pacific than for the late onset years. In addition, there are probably some abnormal precursory signals for the early (late) Meiyu onset years, such as SST being higher (lower) with respect to the mean around the western Pacific warm pool, the Intertropical Convergence Zone more (less) active and the local Hadley circulation stronger (weaker) in previous boreal winter. In northern spring, the convection is probably stronger (weaker) about the Philippines. The North Pacific SST at lower latitudes might be warmer (cooler) from previous April to May, and SST appears higher (lower) over the waters around Australia in proceeding boreal winter and spring.