Rainy season onset over Northeast China and the related atmospheric circulations

摘要

Northeast China is one of the major commodity crop bases in East Asia, thus, it plays an important role in regional and global food security. The transition from a dry to a wet season, that is, the beginning of the rainy season, is critical for agricultural planning in the region. In this study, the rainy season onset (RSO) dates in Northeast China are first determined based on the "anomalous accumulation" (AA) method, and the RSO-related atmospheric circulations are then explored. The results show that the mean RSO date in Northeast China is May 13, with a standard deviation of 13 days. The RSO is related to the sudden changes in atmospheric circulations, including the weakening of the East Asian trough and the East Asian jet stream, and the northward movement of the convergence zone. The RSO dates for Northeast China show strong year-to-year variability. The composite difference in the atmospheric circulations between earlier and later RSO years implies that a meridional dipole pattern reflected by anomalous 500 hPa geopotential height and zonal wind over East Asia is closely related to the RSO interannual variability. This dipole pattern is accompanied by southeasterly (northwesterly) wind anomalies in the middle and low troposphere over Northeast China during the earlier (later) RSO years, consequently enhancing (weakening) moisture transportation and moist static energy over the region and thereby favouring an advanced (delayed) seasonal transition from the dry to wet season. The RSO dates revealed in this study are valuable for guiding spring sowing practices in the region.