Connection of the Summer Onset in Northern Asia with the Precipitation Anomalies in Eastern China during the Meiyu Period

In early June, there normally appears a rapid increase followed by a near steadiness of area-mean temperature of northern Asia, which features the transition of general circulation from winter to summer. Based on the timing of summer onset in northern Asia each year (hereafter referred to as summer onset) from 1951 to 2017, along with NCEP reanalysis data and the observation data in China, the connection of the summer onset with the precipitation and circulation anomalies in China during the Meiyu period is studied. It is attained that in the early (late) summer onset years, there is a “+ − +” (“− + −”) meridional wave train like structure of anomaly geopotential height field at mid-troposphere (500 hPa) ranging from northeast Asia, northeast area of China, and western Pacific Ocean. Similarly, a wave train structure is also found in the anomaly wind field at 850 hPa, i. e. a cyclonic (anticyclonic) circulation over northeastern area of China and an anticyclonic (cyclonic) circulation over the subtropical western Pacific. These circulation anomalies lead to a less (more) precipitation in the Yangtze River basin, and flood (drought) in the Northeast China during the Meiyu period. In the paper, a preliminary investigation on the approach by which the timing of summer onset influences the anomaly circulation and precipitation in Meiyu period is also attempted. It is shown that in the early (late) summer onset years, the ridge over northeastern Asia is established earlier (later) and stronger (weaker) than the normal. The time of the summer onset affects the ridge over northeastern Asia and the circulation and precipitation anomalies in the Eastern China during Meiyu period. The anomalous ridge over northeastern Asia maintains till the end of Meiyu. As the ridge over northeastern Asia is stronger (weaker) favors the formation and maintenance of “+ − +” (“− + −”) meridional wave train along the coast of East Asia, hence the distribution of rain belt.