Subseasonal Change in the Seesaw Pattern of Precipitation between the Yangtze River Basin and the Tropical Western North Pacific during Summer

摘要

There is a well-known seesaw pattern of precipitation between the tropical western North Pacific (WNP) and the Yangtze River basin (YRB) during summer.This study identified that this out-of-phase relationship experiences a subseasonal change;that is,the relationship is strong during early summer but much weaker during mid-summer.We investigated the large-scale circulation anomalies responsible for the YRB rainfall anomalies on the subseasonal timescale.It was found that the YRB rainfall is mainly affected by the tropical circulation anomalies during early summer,i.e.,the anticyclonic or cyclonic anomaly over the subtropical WNP associated with the precipitation anomalies over the tropical WNP.During mid-summer,the YRB rainfall is mainly affected by the extratropical circulation anomalies in both the lower and upper troposphere.In the lower troposphere,the northeasterly anomaly north of the YRB favors heavier rainfall over the YRB by intensifying the meridional gradient of the equivalent potential temperature over the YRB.In the upper troposphere,the meridional displacement of the Asian westerly jet and the zonally oriented teleconnection pattern along the jet also affect the YRB rainfall.The subseasonal change in the WNP-YRB precipitation relationship illustrated by this study has important implications for the subseasonalto-seasonal forecasting of the YRB rainfall.