Asymmetric impact of East Asian jet’s variation on midsummer rainfall in North China and Yangtze River Valley

Abstract

Variations of East Asian subtropical westerly jet (EAWJ) have a close relationship with the midsummer rainfall anomalies in Yangtze River Valley (YRV) and North China (NC). The present study shows that the relationship is asymmetric. When EAWJ is located north (south) of its median position for 1979–2013, EAWJ’s southward displacement is associated with the increased (decreased) rainfall anomalies in NC and no obvious (increased) rainfall anomalies in YRV; and vice versa. The mechanism is as follows. EAWJ’s southward displacement is associated with northward flow anomalies over NC and YRV, when EAWJ is located north of the median position. The associated moisture convergence and warm advection anomalies over NC cause increased rainfall anomalies in NC, while rainfall anomalies in YRV are weak due to the weak thermal advection and water vapor convergence/divergence anomalies. When EAWJ is located south of the median position, anomalous southwestward and eastward flow are observed over NC and YRV, respectively, associated with EAWJ’s southward displacement. The resultant cold advection anomalies over NC cause decreased rainfall anomalies in NC, while the resultant anomalous warm advection and moisture convergence over YRV cause positive rainfall anomalies in YRV. The asymmetric circulation changes responsible for the asymmetric rainfall changes in YRV&NC are quasi-barotropic, implying that EAWJ is among the large-scale environmental forcings for the asymmetric rainfall variability in NC&YRV.