Impact of El Nino and Southern Oscillation on the summer precipitation over Northwest China

Using the reanalysis data and local weather station data, the relationship between El Nino and Southern Oscillation (ENSO) and the summer precipitation over Northwest China (SPNWC) is investigated. A pronounced rainfall response is observed in post-ENSO summer, especially over North Xinjiang and Yili River valley. A possible mechanism is proposed for such a seasonal lagged impact. El Nino could induce Indian Ocean warming that persists into boreal spring and summer although El Nino itself has dissipated. The warmer Indian Ocean sea surface temperature (SST) would heat the tropospheric atmosphere and elevate the tropical geopotential height at the upper level. As a result, the South Asian High (SAH) exhibits a southward extension due to the increasing geopotential height in its southern flank. A southward displacement of subtropical jet is evident associated with such SAH shift, which further induces a barotropic low pressure anomaly over central Asia. Meanwhile, associated with the southward displacement of subtropical jet, the water vapor from tropical Indian Ocean tends to transport across Iranian Plateau and bring wet and warm air into central Asia. The resultant convergence of water vapor with cold and dry air from high latitude leads to the observed rainfall anomaly to the south and east of Balkhash Lake, explaining the lagged relationship between ENSO and SPNWC. Our results have potential applications on the seasonal prediction of SPNWC.