Direct ENSO impact on East Asian summer precipitation in the developing summer

In the developing stage of ENSO, the East Asia summer precipitation (EASP) shows a large variability that is significantly different from that in the decaying summer. In this study, we will focus on understanding the direct El Nino impact on the precipitation over East Asia in the developing summer in the observation. It is found that in its developing summer, the El Nino sea surface temperature anomaly affects the EASP directly from the eastern-central tropical Pacific, with little interference from the rest of the global ocean. The corresponding precipitation anomaly exhibits a tri-pole pattern, with two positive nodes in northeast and southeast China, sandwiched by a negative node in northern/central China. The tri-pole precipitation response is mainly attributed to the El Nino-induced cyclonic anomaly in Northeast Asia and the anticyclonic anomaly in the western North Pacific, which are part of the circulation anomalies of a circumglobal wave teleconnection in the subtropical jet in the Northern Hemisphere and a low level meridional wave train along East Asia coast. These circulation anomalies are generated by the summer El Nino in three pathways: (1) the vertical motion-induced perturbation over the central-eastern tropical Pacific entering into the subtropical jet excites a circumglobal wave train propagation eastward along the jet; (2) the El Nino-induced dipole heating across the equatorial Maritime Continent is mainly responsible for the meridional wave propagation along East Asia coast; (3) the El Nino-induced indirect heating over Northwest India triggers another perturbation in the jet waveguide, all contributing to the precipitation variation in East Asia. Further demonstration indicates the atmospheric response to the El Nino direct heating and perturbation over the tropical Pacific has the major contribution to the El Nino-induced circulation anomaly. As to the El Nino indirect heating over Northwset India, a zonal wave train response in the upper midlatitude which is mainly confined in the Eurasia sector makes a competing contribution to the circulation anomaly over East Asia.