The Linkage between Two Types of El Niño Events and Summer Streamflow over the Yellow and Yangtze River Basins

It is generally agreed that El Niño can be classified into East Pacific (EP) and Central Pacific (CP) types. Nevertheless, little is known about the relationship between these two types of El Niño and land surface climate elements. This study investigates the linkage between EP/CP El Niño and summer streamflow over the Yellow and Yangtze River basins and their possible mechanisms. Over the Yellow River basin, the anomalous streamflow always manifests as positive (negative) in EP (CP) years, with a correlation coefficient of 0.39 (−0.37); while over the Yangtze River basin, the anomalous streamflow shows as positive in both EP and CP years, with correlation coefficients of 0.72 and 0.48, respectively. Analyses of the surface hydrological cycle indicate that the streamflow is more influenced by local evapotranspiration (ET) than precipitation over the Yellow River basin, while it is dominantly affected by precipitation over the Yangtze River basin. The different features over these two river basins can be explained by the anomalous atmospheric circulation, which is cyclonic (anticyclonic) north (south) of 30°N over East Asia. EP years are dominated by two anticyclones, which bring strong water vapor convergence and induce more precipitation but less ET, and subsequently increase streamflow and flooding risks. In CP years, especially over the Yellow River basin, two cyclones dominate and lead to water vapor divergence and reduce moisture arriving. Meanwhile, the ET enhances mainly due to local high surface air temperature, which further evaporates water from the soil. As a result, the streamflow decreases, which will then increase the drought risk.