Simulating Springtime Extreme Rainfall in East Asia during Eastern-Pacific El Niño - Importance of Synoptic-Scale Activities and the Westerly Waveguide

El Niño–Southern Oscillation is the most important source of interannual variability in the tropics; it also exerts great influences on weather and climate systems in local and remote regions through teleconnections. Observed influences of canonical (or eastern Pacific) El Niño on springtime extreme rainfall in East Asia (EA) are studied, and compared with the Coupled Model Intercomparison Project phase 6 (CMIP6) historical runs. Both model and observational data indicate that the anomalous low-level western north Pacific (WNP) anticyclone is the primary contributor to EA springtime extremes during El Niño. On a day-to-day basis, intense daily precipitation is related to enhanced upper-level synoptic-scale waves. Here we use a temperature advection index (TAI) to represent the amount of synoptic-scale activities. It was found that, when EP El Nino occurs, 85% of Yangtze River Basin (YRB)-South Korea (SK)-south of Japan (SP) extreme events are accompanied by instances of positive TAI (as compared to 72% in the climatological sense). However, such a change of association with TAI is not found in CMIP6. Observations further show a stationary wave pattern trapped along the intensified EA westerly jet during EP El Niño, which favors the development of synoptic-scale activity. There is also enhanced moisture transported from WNP to SK-SP, leading to more extreme precipitation in the region. In contrast, the interannual-scale westerly waveguide effect during EP El Niño is poorly simulated in CMIP6 models, resulting in models’ failure in capturing the contemporaneous YRB-SK-SP extreme precipitation changes.