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

<p>El Ni&#241;o&#8211;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 Ni&#241;o 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 Ni&#241;o. 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 Ni&#241;o, 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 Ni&#241;o is poorly simulated in CMIP6 models, resulting in models&#8217; failure in capturing the contemporaneous YRB-SK-SP extreme precipitation changes.</p>