Impacts of Seasonal Transitions of ENSO on Atmospheric River Activity over East Asia

29 Atmospheric rivers (ARs), narrow water vapor transport bands over the mid-latitudes, 30 often cause great socio-economic impacts over East Asia. While it has been shown that 31 summertime AR activity over East Asia is strongly induced by preceding-winter El Niño 32 development, it remains unclear the extent to which seasonal transitions of El Niño 33 Southern Oscillation (ENSO) from winter to summer affect the AR activity. Here we 34 examine the relationship between the seasonal transitions of ENSO and the summertime 35 AR activity over East Asia using an atmospheric reanalysis and high-resolution 36 atmospheric general circulation model (AGCM) ensemble simulations. A rapid transition 37 from preceding-winter El Niño to summertime La Niña results in more AR occurrence over 38 northern East Asia via northward expansion of an anomalous low-level anticyclone over the 39 western North Pacific compared to sustained or decayed El Niño cases. The northward 40 expansion of the anticyclone is consistent with a steady response of the atmosphere to the 41 anomalous condensation heating over the Maritime Continent and equatorial Pacific. 42 Meridional positions of the extratropical AR occurrence and circulation anomalies are 43 different between the reanalysis and AGCM simulations, which is possibly contributed by a 44 limited sample size and/or AGCM biases and suggests that seasonal prediction of 45 AR-related natural disaster risk over East Asia on a regional scale remains a challenge.