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