Influences of Atmospheric Rivers on North Pacific Winter Precipitation: Climatology and Dependence on ENSO Condition

Abstract

Atmospheric river (AR) is a unique mover of moisture from the low latitudes to mid-high latitudes and a potential cause for regional flooding. This study explores ARs' influence on wintertime precipitation (including extreme precipitation) over the pan-North Pacific during 1996-2018 from climatological perspective and during ENSO condition, via distinguishing precipitation and moisture transport between AR-group and non-AR-group. Climatologically, AR-group contributes 30-45% of total winter precipitation, and up to 70% of total extreme precipitation over the eastern Pacific and along the North American west coast between 25°-45°N. The above area is the core region of ARs’ influence on precipitation. AR-group's precipitation in the core region is mostly determined by AR-produced in-situ moisture convergence, while non-AR-group's precipitation is attributed to evaporation from the earth surface.ARs frequency is increased/decreased in the core region during El Niño/La Niña winters. As a result, AR-group contributes more than half of the positive/negative anomalies in total precipitation in the core region, due to AR-related moisture convergence anomalies. Besides, during El Niño winters, AR-group also contributes almost half of negative anomalies in total precipitation over the middle basin between 40°-55°N. An anomalous moisture sink/source always acts to increase/decrease in-situ precipitation, no matter for AR-group or non-AR-group. It is also shown that the moisture transport and its divergence anomalies are primarily decided by changes in wind fields and secondly by moisture. Furthermore, changes in moisture can cause asymmetric features of anomalies in moisture transport and its divergence between El Niño and La Niña but with limited effects.