Diagnosing the connections between western US extreme precipitation and remote forcing

Abstract:

It is critically important to determine how extreme precipitation events may be altered in a changing climate. We present recent research to develop an atmospheric river (AR) detection algorithm to investigate the downstream modulation of the eastern North Pacific ARs by another weather extreme, the East Asian cold surge (EACS), in both reanalysis data and high-resolution global model simulations. It is shown that following the peak of an EACS, atmospheric disturbances of intermediate frequency (IF; 10–30 day period) are excited downstream. This leads to the formation of a persistent cyclonic circulation anomaly over the eastern North Pacific that dramatically enhances the AR occurrence probability and the surface precipitation over the western U.S. between 30°N and 50°N. A diagnosis of the local geopotential height tendency further confirms the essential role of IF disturbances in establishing the observed persistent anomaly. This downstream modulation effect is then examined in the two simulations of the National Center for Atmospheric Research and U.S. Dept. of Energy Community Climate System Model version 4 with different horizontal resolutions (T85 and T341) for the same present-day period (1979–2005). The connection between EACS and AR is much better captured by the T341 version of the model, mainly due to a better representation of the scale interaction and the characteristics of IF atmospheric disturbances in the higher-resolution model. The findings here suggest that faithful representations of scale interaction in a global model are critical for modeling and predicting the occurrences of hydrological extremes in the western U.S. and for understanding their potential future changes.