Explaining the Spatial Pattern of U.S. Extreme Daily Precipitation Change

Observed U.S. trends in the annual maximum 1-day precipitation (RX1day) consist of 15% - 25% increases over the East and slight decreases over the West during the last century. This heterogeneous trend pattern departs from comparatively uniform observed increases in atmospheric moisture availability over the contiguous U.S. Here we present results using an event attribution framework involving parallel sets of global atmospheric model experiments with and without climate change drivers to explain this spatially diverse pattern of U.S. extreme daily precipitation trends. It is shown that the RX1day events in our model ensembles respond to historical climate change forcing differently across the U.S. up to 9% increased intensity over the East but no change over the West. This spatially diverse forced signal is highly correlated with the observed trend pattern. It is shown that the absence of RX1day signals over the West is due to dynamical effects of climate change forcing - via a wintertime atmospheric circulation anomaly that suppresses vertical motion over the West - largely cancelling thermodynamic effects of increased water vapor availability. Concerning the large observed RX1day increases in the East, we show evidence that this is unlikely a symptom of a regional heightened sensitivity to climate change forcing, but most likely resulted from a superposition of strong internal variability with a moderate climate change signal. Implications for future changes in U.S. extreme daily precipitation are discussed.