Contrasting Historical Trend of Atmospheric Rivers in the Northern Hemisphere

Atmospheric rivers (ARs) are essential ingredients of the global hydrological cycle and extreme weather events especially over mid-latitudes. Increasing studies have shown that ARs will become more frequent, intense, and widespread under the future warming. Whether we have already experienced more ARs in recent decades remains elusive. In this study, we show that the AR frequency in the Northern Hemisphere has experienced an overall increasing trend since 1950 and its long-term change is characterized by an inter-basin contrast (i.e., more prominent over the Atlantic than the Pacific) and a meridional dipole pattern over both oceans (i.e., more prominent to the north). Station-based gridded mean and extreme precipitation also exhibit consistent long-term change patterns. We demonstrate that the increase of water vapor with warming (i.e., thermodynamical contribution) can explain the overall AR frequency increase, while changing large-scale circulations (i.e., dynamical contribution) act to shape the spatial contrast of AR change patterns. Our further analysis with large-ensemble climate simulations suggests that such AR change patterns are partly the forced response to the observed sea surface temperature changes, while atmospheric internal variability also plays an important role.