Atmospheric River Activity in a 4xCO2 Climate Simulation

Atmospheric rivers (ARs) are plume-like structures that transport high volumes of water vapor and can result in large amounts of precipitation on land causing damage to the surrounding environment. ARs recently gained media attention during February 2017, when strong AR events impacted the North American west coast, especially the state of California where they caused extensive flooding and damage to roadways and dams. An important societal question is how ARs will change in a warmer climate. To better understand how AR activity on the west coast and other parts of the North Pacific will change in the future, AR activity was diagnosed in a version of the Superparameterized Community Earth System Model (SP-CESM) in pre-industrial and 4xCO₂ climates. ARs were diagnosed using the algorithm of Mundhenk et al. (2016). Overall, U.S. west coast AR activity is projected to increase by 25% in a warmer climate, potentially posing negative societal impacts on populated regions.

Anomalous AR activity as a function of Madden-Julian Oscillation (MJO) phase was also assessed in pre-industrial and 4xCO₂ runs. In a 4xCO₂ climate, land falling AR activity shows increases in amplitude and pattern changes in certain MJO phases compared to pre-industrial climate, and AR frequency is also increased in phases that do not have strong anomalies in current climate.These results highlight that although the MJO can be used as an accurate predictor of AR activity in current climate, changes in AR activity as a function of MJO phase in future climate will present challenges to current statistical prediction models of AR activity based on MJO phase.