Poleward Shift of Atmospheric Rivers in the Southern Hemisphere in Recent Decades

The atmospheric river (AR) frequency trends over the Southern Hemisphere are investigated using three reanalyses and two Community Earth System Model (CESM) ensembles. The results show that AR frequency has been increasing over the Southern Ocean and decreasing over lower latitudes in the past four decades and that ARs have been shifting poleward. While the observed trends are mostly driven by the poleward shift of the westerly jet, fully coupled CESM experiments indicate anthropogenic forcing would result in positive AR frequency trends over the Southern Ocean due mostly to moisture changes. The difference between the observed trends and anthropogenically driven trends can be largely reconciled by the atmosphere‐only CESM simulations forced by observed sea surface temperatures: Sea surface temperature variability characteristic of the negative phase of the Interdecadal Pacific Oscillation strongly suppresses the moisture‐driven trends while enhances the circulation‐induced trends over the Southern Ocean, thus bringing the simulated trends into closer agreement with the observed trends. Plain Language Summary Atmospheric rivers (ARs) are “rivers” in the sky that carry a huge amount of water vapor equivalent to the world's largest rivers on the ground. Because of the amount of water vapor they carry, ARs can cause extreme rainfall and floods upon landfall when the moisture is condensed out by mountain barriers or other favorable conditions. Most of the studies so far have focused on ARs in the Northern Hemisphere. In this study, we found that ARs in the Southern Hemisphere have been occurring at increasingly higher‐latitude regions in the past four decades and that this observed trend is mostly due to winds getting stronger at higher‐latitude regions in the Southern Hemisphere. Using results from climate model simulations, we show that the observed AR trends in the Southern Hemisphere are driven by both human activities, such as greenhouse gas increases and ozone depletion, and the internal variability of sea surface temperatures due to the coupled atmosphere‐ocean system. The poleward shift of ARs in the Southern Hemisphere may have major implications for the amount of moisture and heat transported into Antarctica, thus may also have profound impacts on the Antarctic and in turn, global climate. Key Points Atmospheric rivers (ARs) have shifted poleward in the Southern Hemisphere over the past four decades Anthropogenic forcing evidently plays a role in thi