Poleward Shift of Atmospheric Rivers in the Southern Hemisphere in Recent Decades
Weiming Ma, Gang Chen, Bin Guan
Abstract
Abstract 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.