Understanding the Mechanisms Driving the Poleward Shift of Atmospheric Rivers in the Southern Hemisphere in Recent Decades

Abstract:

The trends in atmospheric river (AR) frequency 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 at around 30 degree S in the past four decades. In line with the trends in AR frequency, the annual total AR-induced precipitation over the Southern Ocean has also been increasing due mostly to the increases in AR frequency. Further analysis on the inter-annual variability of AR centroid latitude, poleward tip latitude and landfall location reveals that AR events have been systematically shifting poleward during this period. To better understand the driving mechanisms, the AR frequency trends are decomposed into the part driven by circulation changes and the part driven by thermodynamic changes by a scaling method. Results show that the observed trends are mostly driven by circulation changes owing to the poleward shift of westerly jet while the thermodynamic contribution confines mostly within the Pacific basin. Decomposing the trends in the fully-coupled CESM experiments indicate that anthropogenic forcing would result in positive trends in AR frequency over the entire Southern Ocean due mostly to moisture changes while the changes in circulation only play a minor role. The difference between the observed trends and the anthropogenically driven trends in the coupled model can be largely reconciled by the inclusion of observed sea surface temperatures (SSTs) in the atmosphere-only CESM simulations: the trends in SSTs toward the negative phase of the Interdecadal Pacific Oscillation (IPO) strongly suppresses the moisture-driven trends while enhances the circulation-induced trends over the entire Southern Ocean, thus bringing the simulated trends into closer agreement with the observed trends.