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

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. 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.

Relationship Between Weather Regimes and Atmospheric Rivers in East Antarctica

“…Here, long-term evolutions mostly involve the latitudinal dimension. Changes in the general circulation and especially the expected meridional shifts of the storm tracks (either driven by increasing greenhouse effect or ozone recovery: e.g., Thompson et al, 2011), low-frequency evolutions in the state of the SAM, and ongoing warming trends, could all modify the intrinsic properties of both regimes and AR events, that is, their recurrence, location (Ma et al, 2020), extension and intensity. Potentially, they could also change their effects on the ice sheet, for example, through stronger extreme precipitation due to Clausius-Clapeyron scaling (Betts & Harshvardhan, 1987;Kharin et al, 2007;Muller et al, 2011;O'Gorman & Schneider, 2009;Pall et al, 2007;Trenberth et al, 2003;Westra et al, 2014), or through enhanced melt occurring under warmer surface air temperatures, especially near the coast (Wille et al, 2019).…”

Atmospheric River Lifecycle Responses to the Madden‐Julian Oscillation

“…Finally, questions have been raised about how ARs will change in the future climate. AR occurrence is projected to increase (Dettinger, 2011;Lavers et al, 2013;Warner et al, 2015) with longer and wider geometric shapes, stronger intensities, and a poleward shift of landfall locations (Espinoza et al, 2018;Ma et al, 2020;Radic et al, 2015;Shields & Kiehl, 2016). The MJO is projected to have deeper and larger convection and to travel further eastward with increasing phase speed (Adames et al, 2017;Chang et al, 2015;Maloney et al, 2019).…”

Winter and spring atmospheric rivers in High Mountain Asia: climatology, dynamics, and variability

“…ARs obtain their high water vapor content from tropical or extratropical moisture sources, moisture convergence, or local evaporation and are often found in the area ahead of the cold front of extratropical cyclones (Rutz et al 2014;Dacre et al 2015). Many studies have shown the global importance of ARs to poleward moisture transport, climate, and water budgets (Zhu and Newell 1998;Guan and Waliser 2017;Paltan et al 2017;Waliser and Guan 2017;Guan et al 2018;Nash et al 2018;Ma et al 2020). Poleward integrated vapor transport (IVT) from ARs makes up over 90% of the total moisture transport in the mid to high latitudes (Zhu and Newell 1998;Guan and Waliser 2015).…”