Responses and impacts of atmospheric rivers to climate change

Abstract

Atmospheric rivers (ARs) are synoptic-scale features characterized by their striking geometry-extending thousands of kilometres in length and an order of magni tude less in width 1-and vertically coherent low-level moisture transport concentrated in the bottom 3 km of the atmosphere 2 (Fig. 1). In total, ARs are estimated to accomplish as much as 90% of poleward moisture transport 3,4 , which, in the North Pacific, averages 700 kg m −1 s −1 (Fig. 1b), more than twice the mean annual discharge found at the mouth of the Amazon River 5. ARs do not describe continuous moisture transport. Rather, they are continually evolving pathways that incorporate moisture from local convergence and evaporation along their track 6,7 or, in select cases, from distant source regions in the tropics or subtropics 8-12. Owing to the complexity of their evolution, our baseline knowledge of AR characteristics at the global scale is uncertain due to the dependency on identification algorithms (Box 1), with factors such as genesis, development and termination only recently being explored 13,14. However, ARs are known to operate as one part of a larger, synoptic-scale dynamical system driving the poleward transport of sensible and latent heat 4,15. They are generally found in the vicinity of extratropical cyclones. Over the North Pacific, for example, 85% of ARs are paired with extratropical cyclones 16 , consistent with their observed relationship with baroclinic instabilities and the mid-latitude storm track 3,6. However, this relationship is nuanced; only 45% of extratropical cyclones over the same region are associated with an AR 16. Similar non-linear relationships are observed in the North Atlantic, where the evolution and life cycle of a single AR can span that of several cyclones 9. While the phenomena are clearly related, their relationship is interactive, with potential implications on the inten sification of storms and the severity of precipitation impacts on land 17,18. Indeed, given their intense moisture transport and moist-neutrality, ARs exhibit conditions that are ideal for forced precipitation, either through interaction with topography or ascent along a warm conveyor belt or frontal boundary 19. Thus, when ARs make landfall, they can have a range of hydrological impacts, including precipitation extremes and related hazards,

Sensitivity of Atmospheric River Vapor Transport and Precipitation to Uniform Sea Surface Temperature Increases

“…The American Meteorological Society (AMS) definition for AR contains little quantitative guidance (AMS, 2019), allowing for flexibility in ARDT development but also contributing to differences across ARDTs. These differences, which include variations in terms of detection variable (e.g., IWV and IVT), thresholds on the intensity of detection variables, geometry, event persistence, and/or other detection considerations, can ultimately affect conclusions about AR characteristics and impacts (O'Brien et al, 2020; Payne et al, 2020; Rutz et al, 2019; Shields et al, 2018). While some ARDTs rely on relative moisture thresholds derived from climatology (e.g., Guan & Waliser, 2015; Lavers et al, 2012), others use absolute thresholds for either IWV (Ralph et al, 2004; Wick et al, 2013) or IVT (Equation ) (Leung & Qian, 2009; Rutz et al, 2013).…”

“…Global‐mean precipitation rates will increase at lower fractional rates than global‐mean IWV due to energetic constraints (2–3% per K surface warming Held & Soden, 2006; O'Gorman & Muller, 2010; O'Gorman et al, 2012); likewise, theoretical considerations for AR precipitation rates also predict lower fractional enhancement than for AR IWV. Since strong ARs are approximately moist‐neutral (Ralph et al, 2017), it is expected that their internal vertical velocities will not change appreciably as the climate warms (O'Gorman, 2015); given this, it can be shown that extreme, nonorographic AR precipitation rates will increase fractionally at the rate of near‐surface e ∗ , which is lower than that for AR IWV (Payne et al, 2020). Since orographic enhancement is a frequent driver of extreme AR precipitation (e.g., Ralph et al, 2006), the previously described changes in the vertical structure of atmospheric moisture have additional consequences.…”

On the crucial role of atmospheric rivers in the two major Weddell Polynya events in 1973 and 2017 in Antarctica

“…In polar regions, where AR activity has been increasing in recent years ( 3 , 4 ), the ability of ARs to rapidly transport large amounts of moisture and heat poleward has significant consequences for both land and sea ice. Their role in short-duration but high-volume melt events over the Arctic and Antarctic has been highlighted in recent years ( 5 ). Research to date has shown that ARs can increase ice melt by several physical mechanisms, including (i) enhancement of the water-vapor greenhouse effect, (ii) the formation of extensive cloud bands that retain outgoing longwave (LW) radiation and re-emit it back to the surface, (iii) the release of condensational latent heat in the advected air mass ( 6 ), (iv) increase in surface melt energy via liquid precipitation ( 7 ), and (v) the generation of turbulent heat fluxes into the ice ( 8 ).…”

Significant increase of global anomalous moisture uptake feeding landfalling Atmospheric Rivers

“…We cannot disregard that under climate change, ARs may be affected by changes in dynamics that could alter the strength of the winds (e.g., 23 ) or increase the anticyclone activity (e.g., 24 ). However, it is generally accepted that thermodynamically driven component dominates 25,26 . In this context, as the amount of moisture in the atmosphere increases, so does the moisture transport.…”