Abstract
Atmospheric rivers (ARs) are critical to the hydrological cycle of the western United States with both favorable and formidable impacts to society based on their landfalling characteristics. In this study, we provide a first-of-its-kind evaluation of how landfalling ARs may respond to several stabilized warming scenarios. To do this we combine a recently developed AR detection workflow with an ensemble of uniform high-resolution (0.25°) Community Earth System Model simulations designed to facilitate detection and attribution of extreme events with global warming. These simulations include a world that might have been in the absence of anthropogenic warming (+0◦C), a world that corresponds to present day warming (+0.85◦C), and several future worlds corresponding to +1.5◦C, +2◦C and +3◦C global warming. We show that warming increases the number of water management relevant landfalling ARs from 19.1 ARs per year at +0◦C to 23.6 ARs per year at +3◦C. Additionally, this warming intensifies the amount of water transported by landfalling ARs resulting in a decrease in the fraction of ARs that are “mostly to primarily beneficial” to water resource management (i.e., 91% of ARs at +0◦C to 78% at +3◦C) and an increase in the fraction of ARs that are “mostly or primarily hazardous” to water resource management (i.e., 2% of ARs at +0◦C to 8% at +3◦C). Shifts in AR character also have important ramifications on flood damages, whereby for every +1◦C of additional warming from present conditions annual average flood damages increase by ~$1 billion. These findings highlight the pragmatic implications of climate mitigation aimed at limiting global warming to under +2◦C.
Highlights
The meridional branch of the atmospheric portion of the global hy drologic cycle occurs primarily via filamentary pulses of anomalously high water vapor (Newell et al, 1992)
Similar to results found in Rhoades et al (2020b), a decrease in the annual average interval between western United States (US) landfalling atmospheric rivers (ARs) is found from +0◦C (21.5 days) to +3◦C (18.0 days) and, +1 AR per ◦C per year occurs within one-week of another
Back-to-back AR events that occur within one-to-two weeks of one another are impactful to water resource managers as they precondition the land-surface to generate amplified and abrupt runoff and can stress-test flood pool assumptions built into reservoir operation (Hatchett et al, 2020; Henn et al, 2020; Sumargo et al, 2020)
Summary
The meridional branch of the atmospheric portion of the global hy drologic cycle occurs primarily via filamentary pulses of anomalously high water vapor (Newell et al, 1992). Feedbacks that alter thermodynamic processes arise primarily from the Clausius-Clapeyron relationship (i.e., column-integrated water vapor increases at approximately +7.5%/K (Held and Soden, 2006)) that in turn influences the magnitude and variability of moisture sources that seed ARs. Feedbacks to dynamical processes occur primarily through modifications to AR lifecycles through synoptic to mesoscale changes in the seasonal storm-track location and its variability and through more localized changes in wind speed, direction, and shear. Feedbacks to dynamical processes occur primarily through modifications to AR lifecycles through synoptic to mesoscale changes in the seasonal storm-track location and its variability and through more localized changes in wind speed, direction, and shear The interaction between these two processes can regionally offset one another (Payne et al, 2020) and lead to more heterogeneous responses that may impact AR families (Fish et al, 2019) or flavors (Gonzales et al, 2019) in different ways. The relative strength and interplay between these processes is emissions scenario dependent and will likely differ across various spatiotemporal scales and time horizons of climate change
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