Recent Warming of Landfalling Atmospheric Rivers Along the West Coast of the United States

Abstract

Atmospheric rivers (ARs) often generate extreme precipitation, with AR temperature strongly influencing hydrologic impacts by altering the timing and magnitude of runoff. Long‐term changes in AR temperatures therefore have important implications for regional hydroclimate—especially in locations where a shift to more rain‐dominated AR precipitation could affect flood risk and/or water storage in snowpack. In this study, we provide the first climatology of AR temperature across five U.S. West Coast subregions. We then assess trends in landfalling AR temperatures for each subregion from 1980 to 2016 using three reanalysis products. We find AR warming at seasonal and monthly scales. Cool‐season warming ranges from 0.69 to 1.65 °C over the study period. We detect monthly scale warming of >2 °C, with the most widespread warming occurring in November and March. To understand the causes of AR warming, we quantify the density of AR tracks from genesis to landfall and analyze along‐track AR temperature for each month and landfall region. We investigate three possible influences on AR temperature trends at landfall: along‐track temperatures prior to landfall, background temperatures over the landfall region, and AR temperature over the coastal ocean adjacent to the region of landfall. Generally, AR temperatures at landfall more closely match coastal and background temperature trends than along‐track AR temperature trends. The seasonal asymmetry of the AR warming and the heterogeneity of influences have important implications for regional water storage and flood risk—demonstrating that changes in AR characteristics are complex and may not be directly inferred from changes in the background climate.

Responses and impacts of atmospheric rivers to climate change

“…At the surface, where ΔT = ΔT s , specific humidity scales with temperature at the rate of the Clausius-Clapeyron scaling factor, α(T s ). This scaling is approximately 6.6% K −1 for surface temperatures characteristic of ARs making landfall in California in the present climate (T s = 13 °C (reFs 52,60 )). Above the surface, however, the IVT response to warming is complicated by two factors.…”

“…For instance, a 20% increase in AR days has been observed in British Columbia 106 . In addition to increased activity, the temperatures of landfalling ARs to the US West Coast have risen since the 1980s, ranging between 0.69 and 1.65 °C over the entire region, depending on season and latitude 60 . Elsewhere, understanding of observed changes is extremely limited.…”

Inter-model agreement on projected shifts in California hydroclimate characteristics critical to water management

“…1), this also implies that intervening dry periods (e.g., between extreme events and between the wet winter seasons) will intensify. Indeed, recent evidence suggests that some of these non-mean state hydroclimate shifts, including drought intensification due to warming (Diffenbaugh et al 2015;Williams et al 2015), warming of atmospheric river storms (Gonzales et al 2019), autumn/spring drying (Swain et al 2018), and increasing inter-seasonal "precipitation whiplash" (Anenberg et al 2019), are already well underway.…”

Meridional Heat Transport During Atmospheric Rivers in High‐Resolution CESM Climate Projections

“…Research has also been conducted on widely different climate regimes from paleoclimate (e.g., Last Glacial Maximum, Lora et al, 2017, andEocene, Kiehl et al, 2018) to future climates under global warming (Chang et al, 2012;Espinoza et al, 2018;Hagos et al, 2016;Mizuta, 2012;Payne & Magnusdottir, 2015;Shields & Kiehl, 2016a, 2016bWarner et al, 2015). However, other than quantifying the thermodynamical contributions of moisture transport under climate change Sousa et al, 2019) and investigating air temperature within the ARs themselves (Gonzales et al, 2019) Supporting Information:…”