Atmospheric rivers in the arctic: Physical processes and impact on the surface energy budget

The Arctic climate system has changed rapidly in recent decades, with mean warming exceeding 2 K, making it the global warming hotspot. This phenomenon, known as Arctic Amplification, is driven by feedback mechanisms reacting to anthropogenic greenhouse effects, including cloud radiative effects, water vapor feedback and temperature feedbacks. Poleward moisture transport also shows changes correlating with Arctic warming. A substantial part of this transport occurs via so-called “Atmospheric Rivers” (ARs). These narrow atmospheric bands are often linked to extratropical cyclones, and transport moist and warm air into polar regions. ARs are linked to sea ice melt, precipitation in both the form of rain and snow, suggesting a warming effect on the Arctic surface. This thesis examines the influence of ARs on the Arctic surface energy budget (SEB) through case studies in early winter and spring, combining ERA5 reanalysis data (1979-2021) and regional simulations with the ICON model. Sensitivity studies explore how lateral moisture inflow affects the SEB, analyzed from Eulerian and Lagrangian perspectives for detailed process insights. Using the Guan-Waliser AR detection algorithm and ERA5, the climatological impacts of ARs on the surface fluxes, as well as moisture, cloud content, and skin temperatures are statistically quantified, with trends assessed between the "old Arctic" (1979-1999) and "new Arctic" (2000-2021). Two AR types are distinguished: moisture-dominated (“wet”) and wind-dominated (“windy”). Furthermore, the occurrence of ARs in the Atlantic Sector is linked to atmospheric circulation regimes. ARs generally warm the Arctic surface, with a strong dependence on the season and surface type. Winter ARs over open ocean show the strongest impact. Overall, the SEB anomalies are mostly dominated by turbulent flux anomalies (latent and sensible heat), i.e. near-surface temperatures and moisture. Radiative effects from increased liquid water and vapor lead to stronger terrestrial downward radiation. In summer, weaker turbulent flux anomalies and the additional cloud albedo effects lead to less strong SEB anomalies, sometimes even leading to less energy uptake of the surface. ARs significantly contribute to the seasonal mean SEB and skin temperatures, particularly in the winter North Atlantic Ocean (up to 20 W m⁻², 1 K). Moisture-dominated ARs induce stronger winter, autumn and spring surface energy gains than wind-dominated ones; and less energy gain in summer. This difference is due to higher liquid water paths, integrated vapor and related stronger radiative effects, and stronger latent heat flux anomalies. Over 45 years, sea-ice decline has influenced AR-related energy flux anomalies, especially by changes in the turbulent heat fluxes. Most ARs occur during the prevalence of the atmospheric circulation regimes NAO+ and SCAN, with SCAN being especially linked to ARs in the central Arctic. These two circulation regimes are further linked to most of the wind-dominated ARs. The regional confinement is especially pronounced in winter. Changes in occurrence frequency of the regimes over the last 45 years correspond qualitatively to AR occurrence changes.