The role of cloud-radiative effects and diabatic processes for the dynamics of the North Atlantic Oscillation on synoptic time-scales

• Main Author: Papavasileiou, Georgios
• Other Authors: Knippertz, P.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: KIT-Bibliothek, Karlsruhe 2020
• Subjects: atmospheric cloud-radiative effects (ACRE); clouds; radiation; cloud–circulation coupling; diabatic processes; latent heating; North Atlantic Oscillation (NAO); ICON; surface pressure tendency equation (PTE); diabatic heating; numerical modeling; CloudSat/CALIPSO; CERES; GERB; ERA5; ERA-Interim; ddc:550; Earth sciences; info:eu-repo/classification/ddc/550; Iceland; North Atlantic; North Atlantic oscillation
• Online Access: https://publikationen.bibliothek.kit.edu/1000123919; https://publikationen.bibliothek.kit.edu/1000123919/88241538; https://doi.org/10.5445/IR/1000123919

Clouds shape weather and climate by regulating the latent and radiative heating in the atmosphere. Recent work demonstrated the importance of cloud-radiative effects (CRE) for the mean circulation of the extratropical atmosphere and its response to global warming. In contrast, little research has been done regarding the impact of CRE on internal variability. During the northern hemisphere winter the dominant mode of atmospheric variability over the North Atlantic and the surrounding continental areas of North America and Europe is the North Atlantic Oscillation (NAO). Here, we study how clouds and the NAO couple on synoptic time-scales during northern hemisphere winter via CRE within the atmosphere (ACRE) in observations and model simulations. A regression analysis based on 5-day-mean data from CloudSat/CALIPSO reveals a robust dipole of cloud-incidence anomalies during a positive NAO, with increased high-level clouds along the storm track (near 45°N) and the subpolar Atlantic, and decreased high-level clouds poleward and equatorward of it. Opposite changes occur for low-level cloud incidence. Satellite retrievals from CloudSat/CALIPSO, CERES and GERB as well as ERA-Interim short-term forecast data show that these cloud anomalies lead to an anomalous column-mean heating due to ACRE over the region of the Iceland low, and to a cooling over the region of the Azores high. To quantify the impact of the ACRE anomalies on the NAO, and to thereby test the hypothesis of a cloud-radiative feedback on the NAO persistence, we apply the surface pressure tendency equation (PTE) to ERA-Interim short-term forecast data. The NAO-related surface pressure tendency anomalies due to ACRE amplify the NAO-related surface pressure anomalies over the Azores high but have no area-averaged impact on the Iceland low. In contrast, surface pressure tendency anomalies due to total diabatic heating, including latent heating and clear-sky radiation, strongly amplify the NAO-related surface pressure anomalies over both the Azores high and the ...