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
• Format: Text
• Language: English
• Published: Karlsruher Institut für Technologie (KIT) 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; Iceland; North Atlantic; North Atlantic oscillation
• Online Access: https://dx.doi.org/10.5445/ir/1000123919; https://publikationen.bibliothek.kit.edu/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 Iceland low, and their impact is much more spatially coherent. This suggests that while ACRE lead to an increase in NAO persistence on synoptic time-scales, their impact is relatively minor and much smaller compared to other diabatic processes. To test the robustness of our PTE-based hypothesis, numerical simulations in ICON are carried out. The PTE analysis in ICON shows results that are qualitatively consistent with the observational analysis, in particular regarding the feedback mechanisms of ACRE and total diabatic heating, which is dominated by latent heating. These PTE-based results are further tested by means of sensitivity simulations in ICON, where a NAO-related diabatic heating pattern is imposed either due to ACRE or total diabatic heating. These heating patterns are based on 5-day-mean NAO regressions of either ACRE or total diabatic heating. The sensitivity simulations confirm the observational hypothesis and show that ACRE feed back positively by up to 1–2% of 1σ NAO, while the total diabatic heating feeds back positively by up to 10% of 1σ NAO. Overall, the observational and modeling work both illustrate the substantial impact of the total diabatic heating for the NAO, while ACRE play a minor role. This highlights that diabatic processes are essential for understanding and accurately modeling the NAO short-term dynamics.