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

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
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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.