Abstract
AbstractClouds shape weather and climate by regulating the latent and radiative heating in the atmosphere. Recent work has 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. Here, we study how clouds and the North Atlantic Oscillation (NAO) couple on synoptic time‐scales during Northern Hemisphere winter via CRE within the atmosphere (ACRE). A regression analysis based on 5‐day mean data from CloudSat/CALIPSO, CERES and GERB satellite observations and ERA‐Interim short‐term forecast data reveals a robust dipole of high‐level and low‐level cloud‐incidence anomalies during a positive NAO, with increased high‐level cloud incidence along the storm track (near 45°N) and the subpolar Atlantic, and decreased high‐level cloud incidence poleward and equatorward of this track. Opposite changes occur for low‐level cloud incidence. The cloud anomalies lead to an anomalous column‐mean heating from 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 for ERA‐Interim short‐term forecast data. The NAO‐generated ACRE anomalies amplify the NAO‐related surface pressure anomalies over the Azores high but have no area‐averaged impact on the Iceland low. In contrast, diabatic processes as a whole, 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 atmospheric cloud‐radiative effects lead to an increase in NAO persistence on synoptic time‐scales, their impact is relatively minor and much smaller than other diabatic processes.
Highlights
The weather and climate of the North Atlantic region and the neighbouring North American and European continental areas exhibit considerable variability on a wide range of spatiotemporal scales
The observational work of Li et al (2014a) hypothesised that cloud-radiative effects (CRE) might represent an important and so far overlooked factor for the North Atlantic Oscillation (NAO) and its variability. We investigate this hypothesis in more detail by combining a suite of satellite observations, reanalysis data, and a dynamical analysis based on the surface pressure tendency equation
In the previous section we found that a positive NAO leads to anomalous atmospheric cloud-radiative effects (ACRE) cooling over the Azores high and ACRE heating over the Iceland low
Summary
The weather and climate of the North Atlantic region and the neighbouring North American and European continental areas exhibit considerable variability on a wide range of spatiotemporal scales. The observational work of Li et al (2014a) hypothesised that cloud-radiative effects (CRE) might represent an important and so far overlooked factor for the NAO and its variability We investigate this hypothesis in more detail by combining a suite of satellite observations, reanalysis data, and a dynamical analysis based on the surface pressure tendency equation. Li et al (2014a) investigated the relation between clouds and the Arctic Oscillation (to which they refer as the Northern Annular Mode/North Atlantic Oscillation; NAM/NAO) on monthly time-scales using satellite observations and reanalysis data They found that the NAM/NAO is associated with robust changes in high-level clouds and top-of-atmosphere CRE, which in the zonal mean works against the temperature anomalies accompanying the NAM/NAO. We remap the fluxes onto a common 0.1◦ grid and derive TOA and BOA long-wave CRE as well as vertically integrated long-wave ACRE
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