The Atmospheric Pathway of the Cloud-Radiative Impact on the Circulation Response to Global Warming: Important and Uncertain

Abstract

Previous work showed that the poleward expansion of the annual-mean zonal-mean atmospheric circulation in response to global warming is strongly modulated by changes in clouds and their radiative heating of the surface and atmosphere. Here, a hierarchy and an ensemble of global climate models are used to study the circulation impact of changes in atmospheric cloud-radiative heating in the absence of changes in sea surface temperature (SST), which is referred to as the atmospheric pathway of the cloud-radiative impact. For the MPI-ESM model, the atmospheric pathway is responsible for about half of the total cloud-radiative impact, and in fact half of the total circulation response. Changes in atmospheric cloud-radiative heating are substantial in both the lower and upper troposphere. However, because SST is prescribed the atmospheric pathway is dominated by changes in upper-tropospheric cloud-radiative heating, which in large part results from the upward shift of high-level clouds. The poleward circulation expansion via the atmospheric pathway and changes in upper-tropospheric cloud-radiative heating are qualitatively robust across three global models, yet their magnitudes vary by a factor of 3. A substantial part of these magnitude differences are related to the upper-tropospheric radiative heating by high-level clouds in the present-day climate. A comparison with observations highlights the model deficits in representing the radiative heating by high-level clouds and indicates that reducing these deficits can contribute to improved predictions of regional climate change.

Cloud‐Radiative Impact on the Regional Responses of the Midlatitude Jet Streams and Storm Tracks to Global Warming

“…The impact of this decorrelation on the climatological circulation is found to be mainly small in our simulations. This is in line with other studies that used the cloud‐locking method to investigate the circulation response to global warming (Ceppi & Hartmann, ; Ceppi & Shepherd, ; Voigt & Shaw, , ; Voigt et al, ).…”

“…The stronger tropical SST increase in PAT compared to UNI leads to a slightly larger change in cloud‐radiative heating in the tropical upper troposphere (Figure S9), but overall, the cloud‐radiative heating change is very similar between UNI and PAT. A very similar pattern of cloud‐radiative heating changes was previously found in aquaplanet simulations in which global warming was mimicked by a uniform 4 K SST increase (Figures 2c and 2d in Voigt & Shaw, ) and in present‐day simulations in a slab ocean setup under quadrupling of atmospheric CO 2 (Figure 2b of Voigt et al, ). Additionally, the pattern is consistent with the atmospheric cloud‐radiative heating changes derived from present‐day COOKIE simulations (Figure 4b in Li et al, ).…”

“…The aquaplanet work of Voigt and Shaw (, ) identified that cloud‐radiative changes are important even when sea surface temperatures (SST) are prescribed, showing that a large part of the cloud‐radiative impact results from the direct atmospheric cloud‐radiative heating. This is supported by the study of Voigt et al (), which investigated the cloud‐radiative impact on the annual‐mean zonal‐mean jet stream response in a present‐day setup. The authors decomposed the cloud‐radiative impact into a surface and an atmospheric pathway, depending on whether SST are interactive or prescribed.…”

“…We study the impact of cloud‐radiative changes on the global warming responses of the midlatitude jet streams and storm tracks in the North Atlantic, North Pacific, and Southern Hemisphere and determine whether the cloud‐radiative impact depends on the ocean basin, season, and pattern of SST increase. For this purpose, we use the atmospheric component of the ICON model and prescribe SST to isolate the impact of cloud‐radiative changes via the atmospheric pathway, that is, the impact of changes in atmospheric cloud‐radiative heating in the absence of a cloud‐radiative impact on ocean surface temperatures (Voigt et al, ).…”

“…The method allows us to break the radiative interactions and feedbacks between clouds and the circulation by prescribing the radiative properties of clouds to the model's radiative transfer scheme (e.g., Voigt & Shaw, ). While originally devised to study the impact of radiative feedbacks on global‐mean and regional surface warming (e.g., Langen et al, ; Mauritsen et al, ; Schneider et al, ; Wetherald & Manabe, ), the locking method has become a helpful tool to investigate the contribution of cloud‐radiative changes to circulation changes (Ceppi & Hartmann, ; Voigt & Shaw, , ; Voigt et al, ).…”