Tropical cloud-radiative changes contribute to robust DJF jet exit strengthening over Europe under global warming

Abstract:

During boreal winter (December to February, DJF), the North Atlantic jet stream is expected to expand eastward over Europe in response to global warming. This eastward expansion and jet exit strengthening is robust across coupled climate models of phase 5 of the Coupled Model Intercomparison Project (CMIP5) and atmosphere-only general circulation models with prescribed sea surface temperatures (SST) and sea ice cover.

Here, we use the atmospheric components of ICON-NWP, MPI-ESM and IPSL-CM5A with prescribed SST and the cloud-locking method to show that cloud-radiative changes significantly contribute to the DJF jet exit strengthening over Europe. This response is robust across all three models. We prescribe SST to isolate the impact of cloud-radiative changes via the atmospheric pathway, i.e. changes in atmospheric cloud-radiative heating, and mimic global warming by a uniform 4K SST increase. We apply the cloud-locking method to break the cloud-radiation-circulation coupling and to decompose the circulation response into contributions from cloud-radiative changes and from the SST increase.

In ICON-NWP, the cloud-radiative impact is dominated by tropical cloud-radiative changes while midlatitude and polar cloud-radiative changes have a minor impact. A further division of the tropics shows that cloud-radiative changes over the tropical Atlantic and Africa hardly contribute to the jet response over Europe because changes in atmospheric cloud-radiative heating under global warming are small in this region. The remaining part of the tropics exhibits large upper-tropospheric cloud-radiative heating so that the Indian Ocean, the tropical West Pacific and the tropical East Pacific all contribute to the jet response over Europe.