Clouds modify jet stream response to global warming
Jet streams are global bands of strong westerly winds in the midlatitudes. They are the reason why, for example, flights towards the east are usually shorter than flights towards the west. Jet streams are an important topic in weather and climate research, because they govern the regional weather and climate as they determine the tracks of extratropical storms. In response to global warming, the location and strength of the jet streams will change. This also affects the frequency and location of strong and damaging storms in the midlatitudes. The response of the jet streams to global warming depends on several factors, including changes in the atmospheric temperature and moisture. Another important factor are clouds and their radiative interactions. The role of clouds is studied in the young investigator group “Clouds and Storm Tracks” of Dr. Aiko Voigt, which is funded by the Federal Ministry for Education and Research (BMBF).
Clouds reflect solar radiation and absorb terrestrial radiation that is emitted by the Earth‘s surface. Thereby, they directly affect the Earth‘s radiation balance and atmospheric temperatures. The effect of cloud changes on the global warming response of the midlatitude jet streams in the German weather and climate model ICON was investigated for different ocean basins and seasons. The so called “cloud-locking method” was used to isolate the effect of cloud changes on the jet stream response to global warming. The results are summarized in Fig. 1 and show that in the annual-mean, about one to two-thirds of the poleward jet shift in the North Atlantic, North Pacific and Southern Hemisphere, and of the jet strengthening in the North Atlantic and Southern Hemisphere can be attributed to cloud changes. In some regions and seasons, the cloud effect can be even larger. In the ICON model, all regional cloud changes contribute to the poleward jet shift, while the jet strengthening is mainly due to tropical cloud changes. These ICON results were further compared to the climate models MPI-ESM and IPSL-CM5A. This showed that the cloud effect on the jet streams is subject to considerable model uncertainties.
Clouds are a major source of uncertainty in climate models. The new results help to understand the response of the jet streams and to improve projections of their response to global warming.
Fig. 1: Annual-mean global warming response of the wind (left) as well as the contribution from global cloud changes (top right) and tropical cloud changes (bottom right). The westwind bands around today‘s jet streams (black lines) get stronger and shift poleward.
BMBF young investigator group „Clouds and Storm Tracks“, M.Sc. Nicole Albern and Dr. Aiko Voigt
Albern, N., A. Voigt and J. G. Pinto (2019): Cloud‐radiative impact on the regional responses of the midlatitude jet streams and storm tracks to global warming. Journal of Advances in Modeling Earth Systems, 11, 1940–1958. https://doi.org/10.1029/2018MS001592
Albern, N., A. Voigt, D. W. J. Thompson and J. G. Pinto (2019): The role of tropical, midlatitude and polar cloud-radiative changes for the regional response of the midlatitude jet streams to global warming, in preparation.
Voigt, A., N. Albern and G. Papavasileiou (2019): The atmospheric pathway of the cloud-radiative impact on the circulation response to global warming: important and uncertain, Journal of Climate, 32, 3051-3067.