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Earth Syst. Dynam., 7, 877-892, 2016
https://doi.org/10.5194/esd-7-877-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
11 Nov 2016
Response of the AMOC to reduced solar radiation – the modulating role of atmospheric chemistry
Stefan Muthers1,2,a, Christoph C. Raible1,2, Eugene Rozanov3,4, and Thomas F. Stocker1,2 1Climate and Environmental Physics, University of Bern, Bern, Switzerland
2Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland
3Institute for Atmospheric and Climate Science, ETH, Zurich, Switzerland
4Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center (PMOD/WRC), Davos, Switzerland
anow at: German Meteorological Service, Research Center Human Biometeorology, Freiburg, Germany
Abstract. The influence of reduced solar forcing (grand solar minimum or geoengineering scenarios like solar radiation management) on the Atlantic Meridional Overturning Circulation (AMOC) is assessed in an ensemble of atmosphere–ocean–chemistry–climate model simulations. Ensemble sensitivity simulations are performed with and without interactive chemistry. In both experiments the AMOC is intensified in the course of the solar radiation reduction, which is attributed to the thermal effect of the solar forcing: reduced sea surface temperatures and enhanced sea ice formation increase the density of the upper ocean in the North Atlantic and intensify the deepwater formation. Furthermore, a second, dynamical effect on the AMOC is identified driven by the stratospheric cooling in response to the reduced solar forcing. The cooling is strongest in the tropics and leads to a weakening of the northern polar vortex. By stratosphere–troposphere interactions, the stratospheric circulation anomalies induce a negative phase of the Arctic Oscillation in the troposphere which is found to weaken the AMOC through wind stress and heat flux anomalies in the North Atlantic. The dynamic mechanism is present in both ensemble experiments. In the experiment with interactive chemistry, however, it is strongly amplified by stratospheric ozone changes. In the coupled system, both effects counteract and weaken the response of the AMOC to the solar forcing reduction. Neglecting chemistry–climate interactions in model simulations may therefore lead to an overestimation of the AMOC response to solar forcing.

Citation: Muthers, S., Raible, C. C., Rozanov, E., and Stocker, T. F.: Response of the AMOC to reduced solar radiation – the modulating role of atmospheric chemistry, Earth Syst. Dynam., 7, 877-892, https://doi.org/10.5194/esd-7-877-2016, 2016.
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Short summary
The Atlantic Meridional Overturning Circulation (AMOC) is an important oceanic circulation system which transports large amounts of heat from the tropics to the north. This circulation is strengthened when less solar irradiance reaches the Earth, e.g. due to reduced solar activity or geoengineering techniques. In climate models, however, this response is overestimated when chemistry–climate interactions and the following shift in the atmospheric circulation systems are not considered.
The Atlantic Meridional Overturning Circulation (AMOC) is an important oceanic circulation...
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