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Impact of a coupled ocean on the stratosphere

Shown in colour are the responses in the zonal-mean zonal winds of the Northern Hemisphere stratosphere to instantaneous doubling of atmospheric CO2. For reference, contours of the control winds are overlaid. The three panels represent three different experiments:

On the right, the atmospheric model is coupled to a simple ocean, so the model atmosphere is at all times in thermal equilibrium with the surface. The effect of doubling CO2 is a strong reduction in the winds of the stratospheric polar vortex.

In the middle, the coupled ocean is gone and monthly-mean sea surface temperatures having interannual variability are prescribed to the atmospheric model instead. The response to CO2 doubling is qualitatively the same as in the coupled case on the right, but has considerably lower amplitude.

On the left, monthly-mean sea surface temperatures without interannual variability are prescribed, and the response to CO2-doubling is very weak.

The relevance of these experiments is the difference between the right and centre panels: state-of-the-art models used in making climate predictions such as published in the IPCC reports are computationally very expensive to run, and cannot afford to coupling to an ocean. Hence they run with prescribed interannually varying monthly-mean sea surface temperatures, an experimental set-up resembling that of our central panel. Our results show that the response found in such experiments may underestimate the true climate response, as the true climate also includes the constant adjustment between air and ocean.

Note that our experiments are process-oriented studies highlighting the importance of simulating accurate daily heat-flux exchange between the ocean and the atmosphere; our experiments are not intended to make specific quantitative predictions about the future of planet Earth.

Please see B. Winter and M.S. Bourqui, Geophys. Res. Lett., 38, L08808, doi:10.1029/2011GL047011, 2011. http://www.agu.org/pubs/crossref/2011/2011GL047011.shtml

About the author

Dr. Barbara Winter is a post-doctoral fellow working with Dr. Michel Bourqui in the field of stratospheric dynamics