Climate Dynamics

, Volume 24, Issue 7, pp 655-666

First online:

Polar amplification of surface warming on an aquaplanet in “ghost forcing” experiments without sea ice feedbacks

  • V. A. AlexeevAffiliated withInternational Arctic Research Center, University of Alaska Fairbanks
  • , P. L. LangenAffiliated withNiels Bohr Institute, University of Copenhagen
  • , J. R. BatesAffiliated withMathematical Physics Department, University College Dublin

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Polar amplification of surface warming has previously been displayed by one of the authors in a simplified climate system model with no ice-albedo feedbacks. A physical mechanism responsible for this pattern is presented and tested in an energy balance model and two different GCMs through a series of fixed-SST and “ghost forcing” experiments. In the first ghost forcing experiment, 4 W/m2 is added uniformly to the mixed layer heat budget and in the second and third, the same forcing is confined to the tropics and extra-tropics, respectively. The result of the uniform forcing is a polar amplified response much like that resulting from a doubling of CO2. Due to an observed linearity this response can be interpreted as the sum of the essentially uniform response to the tropical-only forcing and a more localized response to the extra-tropical-only forcing. The flat response to the tropical forcing comes about due to increased meridional heat transports leading to a warming and moistening of the high-latitude atmosphere. This produces a longwave forcing on the high-latitude surface budget which also has been observed by other investigators. Moreover, the tropical surface budget is found to be more sensitive to SST changes than the extra-tropical surface budget. This strengthens the tendency for the above mechanism to produce polar amplification, since the tropics need to warm less to counter an imposed forcing.