Climate Dynamics

, Volume 24, Issue 7, pp 655–666

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


  • V. A. Alexeev
    • International Arctic Research CenterUniversity of Alaska Fairbanks
  • P. L. Langen
    • Niels Bohr InstituteUniversity of Copenhagen
  • J. R. Bates
    • Mathematical Physics DepartmentUniversity College Dublin

DOI: 10.1007/s00382-005-0018-3

Cite this article as:
Alexeev, V.A., Langen, P.L. & Bates, J.R. Climate Dynamics (2005) 24: 655. doi:10.1007/s00382-005-0018-3


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.

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© Springer-Verlag 2005