Dynamical greenhouse-plus feedback and polar warming amplification. Part I: A dry radiative-transportive climate model
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In this paper, we use a dry radiative-transportive climate model to illustrate that the atmospheric poleward heat transport plays an important role in amplifying the polar surface warming due to anthropogenic greenhouse gases. Because of a much warmer surface in tropics, the anthropogenic radiative forcing is larger in tropics than in extratropics for same amount of anthropogenic greenhouse gases. A direct response to the anthropogenic radiative forcing is an increase in the atmosphere equator-to-pole temperature contrast, leading to a strengthening of the atmospheric poleward heat transport. As a result, part of the extra amount of energy intercepted by the low-latitude atmosphere due to an increase in its opacity is transported to high latitudes. This implies a “greenhouse-plus” (“greenhouse-minus”) feedback to the high (low) latitude surface temperatures, which acts to amplify (reduce) the initial surface warming in high (low) latitudes and may cause a larger surface warming in high latitudes. The Stefan–Boltzmann feedback suppresses the negative dynamical feedback in low latitudes more strongly than the positive in high latitudes, amplifying the global mean surface temperature warming. Because of the oceanic poleward heat transport, the ocean surface temperature in extratropics is warmer in winter compared to the land, implying a larger radiative forcing for the same amount of anthropogenic greenhouse gases. The same dynamical feedback mechanism would also amplify the land surface warming in extratropics by transporting part of the extra amount of energy intercepted by the atmosphere over the ocean surface to land. This explains partly why the observed land surface warming in extratropics is stronger than the surrounding oceans in winter.
The author is indebted to Huug M. van den Dool for his insightful comments and the suggestion of naming the model as a “radiative-transportive model”. The author is grateful to Eugenia Kalnay, F-F Jin, and Antonio Busalacchi for beneficial discussions and constructive suggestions. The insightful and constructive comments and suggestions from Jerry North and an anonymous reviewer, and the editor Ed Schneider are greatly appreciated. The author also thanks Minfeng Zhang for his assistance in plotting the numerical results and Shan Sun at GISS/NASA for her assistant in evaluating the GISS/NASA coupled model climate simulation output. This work was supported by an NSF grant ATM ATM-0403211.
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