Thursday, 13 January 2005
Dynamical Amplifier of Global Warming
A simple coupled atmosphere-land/ocean radiative-transportive model has been put forward to explain the amplified surface warmings in high latitudes. The poleward heat transport by the atmosphere and oceans reduces the equator-to-pole temperature contrast, creating a locally non-radiative equilibrium time mean state. The direct response to an anthropogenic radiative forcing is an increase in the atmosphere and surface equator-to-pole temperature contrasts, thereby strengthening the atmospheric poleward heat transport. As a result, part of the extra amount of thermal energy intercepted by the low-latitude atmosphere due to an increase in its opacity is transported to high latitudes. This effectively implies a “greenhouse-plus” (“greenhouse-minus”) feedback to the high (low) latitude surface temperatures, thereby amplifying (weakening) the direct response to the anthropogenic radiative forcing in high (low) latitudes. In other words, the strengthening of the atmospheric poleward heat transport has a positive (negative) feedback on the high- (low-) latitude surface temperature. Such an asymmetry amplifies the warmings in high latitudes. The Stefan-Boltzmann feedback suppresses the negative dynamical feedback relatively to the positive. As a result, the global mean surface temperature rises further from the warmings due to the anthropogenic radiative forcing. As far as the surface temperature is concerned, the dynamical amplifier is more effective when the poleward heat transport is done by the atmosphere alone. This attributes to a greater surface warming over the land compared to its surrounding oceans. The dynamical amplifier is stronger when the meridional gradient of the incoming solar energy is larger. As a result, the warming asymmetry between high- and low-latitude surface temperatures and between high latitude land and oceans is more evident in winter. For an anthropogenic radiative forcing of 4 Wm-2, the dynamical amplifier alone can give rise to a difference of 0.3 K between high- and low-latitude surface warmings and 0.1 K between high latitude land and oceans in winter. In summer, the warming asymmetry is less than 0.03 K. The dynamical amplifier causes an additional warming of 0.14 K in the global mean winter surface temperature and attributes little additional warming in summer.