A high-latitude convective cloud feedback and equable climates

A convective cloud feedback on extratropical surface temperatures is identified in a zonally averaged two-level atmospheric model. The model contains simplified parametrizations for convection, precipitation, and clouds, and a long-wave radiation scheme that explicitly depends on carbon dioxide, water vapour, and cloud fraction. The convective cloud feedback occurs if the extratropical surface temperature is increased enough to initiate strong atmospheric convection. This results in a change from low to high clouds and from negative to neutral or positive cloud radiative forcing, which further warms the surface and leads to more convection. This positive feedback activates as the CO2 concentration is increased and leads to a climate solution with high boundary-layer temperatures, convection at mid and high latitudes, and an Equator to Pole temperature difference that is reduced by 8-10 degrees C. The reduction in Equator to Pole temperature difference is due to changes in high-latitude local heat balance and occurs despite decreased meridional heat transport. The convective cloud feedback also leads to multiple equilibria and hysteresis with respect to CO2 and other model variables, although these results may be due to the simplicity of the model. The possible connection of the behaviour of the model at high CO2 with equable climates is considered. Copyright (c) 2008 Royal Meteorological Society.