Observed tropical expansion and its impact on the hydrological and energy cycles

Recent analysis of the satellite cloud record indicates that the subtropical dry zone has expanded as the storm tracks have retreated poleward. Decreasing cloud cover at lower latitudes enables more solar radiation to be absorbed by the ocean and thus acts as a positive feedback on the climate system. CMIP3 transient and equilibrium climate model simulations exhibit tropical expansion for increased CO2, but at a much lower rate than has been observed in recent decades. Model experiments suggest that, in addition to CO2, radiative heating from black carbon and tropospheric ozone may be substantial contributors to the observed tropical expansion.