Fig. 4.

Schematic of the proposed mechanism for the double-ITCZ bias. The anomalous energy fluxes and circulation in multimodel mean relative to observations are plotted. Most models simulate too much incoming shortwave radiation over the Southern Ocean due to cloud biases, which results in anomalously high temperatures in the Southern Hemisphere midlatitudes. Similar cloud biases exist in Northern Hemisphere midlatitudes, but to a much smaller degree. The anomalous heating in the Southern Ocean is spread into the Southern Hemisphere tropics by baroclinic eddies. An anomalous Hadley circulation is induced to transport energy from the Southern Hemisphere to the Northern Hemisphere (the red arrow across the equator), and keep the tropical tropospheric temperatures relatively flat. Because water vapor is concentrated in the lower troposphere, this anomalous Hadley circulation transports moisture southward (the blue arrow across the equator), and results in excessive precipitation in Southern Hemisphere tropics. Other biases in energy fluxes at the top of the atmosphere or at the surface that have hemispheric asymmetry can also affect tropical precipitation through this mechanism.