What Balances the Decrease in Net Upward Thermal Radiation at the Surface in Climate Change Experiments?

ABSTRACT

The direct response of surface fluxes to an increase in green house gas concentration is a decrease in net upward long-wave radiation (NLW). This paper examines the responses of the other three surface fluxes, i.e. the latent heat flux (H_L), the sensible heat flux (H_S) and the net short wave radiation (NSW), using a set of IPCC AR4 climate experiments performed with the coupled ECHAM5/MPI-OM AO-GCM. In particular, the questions of whether and how these fluxes compensate the warming effect due to a decrease in upward NLW are studied.

Consistent with the earlier studies, the decrease in upward NLW is strongly compensated by an increase in upward HL. By using the IPCC scenarios and a coupled AO-GCM, two new aspects of this compensation are identified. First, the degree of compensation decreases with the rate of increase in GHG concentration. Secondly, the compensation does not work over the North Atlantic, where the decrease in upward NLW develops parallel to a reduction in upward HL. This leads to large increases in the net downward heat flux over the North Atlantic and a reduction of the MOC. The responses in HS and NSW can further strengthen or suppress the warming effect of NLW, depending on geographical regions considered. There is a general tendency that HS changes in the same direction as NLW over sea, but in the opposite direction over land. For NSW, the response strengthens the NLW changes over land and suppresses the NLW changes over sea.