Simulated

FIGURE 11 Observed surface temperature trend for 1900-1994 and simulated trend for the same period with the ECHAM4/OPYC3 coupled climate model.

simulation, a reasonably good agreement is found (Fig. 11).

We will next investigate the geographical relation between the forcing and the response to forcing in the three transient experiments. This is done by comparing the meridional profiles of zon-ally averaged forcing for the period 2030-2050 to the corresponding meridional profile of the surface temperature (Figs. 12a and 12b, respectively). The result is very much of the same type as in the equilibrium experiment with a slab ocean, suggesting that atmospheric processes are important for the response pattern. The experiment with greenhouse gases only, GHG, has a maximum forcing at around 20°, decreasing both toward the equator and toward higher latitudes. The other experiments have a reduced forcing increasing toward middle latitudes at the Northern Hemisphere due to the emission of SO, in these regions. The meridional profile of response to forcing looks very much different from that of the forcing with the maximum warming taking place at high latitudes of the Northern Hemisphere in the region where the experiments GSD and GSDIO show the smallest forcing! What can be the reason for this?

We believe that the warming pattern is generated following a series of feedbacks in the model. Due to the complexity of the model and the long time scales involved, an analysis at this time can only be tentative. Most models respond to the initial greenhouse warming in the troposphere by increasing the amount of water vapor (most models more or less conserve relative humidity). The altered water vapor enhances the greenhouse effect and

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60n figure 12 (a) Annual mean radiative forcing at the top of the tropopause in the three experiments GHG, GSD, and GSDIO. The figure shows the meridional profiles of zonal averages for the period 2030-2050. (b) Meridional profiles of changes in the annual zonal mean surface air temperatures for the same period (after Roeckner et til., 1999).

30s eq

90n figure 12 (a) Annual mean radiative forcing at the top of the tropopause in the three experiments GHG, GSD, and GSDIO. The figure shows the meridional profiles of zonal averages for the period 2030-2050. (b) Meridional profiles of changes in the annual zonal mean surface air temperatures for the same period (after Roeckner et til., 1999).

positive feedback takes place. The effect is likely to be particularly strong in areas where the moisture content is high, such as in the intertropical convergence zones. Warming over land areas is larger than that over oceans due to the large ocean heat capacity, which delays the warming considerably.

The delayed warming is particularly strong in the southernmost oceans, with their strong oceanic vertical heat exchange. Finally, in the climate warming experiments the storm tracks are moved slightly poleward, particularly over the Northern Hemisphere. The feedback at high-latitude land areas is also enhanced through albedo feedback due to reduced snow cover on the ground in the climate change experiments. We thus anticipate that complex feedbacks such as those suggested here are the probable reasons for the distribution of the warming. Many of the feedback processes are model-dependent and the main cause of the large model variability as that shown in Figure 8.

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