Utilisation of the simulations to assign changes to the boundary

A set of sensitivity simulations was performed in order to untangle the effects of changes in the different boundary conditions. Namely, the boundary conditions are divided into chlorofluorocarbons (CFCs), long-lived greenhouse gases (GHGs) and sea surface temperatures (SSTs) as described in Chapter 2 (see also Appendix A). Enhanced abundance of CFCs lead to increased ozone destruction, and it is known that the

38 4.1. IMPACT OF CHANGES IN GHGS, SSTS AND CFCS 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050 REF−B1 SCN−B2d Ref2000 1960SST+GHG 1960SST 1960GHG 2040SST+GHG 2040SST 2040GHG tropSST midlatSST "PAST" "FUTURE" I 0 XI XII II III IV V VI VII VIII IX X XIII XIV A B C

Figure 4.1: Overview of all simulations used in this study with arrows indicating the differences in boundary conditions: no differences (grey), differences in CFCs (blue), in SSTs (red) and in GHGs (green). The pink arrows indicate differences taken from episodes of the transient simulation SCN-B2d and therefore include changes in all boundary conditions. The numbers on the arrows are used as identifiers later on.

strong ozone depletion that occurred in Antarctica feeds back on the climate through radiative cooling. However, CFCs also act as greenhouse gases. The category of long-

lived GHGs is defined here as the combined effect of CO2, CH4 and N2O and they

effect the atmosphere through trapping of longwave radiation in the troposphere and enhanced infrared emission in the stratosphere. Since SSTs are prescribed and not interactively coupled in the model, they can be regarded as separate forcing. In reality, the changes in SSTs are caused by the effects of GHGs and the two forcings are not necessarily distinguishable. Earlier studies suggest that the main climate effect on the troposphere is forced by the SSTs rather than by the direct effect of changes in GHG concentrations (Fomichev et al., 2007). Therefore, changing either SSTs or GHGs might be regarded as changing the climate in either the troposphere or the stratosphere separately. This gives insights into the processes that drive changes, but care has to be taken in the interpretation since the atmosphere-ocean interaction is neglected.

While in the transient simulations SCN-B2d and REF-B1, the boundary conditions do change over the cause of the simulation, the time-slice simulations are designed to model the mean state of the atmosphere under certain conditions. By incorporating changes in only one of the prescribed boundary conditions, the response of the atmo- sphere to changes in this particular boundary condition can be determined. The set-up of all simulations performed was described in Chapter 2. Fig. 4.1 shows the overview of the simulations repeated from Fig. 2.1, but here including arrows indicative of the differences in boundary conditions between the simulations or between periods within simulations. The response on SSTs, GHGs and CFCs can be obtained by the following comparisons:

i) Response to SSTs

V: 2000-1960 Time-slice Ref2000 minus 1960SST gives the response of the atmo-

sphere to changes in the SSTs from mean 2000 to 1960 conditions, while all other boundary conditions remain at a mean level of year 2000.

VI: 2040-2000 As above, but difference between time-slice 2040SST minus Ref2000,

i.e. giving the response to changes in the SSTs from mean 2040 to 2000 conditions.

VII: 2040-1960 As above, but difference between time-slice 2040SST minus 1960SST,

i.e. giving the response to changes in the SSTs from mean 2040 to 1960 conditions. ii) Response to GHGs

VIII: 2000-1960 Time-slice Ref2000 minus 1960GHG gives the response of the at-

mosphere to changes in the GHG concentrations from 2000 to 1960 values, while all other boundary conditions remain at a mean level of year 2000.

40 4.1. IMPACT OF CHANGES IN GHGS, SSTS AND CFCS

IX: 2040-2000 As above, but difference between Time-slice 2040GHG minus Ref2000,

i.e. giving the response to changes in the GHG concentrations from mean 2040 to 2000 conditions.

X: 2040-1960 As above, but difference between Time-slice 2040GHG minus 1960GHG,

i.e. giving the response to changes in the GHG concentrations from mean 2040 to 1960 conditions.

iii) Response to CFCs

XI: 2000 - 1960 By comparing the time-slice with 1960s conditions in SSTs and GHG

but CFC concentrations of the year 2000 with the period 1960-1969 from SCN- B2d, the effect of changes in CFCs from 1960 to 2000 values can be obtained. Note that here the SSTs and GHG concentrations are transient in one case and stationary in the other and therefore are not identical. However, both episodes represent the state of the atmosphere of the 1960s except for the CFC concen- trations in 1960SST+GHG, so that the differences can be assumed to be largely due to the differences in CFCs.

XII: 2040 - 2000 As above, but comparing the time-slice with 2040s conditions in

SSTs and GHG but CFC concentrations of the year 2000 with the period 2040- 2049 from SCN-B2d, showing the effect of changes in CFCs from 2040 to 2000 values.

iv) Linearity of GHG+SST

IV = VII + X Since individual simulations were performed with changing only SSTs,

only GHGs and both simultaneously to 1960 conditions and to 2040 conditions, the comparison of the sum of the response of the changing SSTs and GHGs only to the combined simulation can give a measure on whether the effects of GHGs and SSTs are adding linearly.