Changes in the Tropical Mean State

The impacts of the increased atmospheric CO2 on the mean state of the Tropics is shown in Fig. 7. Here the difference between 2CO2 and PREIND (2CO2-PREIND), and 4CO2 and PREIND (4CO2-PREIND) seasonal means of SST and precipitation are shown. Panels a, b, e and f indicate that the overall warming of the tropical SST is characterized by regional patterns. Both during JJASO and DJFMA, in fact, the warming is more marked in the western part of the Indian Ocean, in the equatorial Pacific and along the coast of South America, whereas a weaker warming is found in the eastern tropical Indian Ocean and eastern subtropical Pacific. In the tropical Pacific the warming patterns resemble El Nino anomalies. Interestingly, these patterns are similar for the 2CO2 PREIND and 4CO2-PREIND cases, though with different amplitudes.

Similar characteristics exhibit the patterns of difference between the PREIND, 2CO2 and 4CO2 precipitation (panels c, d, g and h). In particular, the increased CO2 induces a remarkable enhancement of precipitation along the ITCZ, from the Indian Ocean, through the Pacific to the Atlantic, during JJASO. Interestingly the increase of rainfall is confined to a relatively narrow region, very close to the equator. In the same season, areas of reduced rainfall are located in the southeastern tropical Indian Ocean and south-central Pacific. During DJFMA, increased precipitation is found south of the equator, along the southern branch of the double ITCZ simulated by the model and discussed in Section 3.1; whereas regions of decreased rainfall are found in the subtropics of both the summer and winter hemispheres. Also in this case, the patterns of precipitation difference a) 2C02-PREIND SST JJASO MEAN

b) 4C02-PREIND SST JJASO MEAN

a) 2C02-PREIND SST JJASO MEAN

b) 4C02-PREIND SST JJASO MEAN

Fig. 7 Differences between the seasonal mean SST and precipitation obtained from the 2CO2 and PREIND experiments (leftpanels) and 4CO2 and PREIND experiments (rightpanels). Panels a, b, e and f show the differences in mean SST; contour interval is 0.25°C. Panels c, d, g and h show the differences in mean precipitation, with a contour interval of 1.0 mm/day

Fig. 7 Differences between the seasonal mean SST and precipitation obtained from the 2CO2 and PREIND experiments (leftpanels) and 4CO2 and PREIND experiments (rightpanels). Panels a, b, e and f show the differences in mean SST; contour interval is 0.25°C. Panels c, d, g and h show the differences in mean precipitation, with a contour interval of 1.0 mm/day

2CO2-PREIND and 4CO2-PREIND exhibit very similar spatial features but different amplitudes.

Table 3 shows the changes in mean temperature, mean precipitation and mean convective precipitation over the entire globe and over the Tropics for the three

Table 3 Global average and tropical average of mean surface temperature, mean total precipitation and mean convective precipitation. The mean have been computed over the 30-year periods considered in the study. Values in parenthesis are the differences (absolute values for temperature and percentage for precipitation) with respect to the PREIND case

Table 3 Global average and tropical average of mean surface temperature, mean total precipitation and mean convective precipitation. The mean have been computed over the 30-year periods considered in the study. Values in parenthesis are the differences (absolute values for temperature and percentage for precipitation) with respect to the PREIND case

PREIND

2CO2

4CO2

T global mean °K

288.37

290.377 (+2.01)

292.72 (+4.36)

T Tropics °K

298.72

300.286 (+1.57)

302.41 (+3.69)

Prec. global mean mm/day

2.757

2.809 (+1.89%)

2.852 (+3.45%)

Prec. Tropics mm/day

3.317

3.353 (+1.09%)

3.354 (+1.12%)

ConvPrec global mean mm/day

1.209

1.164 (-3.72%)

1.106 (-8.52%)

ConvPrec Tropics mm/day

2.117

2.008 (-5.15%)

1.884 (-11.01%)

experiments. Here, the convective precipitation is the precipitation associated with convective processes and produced by the convective parametrization scheme. Interestingly, while a substantial rise in mean surface temperature and mean total precipitation are found when the CO2 is increased, a completely different behaviour is found for the convective precipitation. The latter in fact shows a significant reduction when the atmospheric CO2 concentration has doubled and quadrupled, especially in the tropical region.

A first assessment of the changes in high-frequency (<10 days) convective variability induced by the CO2 forcing has been obtained computing the difference in standard deviation of high-pass filtered OLR anomalies (not shown). Over most of the tropical belt the sign of the difference is negative, indicating a tendency of the model to attenuate the high-frequency convective variability when the atmospheric CO2 is increased. Only over the equatorial Pacific, between about 5°N and 5°S, there is a clear sign of enhanced variability.

These results appear to suggest that increasing the concentration of CO2 in the atmosphere of the model, the general warming of the earth surface is accompanied by a reduction of the (deep) convective activity in the Tropics. The weaker convective activity, in turn, might be due to an enhancement of the vertical stability of the atmosphere. This point will be examined in more detail in Section 5.

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