In this study, a fully coupled high-resolution AOGCM has been used to investigate the possible impacts of greenhouse global warming on the characteristics of tropical cyclones. To our knowledge, this is the first time the impact of global warming on TCs is investigated by means of a state of the art fully coupled GCM.

The simulated TCs have many, basic, gross features similar to the observed ones. However, the rather low intensity of the low-level winds and the too large distance between the cyclone eye and the wind maximum remain unsatisfactory. These shortcomings are likely due to the model resolution, which, though rather high for long climate simulations, is still too coarse for an adequate representation of the tight structures accompanying these phenomena. Despite these problems, the model seems to be able to simulate a reasonable realistic climatology of TCs, both in terms of spatial distribution, seasonal and interannual variability of the TC activity. In particular, the model appears to capture at least some of the links between SST interannual variability and TC activity.

rh7uu jjasu rh700 djfma rh7uu jjasu

rh700 djfma

Fig. 14 4C02-PREIND differences of the mean values of 700-hPa relative humidity (RH700, panels a and b); maximum potential index (MPI, panels c and d) and genesis potential index (GP, panels e and f). Left panels', northern summer; right panels', northern winter. The RH is expressed in %, the MPI in m/s and the GP index in (number of TC)/(unit area x decade). Only statistically significant at a 95% level contours are shown. The significance test has been performed using the boot-strap method

The enhanced concentration of atmospheric CO2 induces a warming of the entire tropical and subtropical upper ocean, accompanied by a redistribution of the tropical rainfall. The increase of the tropical ocean surface temperature, however, is not uniform, and the eastern Pacific exhibits a more pronounced warming with patterns that resemble El Nino SST anomalies. The total precipitation averaged over the Tropics increases with the CO2 increase, but the convective precipitation exhibits a significant reduction.

Along with the attenuated convective activity, our simulations show a substantial and significant reduction of the number of generated TCs, especially over the North West Pacific and North Atlantic tropical regions. Both the decrease in convective activity and and the reduced occurrence of TCs might be due to the larger potential energy barrier found when the CO2 concentration is increased. In the reduction of the TC activity in the ATL region, however, an important role appears to be played also by a considerable increase of the vertical wind shear.

The greenhouse warming is associated with a poleward expansion of the tropical warm SSTs. In particular, the 26°C isotherm, that appears to be crucial for the development of the TCs in the present climate, in the 4CO2 case migrates poleward of almost 10° latitude compared to the PREIND one. In the model, however, the warming of the subtropical and mid-latitudes is not accompanied by a poleward extension of the TC action, consistent with earlier works. The peaks of TC activity remain substantially confined equatorward of 20° latitude in both the Hemispheres.

Despite the reduced number of TCs generated when the CO2 has doubled and quadrupled, there is evidence of an increase in their intensity in terms of precipitation. This might be related with the increase of CAPE found in the warmer climate. The intensity of the simulated TCs expressed in terms of near-surface wind (PDI) and surface pressure, on the other hand, does not appear to be significantly affected by the global warming. However, we think that this result might be actually related to the deficiencies that the model exhibits in reproducing realistic TC intensities.

Acknowledgments The authors are indebted to Gabriel Vecchi, Chiara Cagnazzo and Andrea Alessandri for their precious help, useful suggestions and stimulating discussions. They also want to thank the three anonymous reviewers for their suggestions and constructive criticisms and K. Emanuel for making available the routines to compute the maximum potential index, MPI (http:// This work has been supported by the Euro-Mediterranean Centre for Climate Change and by the European Community project ENSEMBLES, contract number GOCE-CT-2003-505539.

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