An inter-hemispheric comparison of the tropical storm response to global warming
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Model studies do not agree on future changes in tropical cyclone (TC) activity on regional scales. We aim to shed further light on the distribution, frequency, intensity, and seasonality of TCs that society can expect at the end of the twenty-first century in the Southern hemisphere (SH). Therefore, we investigate TC changes simulated by the atmospheric model ECHAM5 with T213 (~60 km) horizontal resolution. We identify TCs in present-day (20C; 1969–1990) and future (21C; 2069–2100) time slice simulations, using a tracking algorithm based on vorticity at 850 hPa. In contrast to the Northern hemisphere (NH), where tropical storm numbers reduce by 6 %, there is a more dramatic 22 % reduction in the SH, mainly in the South Indian Ocean. While an increase of static stability in 21C may partly explain the reduction in tropical storm numbers, stabilization cannot alone explain the larger SH drop. Large-scale circulation changes associated with a weakening of the Tropical Walker Circulation are hypothesized to cause the strong decrease of cyclones in the South Indian Ocean. In contrast the decrease found over the South Pacific appears to be partly related to increased vertical wind shear, which is possibly associated with an enhanced meridional sea surface temperature gradient. We find the main difference between the hemispheres in changes of the tropical cyclones of intermediate strength with an increase in the NH and a decrease in the SH. In both hemispheres the frequency of the strongest storms increases and the frequency of the weakest storms decreases, although the increase in SH intense storms is marginal.
KeywordsTropical cyclones Climate change Tropical Walker Circulation ECHAM5 Southern hemisphere
Simulations were performed at the Norddeutscher Verbund zur Förderung des Hoch- und Höchstleistungsrechnens—HLRN and European Centre for Medium Range Weather Forecasts. Computing support from IFM-GEOMAR is greatly acknowledged. NSK is supported by the Deutsche Forschungsgemeinschaft (Emmy Noether grant KE 1471/2-1), and WT by the Bundesministerium für Bildung und Forschung (Nordatlantik project). Ralf Hand kindly provided code for computing bootstrap significance. The work also contributed to the EU FP7 projects SUMO (No. 266722) and STEPS (PCIG10-GA-2011-304243).
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