Fine resolution simulation of the effect of climate change on tropical cyclones in the South Pacific

Fine-resolution regional climate model simulations of tropical cyclone formation in the South Pacific are conducted to determine the response of the model to impose climate forcings. The GFDL regional Atmospheric Model (ZETAC; Pauluis and Garner 2006) is employed in this work. It is a compressible, nonhydrostatic model with a grid spacing of 1/6 degree (about 18 km) with 45 vertical levels. These are the highest-resolution climate simulations of tropical cyclones conducted to date in this region. No cumulus parameterization is employed but the model uses a five-species cloud microphysical scheme (Lin et al. 1983, Lord et al. 1984). The experimental methodology is similar to that of Knutson et al. (2007). The model is nudged towards the NCEP2 reanalyses at a timescale of two hours in a band five degrees wide around the perimeter of the domain. In addition, a spectral nudging is applied in the interior of the domain over the largest zonal and meridional wave numbers (wave numbers 0, 1 and 2), also with a time scale of two hours. Simulations were analysed for the January-March peak tropical cyclone period, with a run-in period of December. Tropical cyclones were tracked with the GFDL tracking scheme.

The enhanced greenhouse simulations were performed by assuming the present-day interannual variability and imposing a three-dimensional ensemble average climate change simulated by the CMIP3 model suite, for the period 2080-2099, A1B scenario, identical to the methodology of Knutson et al. (2008).

ZETAC simulates well the interannual variability of the difference in tropical cyclone formation between the regions east and west of 170E. The model also generates close to the observed number of tropical cyclones west of 170E but too many east of 170E. Under enhanced greenhouse conditions, the numbers of tropical cyclones decrease substantially, except for the most intense storms with maximum low-level wind speeds greater than 40 ms^-1. Analysis is presented of the association between changes in tropical cyclone formation rate and intensity and changes in large-scale forcing fields.

References

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Knutson, T. R., J.J. Sirutis, S.T. Garner, G.A. Vecchi, and I. Held, 2008: Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geo., 1, 359–364.

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Lord, S. J., H. E. Willoughby, and J. M. Piotrowicz, 1984: Role of a parameterised icephase microphysics in an axisymmetric nonhydrostatic tropical cyclone model. J. Atmos. Sci., 41, 2836-2848.

Pauluis, O., and S. T. Garner. 2006: Sensitivity of radiative-convective equilibrium simulations to horizontal resolution. J. Atmos. Sci., 63, 1910-1923.