Chang, E. K. (2017). Projected Significant Increase in the Number of Extreme Extratropical Cyclones in the Southern Hemisphere. Journal of Climate, (2017).

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Chang, E. K. (2017). Projected Significant Increase in the Number of Extreme Extratropical Cyclones in the Southern Hemisphere. Journal of Climate, (2017).

Projected Significant Increase in Extreme S.H. Extratropical Cyclone Frequency (Chang, 2017)
Link to paper

Extratropical cyclones are responsible for much of the high impact weather in the mid-latitudes, including heavy precipitation, high winds, and coastal storm surges. Thus how these cyclones may change in the future is of much general interest. Previous studies have suggested a poleward shift in the location of these cyclones, but how their intensity may change remains uncertain, especially in terms of maximum wind speed. This study shows that under global warming, state-of-the-art climate models systematically project a significant increase in the frequency of extreme cyclones in the Southern Hemisphere (S.H.).

In the study, projected changes in extreme cyclones in the Southern Hemisphere, based on 26 models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5), have been presented. Multiple definitions of extreme cyclones have been examined, including intensity exceeding constant thresholds of sea level pressure perturbations, 850 hPa vorticity, and 850 hPa winds, as well as variable thresholds corresponding to a top-5 or top-1 cyclone per winter month in these three parameters and the near surface winds. Results show that CMIP5 models project a significant (much more than 10% in most cases) increase in the frequency of extreme cyclones in all four seasons regardless of the definition, with over 88% of the models projecting an increase. Spatial patterns of increase are also consistent, with the largest increase projected between 45°S and 60°S, extending from the South Atlantic across the South Indian Ocean into the South Pacific. Projected increase in cyclone intensity is largely consistent with the projected increase in the temperature gradient in the Southern Hemisphere upper troposphere and lower stratosphere.

(a) Climatology and (b) projected change in the frequency of cyclones with extreme near surface winds in Southern Hemisphere winter. Contours show frequency of cyclone occurrences (in % of time). Shades in panel (b) indicate model agreement, with orange (red) indicating over 2/3 (4/5) of the CMIP5 models project an increase in the number of extreme cyclones. There are no grid boxes at which over 2/3 of the models project a decrease.

(a) Climatology and (b) projected change in the frequency of cyclones with extreme near surface winds in Southern Hemisphere winter. Contours show frequency of cyclone occurrences (in % of time). Shades in panel (b) indicate model agreement, with orange (red) indicating over 2/3 (4/5) of the CMIP5 models project an increase in the number of extreme cyclones. There are no grid boxes at which over 2/3 of the models project a decrease.

Results of this study, together with those from previous studies that suggest cyclone extreme precipitation may increase under global warning, indicate that extratropical cyclones may give rise to more severe impacts in the future. Current research efforts focus on understanding how Northern Hemisphere extreme cyclones may change under the competing influences of the projected increase in the upper level temperature gradient and substantial decrease in the near surface temperature gradient under global warming.—Edmund Chang (Stony Brook University), “Projected Significant Increase in the Number of Extreme Extratropical Cyclones in the Southern Hemisphere,” Journal of Climate30, 4915-4935 (2017).

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January 19, 2017

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