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The Role of Solar Forcing in Australian Tropical Cyclone Activity

Paper Reviewed
Haig, J.E.-A. and Nott, J. 2016. Solar forcing over the last 1500 years and Australian tropical cyclone activity. Geophysical Research Letters 43: 2843-2850.

Writing as background for their study, Haig and Nott (2016) cite the work of Kuleshov et al. (2012), who highlighted a "shortfall of long-range tropical cyclone forecasts in recent seasons from statistical models largely based on the traditional regional, interannual to decadal predictors such as sea surface temperature and the Southern Oscillation Index." The apparent weakness of these models, according to the two Australian researchers, is the short length of the indices they utilize (usually less than 50 years), which fail to capture larger-scale climatic drivers that vary on centennial to millennial time scales.

In an effort to enhance the current limited knowledge surrounding low-frequency drivers of tropical cyclone activity, Haig and Nott set out to compare a relatively new high-temporal resolution paleorecord of tropical cyclone activity (tropical cyclone activity index, CAI) from two sites in eastern and western Australia (Haig et al., 2014) with three known solar-driven indices: Sunspot Number, Southern Hemisphere tree ring δ14C data and 10Be data from the North Greenland Ice Core Project. And what did those comparisons reveal?

Regression analysis between CAI and sunspot extremes for the modern period (1970 to present) revealed a statistically significant negative relationship (i.e., more sunspots lead to less CAI activity) that was further confirmed via cross wavelet analysis. Cross wavelet analysis was also performed for CAI and the two cosmogenic isotope records (δ14C and 10Be), revealing correlations that "potentially follow the 88 year Gleisberg and 205 year de Vries [solar] Cycles."

In commenting on their findings, Haig and Nott write that "the link between tropical cyclone activity in Western and Eastern Australia and solar cycles at decadal and centennial scales is certainly compelling." Indeed it is, and it demonstrates the importance for researchers and policy makers to understand all of the many natural drivers of climate on both short and long time scales, which understanding may well help to further refine model projections and reduce error, such as that observed in tropical cyclone forecasts by Kuleshov et al.

Haig, J., Nott, J. and Reichart, G.-J. 2014. Australian tropical cyclone activity lower than at any time over the past 550-1,500 years. Nature 505: 667-671.

Kuleshov, Y., Wang, Y., Apajee, J., Fawcett, R. and Jones, D. 2012. Prospects for improving the operational seasonal prediction of tropical cyclone activity in the Southern Hemisphere. Atmospheric and Climate Sciences 2: 298-306.

Posted 4 November 2016