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There are fewer Australian tropical cyclones (thanks to climate change?)

Posted By Joanne Nova On April 9, 2014 @ 4:49 pm In Global Warming | Comments Disabled

UPDATE: Cyclone Ita is now Category 5 bearing down on Cooktown in North Queensland, the radars will show it soon. 175km NNE of Cooktown. Winds up to 300km /hr.  931 hPa.  See The BOM warnings. Thoughts for those in the path. (It’s clearly visible in the satellite image on the radar link).

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A new paper by Andrew Dowdy tells us that from 1980 to 2013 the incidence of tropical cyclones around Australia has been falling. If CO2 is influencing cyclones around Australia, presumably this implies we should burn more coal.

Those convinced about the power of CO2 will point out that the models predict an increase in intensity, not frequency. To that end, I say: see the BOM graph below. Note the red bars marked “severe”. Then tell yourself that the science is settled and we should spend billions to change those trends. The BOM say “the number of severe tropical cyclones (minimum central pressure less than 970 hPa) shows no clear trend over the past 40 years.”

Interestingly Callaghan et al 2010 goes back all the way to 1870. It finds the trend of severe land-falling cyclones has fallen by a whopping 62%. (Let’s rush to go back to 1870 levels of CO2, right?) All in all, the main conclusion I draw is that I can’t find any evidence that modern climate science understands what drives storm trends.

In January, Haig et al showed that almost all of the last 1,500 years has been filled with more cyclonic pain around Australia than we have now. I discussed that at the time, with a lot of caveats. But the picture grows that people who generate alarm about cyclone trends in Australia are decidedly unscientific.

(All fun aside, Cyclone Ita is becoming a category four  FIVE between Queensland and PNG. It has killed 21 in the Solomons already, and [now] threatens Cooktown, Port Douglass and Cairns. All the more reason for us to understand these storms instead of using them for political point scoring. Best wishes to all in her path. Expected this weekend Friday night.)

Hat tip: The Hockeyschtick

Figure 3. Time series of the number of TCs in the Australianregion (a) with the influence of ENSO removed based on NIN (b) and SOI (c). Linear fits to these data are shown.

 

I would not call this study “long term”, though it’s fair enough that Dowdy uses the longest satellite data he could.

 

Graph showing the number of severe and non-severe tropical cyclones from 1970–2011 which have occurred in the Australian region. Severe tropical cyclones are those which show a minimum central pressure less than 970 hPa. | Source BOM site.

Abstract

Tropical cyclone (TC) observations are used to examine changes in the TC climatology of the Australian region. The ability to investigate long-term changes in TC numbers improves when the El Niño-Southern Oscillation (ENSO) is considered. Removing variability in TC numbers associated with ENSO shows a significant decreasing trend in TC numbers at the 93–98% confidence level. Additionally, there is some indication of a temporal change in the relationship between ENSO and TC numbers, with ENSO accounting for about 35–50% of the variance in TC numbers during the first half of the study period, but only 10% during the second half.

From the Discussion and conclusions

A decreasing trend is suggested by the results presented here for the Australian region. It is noted that this is not representative of other regions or the global average.

The correlation between TC #s and the ENSO indices is weaker during the second half of the study period (from 1998 to 2013) than during the first half (from 1982 to 1997). ENSO was found to account for about 35–50% of the variance in TC #s during the first half of the study period (36% for SOI and 49% for NIN), but only about 10% during the second half (9% for
both NIN and SOI).

In Australia, the La Nina state means more cyclones, so we ought be getting more at the moment shouldn’t we, rather than less?

In the Australian region, one possible physical explanation for a decreasing trend in TC #s could be a potential long-term shift in environmental conditions towards a more El Niño-like state, given that fewer TCs tend to occur in the Australian region during El Niño than La Niña conditions.

Someone thought rising CO2 might have made an El Nino type event (called a Modoki event) a bit more common:

Some studies have hypothesized that increasing greenhouse gas concentrations could potentially influence the occurrence of Modoki events,with Ashok et al. (2012) suggesting that further increases in global warming in the tropical Pacific may result in more basin-wide warm events in place of canonical El Niño events, along with the possible occurrence of more intense El Niño Modoki events.

But the researchers looked for this and found nothing:

…There was virtually no trend in the EMI data used here, such that a temporal change in the
occurrence frequency of Modoki events is not likely to be the cause of the downward trend in TC #s (Figure 3)  or the variation between the two halves of the study period (Figure 4).

Another longer term study, Callaghan et al, also found cyclones have decreased, and substantially. Looks like there is no joy for alarmism in Australian storm trends. By this measure, the invention of electric power plants, has if anything, improved the climate around Australia:

Callaghan and Power (2010) also reported a decreasing trend in TCs making landfall over eastern
Australia, significant at the 90% level, based on reports from numerous historical sources (including peer-reviewed publications, newspapers, sea-faring observations and other media reports dating back to the late 19th century).

In Callaghan 2010, the abstract says there has been a 62% decline in since 1870 in severe cyclones making land-fall (wow):
The linear trend in the number of severe TCs making land-fall over eastern Australia declined from about 0.45 TCs/year in the early 1870s to about 0.17 TCs/year in recent times—a 62% decline. This decline can be partially explained by a weakening of the Walker Circulation, and a natural shift towards a more El Niño-dominated era. The extent to which global warming might be also be partially responsible for the decline in land-falls—if it is at all—is unknown.

REFERENCES:

BOM page: Tropical Cyclone Trends

Callaghan J, Power S. 2010. Variability and decline in the number of severe tropical cyclones making land-fall over eastern Australia since the late nineteenth century. Climate Dynamics 37: 647–662, DOI: 10.1007/s00382-010-0883-2.

Dowdy. A. (2014) Long-term changes in Australian tropical cyclone numbers, Atmospheric Science Letters, DOI: 10.1002/asl2.502 [abstract]

Haig, J., Nott, J. and Reichart, G. (2014)   Australian tropical cyclone activity lower than at any time over the past 550–1,500 years, Nature 505, 667–671 doi:10.1038/nature12882 [Abstract]

 

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^Yes, this is satirical poke. I see no evidence at all that CO2 is making a significant difference to storms either way.

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