March 30, 2010

Southern Hemisphere Hurricanes – Not Changing?

Filed under: Climate Extremes, Hurricanes

Have you heard of all the hurricane activity going on right now in the Southern Hemisphere? We are moving into the hurricane season for the Southern Hemisphere, and if you haven’t heard much about it, the reason is that right now there is zero hurricane activity anywhere on the planet. Of course, there will be a hurricane in the Southern Hemisphere in the coming weeks, and some reporter will immediately invoke global warming – this is our guaranteed prediction!

The latest research on trends in hurricanes in the Southern Hemisphere comes from a team of seven scientists from Australia’s National Climate Centre and the Centre for Australian Weather and Climate Research. Kuleshov et al. begin with the usual proclamation that “Trends in tropical cyclone (TC) occurrences and intensity, and possible physical mechanisms for change, have been discussed widely in recent years.” That is an understatement – we at World Climate Report have posted no end of essays on the topic based on articles on the subject that seem to appear in the literature over and over. Kuleshov et al. review key articles suggesting a substantial increase in intense TC’s for the globe, the North Atlantic, and the northwest Pacific. However, they state “Other authors have rejected these findings, mainly on the basis of the argument that changes have been so great in observation technologies and analysis techniques that the reported changes are artificial, and not due to any actual trends.”

Kuleshov et al. gathered tropical cyclone data for the Southern Hemisphere (SH), the South Indian Ocean (SIO), the South Pacific Ocean (SPO), and the Australian Region (AR). They defined a tropical cyclone as “a tropical system that attains minimum central pressure of 995 hPa or lower.” They also defined severe tropical cyclones as those that attain a minimum central pressure of 970 hPA.

As seen in the key figure below (Figure 1), there are simply no trends in their data, and the authors conclude “For the 1981–1982 to 2006–2007 period, there are no apparent trends in the total numbers of TCs reaching minimum central pressures of 995 hPa or lower, nor in the numbers of severe TCs in the five subdomains of the SH examined”. Once again, a study appears in a major journal showing no trend whatsoever in hurricane activity—had they found trends upward, no doubt their work would have featured in news outlets throughout the world.


Figure 1. Annual numbers of tropical cyclones with lifetime minimum central pressure of 995 hPa or lower for the SH (Southern Hemisphere) (squares, right axis), SIO (Southern Indian Ocean) (diamonds, left axis), SPO (South Pacific Ocean) (circles, right axis), and AR (Australian Region) (triangles, left axis), 1981–1982 to 2006–2007
seasons, with linear trends (from Kuleshov et al, 2010)

So a first glance, it appears we once again have a situation where the observations are not consistent with the predictions for more hurricane activity. That assumes the climate models are predicting more hurricane activity given the ongoing buildup of greenhouse gases; a recent article in the Journal of Climate suggests that the findings from the Kuleshov et al. research may be quite consistent with model predictions.

The latest modeling effort is by four scientists associated with the NOAA/Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. Zhou et al. begin with an interesting sentence as they note “As global atmospheric climate models move to finer horizontal resolution, the hope is that simulations of the climatology of tropical storms will improve to the point that they can be used to reliably study the impact of changing climate conditions (including global warming) on storm statistics.” We at World Climate Report have long questioned whether the models are up to the task of predicting future “storm statistics” and in the first sentence of their article, Zhou et al. appear to confirm our suspicions.

The problem to date is that the global climate models run at a very course spatial resolution that does not allow features of a synoptic-scale event such as a hurricane to be captured in the model. Zhou et al. state that “studies suggest that atmospheric resolutions in the range of 20–100 km may be sufficient to study many aspects of genesis and storm distribution.” They note that “The standard version of the Geophysical Fluid Dynamics Laboratory (GFDL) Atmospheric Model version 2.1 has a horizontal grid spacing of 2° latitude by 2.5° longitude”. Zhou et al. improve on this resolution and use a new version of the GFLD model with a spatial resolution of 0.5°, which is approximately 50 km and well within the desired range for capturing elements of tropical cyclones. We completely agree with their statement “It is important that a model utilized to project tropical storm statistics into the future be capable of simulating observed trends in storm frequency.”

Using the period from 1981 to 2005, they compared the model predictions to observed storms. In the Atlantic, the model worked well with approximately 65% of the variation correctly reproduced by the model. The explained variance levels were lower in the east, west, and south Pacific, but still highly statistically significant. For a variety of reasons, the model did not reproduce the storm counts very well in the Indian Ocean. Not lost in our eyes was the model predicting an 8% reduction in global hurricane activity over the 1981 to 2005 period.

Now for the really good stuff. Zhou et al. explain “To explore potential changes in hurricane frequency from anthropogenic climate change, we perturb this model with SST anomalies taken from various models of the climate near the end of the twenty-first century” (SST stands for sea surface temperature). They found an 18% reduction in hurricanes globally, with an especially large reduction in the Atlantic. They state “the global mean reduction (~15%) appears to be close to the value one would estimate by extrapolating the reduction simulated by the model for the period 1981–2005 (~8%)”. A particularly interesting finding is that the reduction in hurricane activity in the Southern Hemisphere is twice as large as the reduction in the Northern Hemisphere.

So we put the two articles together and find they are quite consistent with one another. The observed reduction in hurricanes in the Southern Hemisphere is consistent with the prediction for fewer hurricanes in the future!

References:

Kuleshov, Y., R. Fawcett, L. Qi, B. Trewin, D. Jones, J. McBride, and H. Ramsay. 2010. Trends in tropical cyclones in the South Indian Ocean and the South Pacific Ocean. Journal of Geophysical Research, 115, D01101, doi:10.1029/2009JD012372.

Zhou, M., I.M. Held, S.-J. Lin, and G.A. Vecchi. 2009. Simulations of global hurricane climatology, interannual variability, and response to global warming using a 50-km resolution GCM. Journal of Climate, 22, 6653-6678.




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