While on a global level tropical cyclones are expected to slightly reduce in frequency overall, because of regional conditions some areas will actually experience an increase in tropical cyclone frequency. Hawaii happens to be one of those areas.
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The Tropical Cyclones that threaten Hawaii mostly form in the eastern Pacific just south of the Baja California peninsula where higher sea surface temperatures (SST), lots of moisture and weak vertical wind shear provide ideal conditions for storm formation from June to November.
Currently many tropical cyclones that form in the eastern Pacific fizzle out before reaching Hawaii due to drier conditions over the central sub-tropical Pacific and wind shear from the westerly subtropical jet- stream. But global warming is projected to move the upper-level westerly subtropical jet-stream poleward, enhancing the mean steering flow easterly for cyclones so that more will travel the distance to hit Hawaii.
The study - Projected increase in tropical cyclones near Hawaii (abstract) - was published online in Nature Climate Change on 5 May 2013. Climate researchers Hiroyuki Murakami, Bin Wang and Tim Li from the Meteorology Department, University of Hawaii at Manoa teamed up with Akio Kitoh at the Meteorological Research Institute, the University of Tsukuba in Japan to write the study.
The IPCC special report on extreme weather events (SREX) released November 2011 reported that for Tropical Cyclones around the world, Intensity may increase but frequency may stay the same or even decrease. Wind speeds are likely to increase, although perhaps not in all ocean basins. There was medium confidence in a projected poleward shift of extra-tropical storm tracks. Storm frequency and intensity may vary with local and regional conditions in different ocean basins.
The scientists in this study used a state-of-the-art, high-resolution global climate model, and data on the recent history of tropical cyclones in the North Pacific. They projected cyclone frequency and tracks for a 2075-2099 scenario, based upon greenhouse gas emissions continuing to rise resulting in temperatures about 2°C higher than today. With the lack of current global mitigation action this is probably a quite conservative scenario estimate.
FIGURE 5 Schematic diagram showing calculation of prescribed lower boundary conditions for the future projections. The abbreviations are as follows: SST (Sea Surface Temperature), HadISST (Hadley Centre observational SST), PDTR (trend in the present - day SST), PDIV (inter - annual variation in the present - day SST), GWTR (trend in the ensemble mean of future SSTs), GWIV (inter -annual variation in the ensemble mean of future SSTs), GWDT (averaged future changes in the ensemble mean of SSTs), CGCM (Coupled General Circulation Model), SRES (Special Report on Emission Scenarios), and C20C (Climate of the Twentieth Century).
"In our study, we looked at all tropical cyclones, which range in intensity from tropical storms to full-blown category 5 hurricanes. From 1979 to 2003, both observational records and our model document that only every four years on average did a tropical cyclone come near Hawaii. Our projections for the end of this century show a two-to-three-fold increase for this region," explains lead author Hiroyuki Murakami.
Ironically, the study found that fewer tropical cyclones will form in the eastern Pacific, but those that do form are more likely to travel the 3000 miles to Hawaii due to more favourable conditions.
"Computer models run with global warming scenarios generally project a decrease in tropical cyclones worldwide. This, though, may not be what will happen with local communities," said Murakami.
The historical incidence of tropical cyclones landing on Hawaii is fairly low - about one in every four years. But the study found that the frequency is likely to increase to perhaps an average of one every year or two striking Hawaii. This was a robust and statistically significant finding.
"Our finding that more tropical cyclones will approach Hawaii as Earth continues to warm is fairly robust because we ran our experiments with different model versions and under varying conditions. The yearly number we project, however, still remains very low," reassures study co-author Wang.
The study focussed on storm frequency, but we know that in a warming world storm systems also carry more moisture. The torrential rain that can be unleashed can cause devastating flash flooding, damaging crops and human infrastructure and resulting in deaths.
The study abstract says in full:
Projections of the potential impacts of global warming on regional tropical cyclone activity are challenging owing to multiple sources of uncertainty in model physical schemes and different assumptions for future sea surface temperatures. A key factor in projecting climate change is to derive robust signals of future changes in tropical cyclone activity across different model physical schemes and different future patterns in sea surface temperature. A suite of future warming experiments (2075-2099), using a state-of-the-art high-resolution global climate model robustly predicts an increase in tropical cyclone frequency of occurrence around the Hawaiian Islands. A physically based empirical model analysis reveals that the substantial increase in the likelihood of tropical cyclone frequency is primarily associated with a north-westward shifting of the tropical cyclone track in the open ocean south-east of the islands. Moreover, significant and robust changes in large-scale environmental conditions strengthen in situ tropical cyclone activity in the subtropical central Pacific. These results highlight possible future increases in storm-related socio-economic and ecosystem damage for the Hawaiian Islands.
- Adapted from Eurekalert media release, May 5, 2013 - More hurricanes for Hawaii?
- Hiroyuki Murakami, Bin Wang, Tim Li, and Akio Kitoh: Projected increase in tropical cyclones near Hawaii. Nature Climate Change, May 5, 2013, Projected increase in tropical cyclones near Hawaii (abstract)
- Images from the study Suplementary information Courtesy Hiroyuki Murakami, Nature Climate Change. Figure 1 - projected change in number of tropical cyclones per year by the last quarter of this century. The green stippling indicates statistical significance at the 99 percent confidence level.