Climate Change May Increase the Number of Hawaiian Hurricanes

The Eastern Pacific is a busy place for tropical storms and hurricanes, with an average of 15 named storms, 8 hurricanes, and 3 intense hurricanes forming each year. However, these plentiful storms rarely affect Hawaii. The predominant storm track is well to the south of the Hawaiian Islands, and the air tends to be dry and ocean temperatures relatively cool near the islands, making it difficult for a storm to make it there intact. But with two tropical storms potentially threatening the islands in the coming week, and Tropical Storm Flossie having passed with 100 miles of the islands in 2013, it is fair to ask, could climate change be increasing the odds of tropical storms and hurricanes affecting the Hawaiian Islands? A 2013 modeling study published in Nature Climate Change, "Projected increase in tropical cyclones near Hawaii", found that global warming is expected to increase the incidence of tropical storms and hurricanes in Hawaii. Lead author Hiroyuki Murakami, from the International Pacific Research Center at the University of Hawaii at Manoa, commented in a press release accompanying the paper: "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."


Figure 1. Projected change in number of tropical cyclones per year by the last quarter of this century in the 2013 Murakami et al. modeling study published in Nature Climate Change, "Projected increase in tropical cyclones near Hawaii" (in this study, tropical cyclones were defined as only tropical storms and hurricanes, though the general term "tropical cyclones" usually includes tropical depressions as well.) The frequency of a tropical cyclone in a 5°x5° area over the Hawaiian Islands increased from about 0.7 - 1.2 storms per year to about 2 - 3 storms per year. Note that the research projects that the heavily populated Mexican Pacific coast will see a decrease in tropical storms and hurricanes--about one less storm per year. The green stippling indicates statistical significance at the 99 percent confidence level. Image credit: Press release from the University of Hawaii, Hiroyuki Murakami, and Nature Climate Change (2013).

Why an Increase for Hawaii?
Even though their model predicted that fewer tropical cyclones would form in the Eastern Pacific in a future climate with global temperatures 2°C (3.6°F) warmer than at present, more of these storms made their way to Hawaii. This occurred because of three factors:

1) A shift in the upper air steering currents, caused by movement of the upper-level westerly subtropical jet poleward so that the mean steering flow near Hawaii became more east-to-west.

2) A tendency for storms near Hawaii to be stronger (stronger hurricanes tend to move more to the northwestward in the Northern Hemisphere, due to a phenomenon known as beta drift, caused by the variation in the Coriolis parameter across the width of the storm.)

3) A northwards shift in the genesis location where Eastern Pacific tropical storms formed, due to warming of the ocean waters.

"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," reassured study co-author Wang in the press release. Only three tropical storms or hurricanes have made landfall in the islands since 1949, an average of one every 27 years, so an increase by a factor of 2 - 3 would imply a landfall every 9 - 14 years. With such a low incidence of storms, it will be very difficult to determine if they are indeed changing due to a changing climate without several decades of data, though.


Figure 2. Double trouble for Hawaii: True-color VIIRS image of Hurricane Iselle (left) and Tropical Storm Julio (right) approaching Hawaii, taken between 3 - 6 pm EDT August 5, 2014. At the time, Iselle was a Category 2 hurricane with 110 mph winds, and Julio had 65 mph winds. Image credit: NOAA Visualization Lab.

I'll have a new post Wednesday morning.

Jeff Masters