Pacific Ocean typhoons could be intensifying more than previously projected

November 21, 2018 by Sandra Ortellado, American Geophysical Union
Super typhoon HaiyanCredit: NASA Earth Observatory

Changes to the uppermost layer of Earth's oceans due to rising temperatures are likely causing an increase in intense Pacific Ocean typhoons, suggesting strong typhoons may occur more frequently than scientists project in the coming decades, according to new research.

The surface layer of the , known as the ocean mixed layer, is approximately 200 meters (656 feet) deep on average. This layer is constantly exchanging gases with the atmosphere and experiencing mixing caused by winds, heat transfer, evaporation and changes in salinity.

Intense typhoons, like 2013's Super Typhoon Haiyan, are classified based on high wind speeds, often of 130 mph or more. Typhoon Haiyan was one of the strongest tropical cyclones ever recorded, and scientists have noticed an increase in the proportion of intense typhoons occurring each season in the Pacific Ocean since the 1980s but have not been able to explain why.

A new study published in Earth's Future, a journal of the American Geophysical Union, finds the ocean mixed layer deepened along tropical cyclone tracks by 1.7–2.0 meters from 2002-2015, while other factors changed only marginally. The authors conclude this deepening could be responsible for the uptick in intense typhoons from 1980 to 2015, and they project the increase of intense typhoons will continue at a greater rate than previously projected in the coming decades.

Understanding how climate variability within a climate system contributes to the observed increase in the proportion of intense typhoons helps scientists understand the hazards posed by devastating storms in the north Pacific Ocean, especially as the ocean and atmosphere continue to warm, according to the researchers.

"In the context of global warming, it's important to know how the ocean responds to global warming, which factors contribute, and how that changes the structure of the ocean," said Liguang Wu, lead author of the study and professor at Nanjing University of Information Science and Technology in Nanjing, China. "We need to be aware of how conditions are changing around us in order to prepare."

A combined image of clouds and sea surface temperatures in the Atlantic on August 27, 1998, which shows Hurricane Bonnie over the East Coast of the United States and the cool water track that Bonnie left in its wake. Ocean surface cooling is a function of the ocean mixed layer depth, according to the new study. Credit: NASA Scientific Visualization Studio
Why are typhoons becoming more intense?

Scientists have been studying the changes in typhoon over the past several decades, but the reasons for the increase in typhoon intensity is still a subject of controversy, according to Wu.

In the new study, Wu and his team examined the contributions of various factors controlling typhoon intensity change, such as , the temperature of outward flowing air and water, ocean mixed layer depth, and vertical wind shear, as well as shifts in the tropical cyclone tracks. They used computer simulations to compare each factor with observed tropical cyclone intensities in the western North Pacific basin for each year from 1980-2015.

After quantifying the contribution of each environmental factor to tropical cyclone intensity, they determined that the increase in the proportion of intense typhoons was largely due to a deepening of the ocean mixed layer. This deepening is in turn caused by variations in ocean and atmospheric conditions.

The deepening of the ocean mixed layer is just one of many substantial changes to atmospheric and ocean circulations that have occurred in the western North Pacific since 2000 as a result of climate change, according to the researchers. Deepening of the ocean mixed layer is likely the major reason for the sudden increase in the proportion of intense typhoons in 2001, Wu said.

Because previous studies have not accounted for ocean mixed layer depth in their projections, the authors conclude that future typhoons in the North Pacific may be increasingly intense, and to an even greater degree than previously thought.

"Previous studies have focused mainly on the role of sea surface temperature in changing typhoon intensity," Wu said. "But they did not account for ocean mixed layer depth as we have, which is why we think that those projections might have underestimated the increase in intensity."

Explore further: Increasing rainfall in a warmer world will likely intensify typhoons in western Pacific

More information: Liguang Wu et al. Dominant Role of the Ocean Mixed Layer Depth in the Increased Proportion of Intense Typhoons During 1980-2015, Earth's Future (2018). DOI: 10.1029/2018EF000973

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1 / 5 (4) Nov 21, 2018
Another climate article that "could be" full of chit.
5 / 5 (5) Nov 21, 2018
Another climate article that "could be" full of chit.

Or it "Could be" more likely to occur. Given science's track records, I would choose the later. ie; more likely to occur.
5 / 5 (1) Nov 23, 2018
well, the old_coot continues to "chit" all over the internet. It's why he refuses to let the orderlys change his diapers and clean him up. Smearing the feces of primitive religions all over a public forum is his fetish.
not rated yet Nov 24, 2018
There is no increase in typhoon activity since 1950. In fact available data shows small decrease in activity, by number of major typhoons, by days when major typhoons were active, and by total season ACE. At least that's what I get by putting a trendline on data from here: http://tropical.a...tpacific
5 / 5 (2) Nov 24, 2018
There is no increase in typhoon activity since 1950. In fact available data shows small decrease in activity, by number of major typhoons, by days when major typhoons were active, and by total season ACE.

I've said before that most raw data is meaningless without making the appropriate adjustments, and this is true of the JTWC data you used at Colorado State. You should at least have clicked on the "Data Quality" tab to get an idea of some of the problems with the data. In actual fact, there's been an increase in typhoon intensity in both the western North Pacific and the North Atlantic and it correlates very closely to the SST in those areas (http://www.atmose...anes.pdf ). That paper also details why the raw data isn't useful in the supplementary information (https://www.natur...ure03906 ).
5 / 5 (2) Nov 24, 2018
Additionally, ACE is a bad way to look at hurricane intensity. It's hard, just looking at that value to tell if the hurricanes have increased in size or not. From the above paper:
the peak wind speeds exhibit little if any correlation with measures of storm dimensions
I'd prefer that the paper used power dissipation which includes the area of the storm, but it seems the cube of max velocity correlates in some way to the monetary loss due to the storm. As we've seen recently, storms are getting larger and that's directly related to the increase in SST and OHC (https://agupubs.o...EF000825 ).
not rated yet Nov 24, 2018
You should at least have clicked on the "Data Quality" tab to get an idea of some of the problems with the data

Which, along other things, also states:

The Dvorak technique is the primary technique currently used today to estimate tropical cyclone intensity from satellite imagery. This technique was first initiated in 1972; however, the quality of satellite data in the 1970s through the mid-1980s likely led to significant underestimates in intensity, especially for the most intense systems.

so, if anything, it underestimates for that period.

Of course, ACE is just a rough estimate, and integrating windfield over entire surface area would be better, but other stats have similar trendlines. The increasing activity trend is present in the North Atlantic, though.
5 / 5 (2) Nov 24, 2018
so, if anything, it underestimates for that period.

Yes, but only for the period of 70s to mid-80s. Looking at Emanuel's paper, even a significant increase in PDI for those periods doesn't markedly change the results: Warmer ocean temperatures lead to more severe typhoons.

As for ACE, I agree that it's not a very useful metric for climate change, especially as it's expected that increasing temperatures lead to fewer typhoons. Another recent paper on the recent increase in typhoon intensity (https://journals....5-0585.1 ) said it best:
As the increasing intensities compensate for the loss of ACT by decreasing number of TCs, the ACT remains largely unchanged.

PDI improves on that a bit because of the cube of max velocity, but I'd still prefer to see the area included. ACE/ACT and PDI don't include the storm area increase that seems to have occurred recently.

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