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Book Review

A Cluster of Typhoons: Intensification over the last three decades

Paper: Mei, Wei; Xie, Shang-Ping. Intensification of landfalling typhoons over the Northwest Pacific since the late 1970s. Nature Geoscience, v9: 753–757.

Super Typhoon Nina approaching the Philippines, 1987. Image produced from NOAA data. Credit: NOAA, wikipedia.

Super Typhoon Nina approaching the Philippines, 1987. Image produced from NOAA data. Credit: NOAA, wikipedia.

In recent decades, it seems we’ve become all too familiar with the devastation that a hurricane can leave behind: ruined infrastructure, swept-away homes, and even fatalities. We’ve become so familiar with them that as this post is written, Florida is bracing itself for Category 4 (Cat 4) Hurricane Matthew, which pummeled Haiti. By now, it almost seems routine to have storm after severe storm. But has the frequency of intense hurricanes, typhoons, and all tropical cyclones always been so high? Luckily, long-collected data can help answer this question.

Although the damage caused by tropical cyclones (TCs) is associated with TC landfalls, the root of the damage comes from the formation of the TC over the ocean, specifically the conditions under which one forms. TCs typically form over large bodies of relatively warm water, such as the equatorial Atlantic or western Pacific. They gain their energy through the evaporation of water from the ocean surface, and then use this acquired energy to cause the devastation we are all too familiar with.

For the Northwest Pacific, where TCs are most active and threaten the large population of Southeast Asia, studies of TC intensity have been hindered by discrepancies between different TC data sets. In particular, the data sets of the Joint Typhoon Warning Center (JTWC) and the Japan Meteorological Agency (JMA) – the two most widely used data sets in typhoon research – show differing trends and annual counts of Cat 4 and 5 typhoons starting from 1977.

Researchers from Scripps Institution of Oceanography and the University of North Carolina set out to reconcile the discrepancies between the data sets to better understand TC intensification trends over recent decades, and to hopefully create more accurate models for future storms. The discrepancies mainly arose from differing methodology starting in 1977: JTWC provided maximum sustained wind speeds averaged over 1 minute periods, while JMA used 10 min periods. The researchers (to put it very simply) adjusted the JMA data from 10-min mean values to 1-min mean values to create better correlation between the two sets of data.

Through the adjusted data sets, they found that the average number of Cat 4 and 5 TCs in the northwest Pacific has increased by more than two per year over the past 38 years – it used to be about 5 per year and now it is up to 7. Additionally, the proportion of Cat 4 and 5 TCs to all typhoons has more than doubled during 1977–2014. These findings all suggest a significant shift towards more intense TCs.

Typhoons by the numbers

Figure 1: (a) Annual number of Cat 4 and 5 typhoons. (b) Ratio of the annual number of Cat 4 and 5 typhoons to that of all typhoons. (c) Annual mean typhoon lifetime peak intensity in the northwest Pacific as a function of time from the JTWC data (black curve) and adjusted (adj.) JMA 1-min wind data (red curve; see Methods). Green curve in a shows results for the original (orig.) JMA 1-min wind data (obtained using JMA 10-min wind data and a conversion used in previous studies). Thick dashed lines in each panel show linear trends during 1977–2014. Credit:

Figure 1: (a) Annual number of Cat 4 and 5 typhoons. (b) Ratio of the annual number of Cat 4 and 5 typhoons to that of all typhoons. (c) Annual mean typhoon lifetime peak intensity in the northwest Pacific as a function of time from the JTWC data (black curve) and adjusted (adj.) JMA 1-min wind data (red curve). Green curve in a shows results for the original (orig.) JMA 1-min wind data. Thick dashed lines in each panel show linear trends during 1977–2014. Credit: Mei & Xie, 2016.

The researchers also wanted to study changes in typhoon intensity with regard to regional characteristics. They grouped TCs of the northwest Pacific into 4 clusters and analyzed each cluster by both the JTWC and the adjusted JMA data sets. Interestingly, and unfortunately for the millions living along Asian shorelines, their analysis showed that typhoon groups with the largest landfall rates show the highest rates of intensification.

The first cluster, Cluster 1, constitutes around 34% of the northwest Pacific typhoons. These typhoons form east of the Philippines, and roughly 75% of them make landfall near China, Taiwan, Korea, and Japan. This cluster had the largest increase of annual mean peak intensity, having risen by ~15% during 1977–2013. Additionally, the number of Cat 4 and 5 typhoons have increased by nearly four times (from less than one per year in the late 1970s to more than four per year in recent years). Cluster 2 makes up nearly 30% of the typhoons in the study area, forming over the South China Sea. 97% of these strike Southeast Asia or South China. Typhoons in this group on average have intensified by ~12%, and Cat 4 and 5 typhoons have approximately doubled both in number and in proportion of all typhoons(from more than 10% to nearly 30%). Clusters 3 and 4 are formed farther east and generally stay over the open water during their lifetime, with a much smaller chance of making landfall. These have only had some increases in intensity compared with the first 2 clusters.

Recall earlier when we said that TCs typically form over large bodies of relatively warm water. You are probably also aware that the global ocean sea surface temperature (SST) is on the rise due to climate change. Well, the researchers found that changes in TC intensification rate (speed at which a TC reaches peak intensification) of these clusters are tied to the SST warming pattern. It certainly appears that with changing global temperatures comes more intense, damaging cyclones. And although the SST change pattern can be affected by changes in ocean currents and surface winds, there are both natural and anthropogenic components. The study encourages more experiments in order to help separate these components for more accurate climate and storm models.

All in all, these researchers were able to reconcile two differing data sets to show that land-falling typhoons have significantly intensified since the late 1970s (between 12% and 15%), and the change in intensification rate is due to warming of Pacific Ocean SSTs. The study supports what climate scientists have been saying for a long time: that with warming oceans comes more frequent and more extreme weather. Typhoons striking Eastern Asia will only intensify, threatening physical, social, and economic impacts to the region. As such, studies like this are paramount to preparation for these threats, and may translate back to TCs like Hurricane Matthew.

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