PNNL

Results: Typhoon Alley, an area of the western tropical Pacific, already has destructive storms that rip through the region. That area may see more and more intense storms, according to researchers at Pacific Northwest National Laboratory.  Their analysis of the strongest tropical storms over the last half-century—known as super typhoons—reveals that they are intensifying. Rain that falls on the ocean reduces its salinity and allows typhoons to grow stronger.

"This work has identified an extremely important region affected by this, the western tropical Pacific known as Typhoon Alley. These storms are really destructive over that region," said oceanographer Dr. Karthik Balaguru of PNNL, who published the work in Nature Communications.

Why it Matters: Typhoons need heat to grow. Known in the Atlantic as hurricanes, typhoons normally have a natural check on how intense they become. The oceans store a lot of the sun's energy as heat, and storms rely on that heat to build. As their strong winds whip up the ocean's surface, they churn deeper colder water to the surface, which cools off the surface and weakens the typhoon's power.  

Previous studies suggested that as the planet warms, so does the surface of the ocean. As the temperature difference between surface ocean water and deeper water increases, ocean churning by typhoons would cool the surface more strongly. Ultimately, that could decrease the intensity of tropical storms in the future.

But a warmer atmosphere also brings more rainfall to the ocean—and freshwater is less dense than saltwater. As the freshwater collects on the ocean's top, it prevents the normal churning, thereby keeping the surface warmer. Thus, a lack of ocean water mixing might mean more surface warmth and a more intense storm.

Methods: The team focused on the western Pacific Ocean, where almost a third of tropical storms form.  Looking the salinity of the top layer of ocean, they saw that between 1958 and 2013, the ocean there did become less salty during typhoon season, and most of this decrease was in the top 50 meters of ocean. A quick overlay of storm tracks showed that they fell along the areas of lower ocean salinity.

The team then looked at how the surface salinity changes affected the strength of super typhoons, storms that are as strong as category 4 or 5 hurricanes. They analyzed the wakes of cold water the super typhoons left on the ocean as they passed, and which of two competing factors—the typhoon intensity bump from a decrease in salinity or an intensity decrease from a larger gradient in the ocean temperature—played a bigger role in modulating the intensity of the super typhoons. They found that the influence of the salinity change to fuel storms was about 50 percent stronger than the ocean temperature effect that decreases the intensity of super typhoons. The strongest super typhoons are most affected by the changes because they strongly rely on the ocean's heat as their fuel. When they plugged those relationships into climate model projections for the future, they team found that as greenhouse gases and temperatures rise, the increase of rainfall over the oceans will ultimately lead to more intense storms. They verified that effect using almost 20 different climate models.

What's Next? This work identifies a need to study upper ocean salinity in addition to temperature when examining the intensity of typhoons.

For more information, read the PNNL news release, "Increasing rainfall in a warmer world will likely intensify typhoons in the western Pacific"

Acknowledgements

Sponsor: The research was supported by the U.S. Department of Energy's Office of Science, Office of Biological and Environmental Research for the Regional and Global Climate Modeling program; and the U.S. National Oceanic and Atmospheric Administration's Atlantic Oceanographic & Meteorological Laboratory.

Reference: Balaguru K, GR Foltz, LR Leung, and KA Emanuel. 2016.  "Global Warming-Induced Upper-Ocean Freshening and the Intensification of Super Typhoons." Nature Communications 7:13670. DOI: 10.1038/ncomms13670