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Science

Tropical Cyclones Intensify When They Hit Freshwater

A new study has been published which analyses a decade’s worth of tropical cyclones and found that, when a hurricane blows over ocean regions high in freshwater content, it can unexpectedly intensify.

The probability that a hurricane will ever encounter such conditions is relatively low, ranging from 10 to 23 percent, but the effect when it does happen is relatively large: Hurricanes can intensify by up to 50 percent.

Cyclone Nargis on May 1, 2008, as a category 2 storm.

Authors Note: for the purposes of this article, the term ‘hurricane’ will be used instead of tropical cyclone. For a full explanation of tropical cyclone naming conventions, click here

“Sixty percent of the world’s population lives in areas affected by tropical cyclones,” said ocean scientist Karthik Balaguru at the Department of Energy’s Pacific Northwest National Laboratory. “Cyclone Nargis killed more than one hundred and thirty eight thousand people in Burma in 2008. We can predict the paths cyclones take, but we need to predict their intensity better to protect people susceptible to their destructive power.”

Conditions likely to affect a hurricane in this way require large amounts of freshwater to be deposited into the path of a hurricane. For example, where the Amazon River system or the Ganges River system reach the ocean, or in regions such as the western Pacific Ocean where tropical storms deposit rain into the ocean.

In a normal situation, where a Hurricane will simply move along the ocean, the storm’s intensity will decrease as the ocean water cools off due to mixing by the strong winds under the storm, which in turn pumps less heat into the storm.

However, as Balaguru and his PNNL colleagues and researchers led by Ping Chang at Texas A&M University and Ocean University of China in Qingdao, China discovered, when there is enough freshwater to create a barrier layer, typically 50 metres below the surface, the ocean water can’t cool as much and continues to pump heat into the storm.

So instead of dying out, the storm can actually intensify by up to 50 percent.

The barrier layer has this effect on storms, Balaguru said, because it insulates the surface layer from the colder water below, preventing the storm’s access to cooling water. When fresh water dumps into the salty ocean, it makes the surface layer less salty, creating the barrier layer below it. When a passing storm causes the surface layer to pull up water from below, the water comes from the barrier layer rather than the much colder water beneath.

The research team ended up investigating 587 tropical storms and cyclones between 1998 and 2007 in the western tropical Atlantic, the western Pacific and the northern Indian Oceans. In addition, they supported their observational analysis by creatign a computer model that compared tropical cyclones over regions with and without barrier layers.

Source: Pacific Northwest National Laboratory
Image Source: Photo courtesy of NASA




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