Research may help increase accuracy of hurricane intensity forecasts
By Summit Voice
SUMMIT COUNTY — Hurricanes can intensify significantly when they move over areas with high concentrations of fresh water, including near-coastal areas where large rivers empty into the oceans.
Researchers with the Department of Energy’s Pacific Northwest National Laboratory, Ocean University of China and Texas A&M studied 10 years worth of hurricane data, finding that significant quantities of fresh water can form a barrier layer near the surface of the ocean that prevents cooler water from mixing to the surface. Typically, hurricanes can weaken themselves as they churn up cooler water from the depths.
The probability that hurricanes will hit such conditions is small, ranging from 10 to 23 percent, but the impacts for affected populations can be significant, according to the study published this week in the Proceedings of the National Academy of Sciences Early Edition.
The study results might help improve predictions of a hurricane’s power in certain regions. The intensification may occur where large river systems pour fresh water into the ocean, such as by the Amazon River system, the Ganges River system, or where tropical storms rain considerably, as in the western Pacific Ocean.
“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.”
A rough estimate for the destruction wreaked by a hurricane is the cube of its intensity. “A 50 percent increase in intensity can result in a much larger amount of destruction and death,” said Balaguru.
Satellites are very useful for tracking and helping to predict the path of tropical storms as they move across the ocean and develop into cyclones, as well as predicting where the storms will make landfall.
But current technology isn’t as good at predicting how intense the storm will be when it does. Satellites can only see the ocean from above, but it’s the ocean’s heat that feeds the storm. So Balaguru decided to look at the ocean itself.
To do so, Balaguru started with one hurricane: Omar. Omar nearly topped the scales as a Category 4 storm in the eastern Caribbean Sea in October 2008, causing $79 million in damages. Balaguru and colleagues collected data about ocean conditions including water temperature, salt content, and water density, and compared that data to the intensity of the storm.
By comparing observations of the storm’s intensity with data on ocean temperature and salinity, Balaguru was able to determine that Omar’s most intense episodes occurred when it was over these thick barrier layers.
But Omar was just one storm. To determine whether the barrier layer connection was real, Balaguru and colleagues examined 587 tropical storms and cyclones between 1998 and 2007 in the western tropical Atlantic, the western Pacific and the northern Indian Oceans.
They found that the tropical storms over thick barrier layers cooled off 36 percent less than storms over areas lacking barrier layers, and barrier layer storms drew 7 percent more heat from the ocean than other storms. That translated into 50 percent more intense hurricanes on average.
The team supported their observational analysis with a computer model, comparing tropical cyclones over regions with and without barrier layers. The model found a similar decrease in cooling by the barrier layer storms, more heat transferred from the ocean to the storm, and a similar intensification.
This work addressed what happens to hurricanes now, under current climate conditions. Scientists predict that global warming will have an effect on the ocean water cycle. Future research could explore how the distribution of the barrier layers changes in a warmer world.
