As we head into the teeth of hurricane season, scientists have identified a new phenomenon that could intensify a tropical cyclone's strength once it reaches land. It's called the "brown ocean" effect, and it could signal a new level of potential devastation for hurricanes as they make landfall.
During hurricane season, storm watchers and researchers closely study the forces that transform African storms into tropical cyclones as they gather strength off shore in deep ocean waters and move towards land. They try to predict their intensity as they make landfall so that coastal communities are prepared. Hurricanes feed off the warm, moist air during the summer over ocean waters.
Right now, for instance, even though the peak of the hurricane season is still a month away, four named storms have already formed and are picking up strength in the deep oceans. This is unusual. Since 1950, there have only been five other hurricane seasons when more than one storm formed in the deep tropics before Aug. 1. That means that this hurricane season is likely to be more intense than usual. August, September and October are typically the most active months in the hurricane season.
When tropical superstorms hit major cities, they can cause major destruction – like Hurricane Katrina in New Orleans and Superstorm Sandy in New York and New Jersey. A saving grace for these destructive hurricanes in the past has been that they lose strength as they hit land and are unable to pick up warmth and moisture from the oceans.
No more. That saving grace may be disappearing as the world warms.
New research recently published in the International Journal of Climatology shows that nearly a fifth of such tropical superstorms may actually increase their intensity – not lose it – when they cross land. The NASA-funded study is the first global assessment of the post-landfall strength and structure of inland tropical cyclones, and the weather and environmental conditions in which they occur.
In short, some significant percentage of tropical cyclones will pick up warmth and moisture from the soil of the land – treating the land as if it was a "brown ocean" – because of changes occurring in Earth's climate that are affecting land and oceans alike. This "brown ocean" effect means that some percentage of hurricanes and superstorms will continue to cause destruction as they move inland.
"Inland flooding is one of, if not the most, deadly aspects of tropical cyclones or hurricanes. We already know that it doesn't take a major hurricane to create hazardous or flood conditions," one of the study's authors, Dr. J. Marshall Shepherd, told me.
"This work highlights that we can't always take for granted that threats from a hurricane diminish just because it makes landfall," said Shepherd, a University of Georgia professor of geography and meteorology, and the current president of the American Meteorological Society.
What the research showed was that some tropical storms will actually pick up strength and energy from very moist and warm soils and increase their energy and intensity inland. The researchers called this the "brown ocean" effect to distinguish it from what we know about how tropical storms become hurricanes out over the waters of deep oceans.
"Ample evaporation from very moist soils feeds into tropical cyclones inland, supplying energy to sustain cyclones that would otherwise decay," the study's lead author, Dr. Theresa Anderson, a UGA post-doctoral researcher, told me. "Knowledge of prior rainfall and soil moisture conditions can provide clues as to how a tropical cyclone might behave post-landfall".
Such inland tropical storms are mimicking the moisture-rich environment of the ocean where the storm first began its journey, she said.
None of this is good news for hurricane watchers or communities hundreds of miles from coastlines. If tropical storms are able to cause destruction both as they hit the coast – and then actually increase their intensity by picking up energy inland over the "brown ocean" of land and soil, it may signal a new era for tropical cyclones and hurricanes.
There has already been some evidence of this "brown ocean" effect in the United States. In the summer of 2007, for instance, Tropical Storm Erin confounded meteorologists because it intensified as it tracked through Texas. It actually formed a hurricane "eye" over Oklahoma, and was more intense inland than it had ever been over the ocean.
Erin was an example of this newly defined type of inland tropical cyclone that Shepherd and Anderson are defining as "brown ocean" storms capable of picking up energy from warm, most soils inland.
Anderson and Shepherd studied tropical storms that survive beyond landfall by analyzing data from NOAA's National Climatic Data Center gathered during the past 30 years that had tracked at least 220 miles inland and then compared it to NASA atmospheric and environmental data sets.
Of the 227 inland tropical cyclones that were identified by Shepherd and Anderson, 45 of them maintained or increased strength – exhibiting this "brown ocean" effect. They also found that not all such inland storms fueled by the "brown ocean" effect are the same – other factors come into play as well that increase or diminish the inland storm's effect.
While most inland tropical cyclones occur in the United States and China, Shepherd and Anderson found that hot spots during the 30-year study period turned out to be in Australia. When they investigated the environment and conditions surrounding the "brown ocean" phenomenon in Australia that gave rise to the inland storms, they were able to pin down conditions that drive them. If soils have plenty of moisture or release latent heat, for instance, that can fuel inland storms.
So as the Earth's climate changes – as dry areas get drier and wet areas get wetter – such inland superstorms will only become more likely because of the "brown ocean" effect. And hurricane-watching won't be confined just to communities along the coast. We'll all need to pay attention.