August 1, 2008 Meteorologists can make long range predictions about hurricane activity months in advance using what they call the Atlantic Meridional Mode. It is the relationship between sea surface temperature, wind speed and direction, and the amount of moisture in the atmosphere. Researchers can make a forecast based on winter conditions. A negative rating translates to a quieter hurricane season.
- Saffir-Simpson Hurricane Scale
- Meteorological history of Hurricane Katrina
- Hurricane Opal
- List of major natural disasters in the United States
The 2008 hurricane season is underway. Will it be an active season, or a quiet one? Scientists are looking at a new atmospheric model that could hold some of the answers, and one day allow them to predict hurricane activity months in advance, providing an earlier warning for those in harm's way.
Former New Orleans chef John Roussos serves up a taste of the Crescent City in his Wisconsin restaurant. Though he was hundreds of miles away, he says Hurricane Katrina hit close to home.
"It'll be awhile before I go back - maybe a long while," says Roussos.
University of Wisconsin atmospheric scientist Jim Kossin, Ph.D., wasn't just close to a hurricane -- he was inside one. "In Hurricane Gilbert, that we flew into, the eye was very clear because it was such a strong storm," Dr. Kossin says.
Now, Dr. Kossin's research focuses on a new way to understand hurricane activity by combining key ocean and atmospheric factors. It's called AMM or Atlantic Meridional Mode -- a relationship between sea surface temperature, wind speed or direction and the amount of moisture in the atmosphere. Dr. Kossin says a positive or warm AMM sets the stage for an active hurricane season.
"During the AMM warm phase we have warmer water than average, weaker shear than average, many hurricanes that form and become major hurricanes, " Dr. Kossin says. A negative AMM means colder water, stronger winds, and dryer air -- conditions that stop hurricanes. "And you can see that the number of hurricanes that are forming during this phase are much less," says Dr. Kossin.
Concentrating on how the atmosphere and ocean work together may help scientists better understand hurricanes themselves.
"The number of hurricanes that we get, how long they last, how strong they get, where they track, which I think will have. We're still working on it, but I think it will have a lot of implications for landfall, which of course we care most about," Dr. Kossin says.
It's one more step toward forecasting one of the earths' most powerful natural phenomena.
ABOUT HURRICANES: A hurricane is a type of tropical cyclone, a low-pressure system that usually forms in the tropics and has winds that circulate counterclockwise near the earth's surface. Storms are considered hurricanes when their wind speeds surpass 74 mph. Every hurricane arises from the combination of warm water and moist warm air. Tropical thunderstorms drift out over warm ocean waters and encounter winds coming in from near the equator. Warm, moist air from the ocean surface rises rapidly, encounters cooler air, and condensed into water vapor to form storm clouds, releasing heat in the process. This heat causes the condensation process to continue, so that more and more warm moist air is drawn into the developing storm, creating a wind pattern that spirals around the relatively calm center, or eye, of the storm, much like water swirling down a drain. The winds keep circling and accelerating to form a classic cyclone pattern.
RATING HURRICANES: Hurricanes are categorized according to the strength of their winds according to the Saffir-Simpson Hurricane scale. They are rated from lowest wind speeds (Category 1) to highest (Category 5). But even lower category storms can cause a great deal of damage, mostly from storm surges -- when water is pushed towards the show by strong winds and combines with normal tides to create hurricane storm tides -- and the resulting flooding. The worst devastation from hurricane Katrina, for example, occurred when flooding caused the New Orleans levees to fail.
EFFECTS OF CLIMATE CHANGE: If it seems like hurricanes are becoming more frequent and severe, that's because they are, thanks to rising air and water temperatures around the world, which make it easier for hurricanes to form. Some scientists attribute this to global warming and human activity, such as deforestation and greenhouse gas emissions. Others think that it is due to natural changes deep in the Atlantic, part of a natural cycle that shifts every 40-60 years.
The American Geophysical Union and the American Meteorological Society contributed to the information contained in the TV portion of this report.