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Hurricane Seasons Grow Longer

By Andrea Thompson,
LiveScience.com
Posted: 2008-07-15 15:30:52
Filed Under: Science News
LiveScience.com


(July 15) -- Hurricane seasons have been getting longer over the past century and the big storms are coming earlier, LiveScience has learned. The trend has been particularly noticeable since 1995, some climate scientists say.

Further, the area of warm water able to support hurricanes is growing larger over time. The Atlantic Ocean is becoming more hurricane friendly, scientists say, and the shift is likely due to global warming.

"There has been an increase in the seasonal length over the last century," Jay Gulledge, a senior scientist with the Pew Center on Global Climate Change, told LiveScience. "It's pretty striking."

A study Gulledge co-authored with other climate scientists found a five-day increase in season length per decade since 1915.

Hurricane season officially starts June 1, but the first named storm of the 2008 season, Tropical Storm Albert, formed on May 31. The first hurricane of the season, Hurricane Bertha, formed on July 1, reaching hurricane strength on July 7, relatively early in the season for a major storm.

In the last decade, more strong storms have been forming earlier in the season, said hurricane researcher Greg Holland of the National Center for Atmospheric Research in Boulder, Colo.

While this trend hasn't been formally linked to global warming because climate models can't reproduce individual storms, Holland thinks it's likely that the warming caused by manmade greenhouse gases is a major factor in the seasonal shift based on observations of changes in recent decades and the predictions models are making for the changing conditions in the Atlantic basin.

The length of the hurricane season is "one of the potentially big signals" that could change in response to global warming, Holland said.

Defining the season

The definition of the hurricane season depends on who you ask: For hurricane forecasters and coastal residents living in an area prone to hurricane landfalls, the standard dates are June 1 to Nov. 30. The National Hurricane Center uses these dates because historically most storms occur within that span of six months and because having a definitive time frame helps to heighten the public's awareness of the dangers of hurricanes.

But for researchers looking at how hurricane activity has changed over time, those dates don't really matter — meteorologists look at the dates of the first and last named storms in a given year, which allow them to evaluate the actual length of each hurricane season.

Since 1995, hurricane seasons have been increasing in length based on the latter definition, Holland said, with stronger storms that typically wouldn't be seen until mid-August showing up in July (Bertha, which became a Category 3 storm in the Atlantic last week, is one example).

Expanding warm pool

Like a hurricane's intensity, the length of the hurricane season is affected by the temperature of the ocean that fuels the storms. The warmer the water, the more energy a storm has to draw from.

Hurricanes and tropical storms have been forming earlier in the season recently because "we now get warmer sea surface temperatures earlier in the year," Holland explained. "The whole season has extended out."

Peter Webster of Georgia Tech put a finer point on it. "There is some work that says that the length of the North Atlantic hurricane season has become longer as SSTs [sea surface temperatures] warm up more quickly early in the season," he said.

Tropical storms and hurricanes need water of at least 80 degrees Fahrenheit (26.5 degrees Celsius) to form. The area where water temperatures meet or exceed that mark is called the warm pool.

In recent years, the warm pool has expanded, creating a larger area over which hurricanes can develop and strengthen, Holland told LiveScience. It is now reaching all the way to the coast of Africa, allowing storms to form farther east, and so giving them more time to strengthen as they traverse the Atlantic.

Bertha, for example, formed farther east than any other July storm on record.

These storms that form so far over in the eastern Atlantic are called "Cape Verde-type" storms, after the chain of islands off the western coast of Africa. Cape Verde-type storms account for a major proportion of all major hurricanes (Category 3 and higher), Holland said.

More News From LiveScience.com

These storms tend to take a straight westward path across the Atlantic, avoiding land and cooler waters, which can kill a storm. Hurricane Andrew, which devastated southern Florida in 1992, and 2007's Hurricane Dean, which wreaked havoc in the state of Yucatán in Mexico, were both Cape Verde-type storms, along with Bertha.

Holland thinks that the growth of the warm pool will be a factor in the length of future hurricane seasons by promoting these and other early-forming storms.

Outliers

Other early storms, outliers to the standard June 1 to Nov. 30 season, such as this season's Tropical Storm Arthur or last year's Subtropical Storm Andrea (which formed on May 9), aren't all that unusual. Such early birds were seen even before global warming became an issue — the earliest-forming storm in recorded weather history was observed on March 7, 1908.

"There's always been the odd one out," Holland said, adding that we'll likely see more of these in a warming world.

"We have to expect that they'll be more outliers," he said, though he doubts that the official dates of hurricane season will change, since most will still lie within that window.

But these aren't the early-forming storms that Holland is worried about, because they tend to be weaker. It's the major storms, like the Cape Verde-type, that are forming in July and later that are the ones to watch out for, he said.

These shift to more major storms is also cause for concern because the Atlantic historically had fairly timid hurricane seasons compared to other storm-producing basins such as the Indian Ocean. Because the Atlantic basin wasn't optimized for hurricane formation already, "it didn't take much of a change to see a difference," Holland said.

One other way the Atlantic basin is becoming more hurricane-friendly, besides warmer oceans, is more favorable atmospheric conditions. Warming ocean temperatures also change atmospheric circulation patterns. Holland said some changes are already happening over the Atlantic and climate models predict that these changes will also tend to promote the development of storms off the coast of Africa.

"All of the stars are lining up," he said.

(c) 1999-2007 Imaginova Corp. ALL RIGHTS RESERVED.

2008-07-15 11:21:30
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indyplumbing 11:17:18 AM Jul 16 2008

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These "fellows" have no idea what their talking about. I honestly think they start with a conclusion and work backwards to find or create evidence to support their theories. They been wrong about the last two seasons. While I believe we should do what we can to help the earth be cleaner, the earth changes and has forever and will continue to do so............ no human can stop that. I think Global Warming is a crock!!! People who believe in it are, therefore, "crockpots."

jkanon 10:34:21 AM Jul 16 2008

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Where are the deniers?

jkanon 07:21:23 AM Jul 16 2008

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tjfoster965 01:06:56 AM Jul 16 2008 Report This! legodaddy, your emergency preparedness ideas are right on target. Do you want to pay the extra two percent each in municipal and federal taxes to support it?-------------------------------------MYTH: Even if global warming is a problem, addressing it will hurt American industry and workers.FACT: A well designed trading program will harness American ingenuity to decrease heat-trapping pollution cost-effectively, jumpstarting a new carbon economy.Claims that fighting global warming will cripple the economy and cost hundreds of thousands of jobs are unfounded. In fact, companies that are already reducing their heat-trapping emissions have discovered that cutting pollution can save money. The cost of a comprehensive national greenhouse gas reduction program will depend on the precise emissions targets, the timing for the reductions and the means of implementation. An independent MIT study found that a modest cap-and-trade system would cost l

benstraight1980 07:20:42 AM Jul 16 2008

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benjamin straight writes: At least it has been a quiet season so far.

jkanon 07:19:33 AM Jul 16 2008

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For instance, we might not know if it will be below freezing on a specific December day in New England, but we know from our understanding of the region's climate that the temperatures during the month will generally be low. Similarly, climate tells us that Seattle and London tend to be rainy, Florida and southern California are usually warm, and the Southwest is often dry and hot. Today’s climate models can now reproduce the observed global average climates over the past century and beyond. Such findings have reinforced scientist’s confidence in the capacity of models to produce reliable projections of future climate. Current climate assessments typically consider the results from a range of models and scenarios for future heat-trapping emissions in order to identify the most likely range for future climatic change.

jkanon 07:17:57 AM Jul 16 2008

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allpropgh 03:08:48 AM Jul 16 2008 Report This! oh yeah just like the year after Katrina was supposed to be really bad. (Didn't happen) these guys barely get the weather right for the next day, how do they know anythingMYTH: Accurate weather predictions a few days in advance are hard to come by. Why on earth should we have confidence in climate projections decades from now?FACT: Climate prediction is fundamentally different from weather prediction, just as climate is different from weather.It is often more difficult to make an accurate weather forecast than a climate prediction. The accuracy of weather forecasting is critically dependent upon being able to exactly and comprehensively characterize the present state of the global atmosphere. Climate prediction relies on other, longer ranging factors. For instance, we might not know if it will be below freezing on a specific December day in New England, but we know from our understanding of the region's climate that the temperatures during

fopdaddy1 06:33:50 AM Jul 16 2008

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what does it all mean? tune in next century, oh, yeah we`ll all be dead.

tilllarsen 06:08:37 AM Jul 16 2008

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Sceince has improved, however the understanding of what is being stated has diminish so much that the acuracy rarely matters because to many don't take the time to listen!

allpropgh 03:08:48 AM Jul 16 2008

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oh yeah just like the year after Katrina was supposed to be really bad. (Didn't happen) these guys barely get the weather right for the next day, how do they know anything

tjfoster965 01:06:56 AM Jul 16 2008

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legodaddy, your emergency preparedness ideas are right on target. Do you want to pay the extra two percent each in municipal and federal taxes to support it?

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