Wednesday, October 25, 2006

Could Saharan Dust Storms Slow Hurricane Season?

No one would have believed, in the last years of the 19th century, that human affairs were being watched keenly and closely by intelligences greater than man’s and yet as mortal as his own.

So begins the H.G. Wells classic The War of the Worlds, the story of an alien invasion of our planet. Equipped with giant machines and powerful weapons, the Martians carve a path of destruction through England.

Yet, in the end, the alien invaders are defeated, not by the “weapons and guile of man, but by the simplest of all things, tiny microbes” which the Martians lacked any resistance.

In Wells’ timeless science fiction novel the Martians learned the hard way that one should never underestimate the power of something small. Now, researchers studying hurricanes are applying those same principles when it comes to Saharan dust. Tiny specks of dust may be suppressing hurricane activity just like the microbes that brought down the Martian machines.

Researchers at the Hurricane Research Division in Miami and the University of Wisconsin studied the past 25 years of satellite data of the Atlantic Ocean. They found that during times of intense hurricane activity the large clouds of dust that periodically blow westward from the Saharan Desert are relatively scarce.

Yet, in years when they are few hurricanes, the dust storms were stronger and tended to spread over much of the Atlantic Ocean and Caribbean Sea.

“The research is still in its infancy,” said James Dunion, a hurricane researcher at the Hurricane Research Division. “The results of the most recent study are very encouraging.”

In 2002, Dunion developed a technique for detecting the large dust clouds by using special infrared imagery from weather satellites. Since then, Dunion and researchers at the University of Wisconsin have been looking at the large dust clouds and their impact on hurricanes.

This past summer Dunion and his colleagues conducted new research in the eastern and central Atlantic. Using Hurricane Hunter aircraft, Dunion’s group flew eight missions to study the interactions between developing hurricanes and Saharan dust storms.

“We were able to focus our efforts over a part of the ocean basin that is rarely sampled by aircraft,” said Dunion. “Yet, this area (in the eastern Atlantic) is the breeding ground for hurricane seedlings that account for over half of the tropical storms and hurricanes that we see in the Atlantic each year.”

Some of the most powerful hurricanes in history have formed in the eastern Atlantic Ocean. Storms like Andrew, Camille and, most recently, Frances, all began life as a tropical disturbance in the eastern Atlantic.

“This research has helped us learn more about what causes hurricanes to intensify or weaken,” said Dunion.

The Saharan dust storms are part of a feature called the Saharan Air layer (SAL). SAL outbreaks tend to be most intense in the early summer and are responsible for ejecting vast amounts of dry, dusty air into the Atlantic throughout the hurricane season.

Some of the dust storms can cover an area the size of the lower 48 states and can travel as far west as Central America, the Gulf of Mexico and here in South Florida.

Dunion will spend the coming months analyzing his data trying to understand how the various workings of the dust storms-dry air, strong winds and suspended dust-can act to suppress hurricane formation and intensification.

Like those tiny microbes that conquered the Martin invaders in Wells’ science fiction classic, Dunion hopes to discover how tiny specks of dust can bring down the great storm on Earth.

Posted at 12:10 PM

Tuesday, October 17, 2006

When Will It Be Our Turn?

Top down or top up? When I drive to the television station each weekday afternoon the top of my convertible is still up, the windows still shut and the air conditioning (thank goodness) still humming.

The calendar may say autumn but it still summer in South Florida. But, it won’t be long before I can put the top down on my car and enjoy the cool, dry air that (before Frances, Jeanne and Wilma came along) made South Florida famous.

“Most Floridians await the arrival of fall with the same anticipation that is common among northerners who long for the first warm days of spring,” writes Morton Winsberg, author of Florida Weather (University of Central Florida Press). “After a season of heat and humidity, the first break in high temperatures comes as a great relief.”

In recent weeks, we’ve seen a slight moderation in the temperature and, more importantly, the humidity. A series of weak “cool” fronts have swept through the area ushering in slightly drier air that, especially in comparison to the brutal humidity of summer, feels pretty good.

Still, the weak fronts have not had much of an impact on temperatures. Sure, the morning lows are a few degrees cooler but each afternoon the mercury is nipping at 90 degrees, a bit too warm for the top to come down on my car especially when I’m wearing a dress shirt and a tie.

So, when can we expect that first genuine cold front of the season? The first front that will knock down the temperature, eliminate humidity and put a smile on everyone’s face?

In the last three years, that first mighty cold front has arrived as early as October 16 and as late as November 6th, which fits in nicely with a study Winsberg did for his book. Winsberg, a professor of geography at Florida State University, examined weather records for a 30-year span for three areas across the Sunshine State: the panhandle, central Florida and southern Florida.

He found that the first cool spell of the season arrives in Tallahassee between September 28 and October 6, in Orlando between October 18 and October 29 and in South Florida between October 24 and November 17th.

Right on time, folks in Tallahassee and Orlando have already experienced their first taste of fall with comfortable temperatures and low humidity. Yet, while the early season cold fronts have brought delightful weather to northern and central Florida, they have not been strong enough to push that cool air into South Florida.

When will it be our turn?

Of course, none of us will forget the arrival of last year’s first big cold front. It was the afternoon of October 24th and came just hours after Hurricane Wilma tore through South Florida.

Wilma knocked out power to 98% of the area but, thanks to the front, overnight lows dropped into the 50’s making the cleanup manageable. We didn’t have power but the dry, clear air behind that first cold front gave us spectacular views of the night sky, while the sunny, comfortable days made the recovery from Hurricane Wilma a bit easier.

This year’s first significant cold front is probably just a few weeks, maybe even days away from South Florida. It is likely taking shape in the Prairie Provinces of central Canada preparing to drive south into the United States and eventually Florida, bringing an end to our wet season and beginning our really great weather season.

Top down or top up? It won’t be long now until the answer is top down.

Posted at 12:31 PM

El Niño: He's Back

It was a clear, calm day at the bottom of the Earth when an iceberg known a B15A suddenly broke into a half-a-dozen pieces. Scientists, monitoring the Antarctica ice shelf from satellite photographs, were stunned and confused. What could cause the 60-mile-long iceberg to fall apart?

The answer was even more surprising. Seismometers planted in the ice suggested the iceberg had been moving up and down and from side to side before it broke apart. The scientists figured a storm somewhere might have generated strong waves that destroyed the iceberg.

That “somewhere” turned out to be more than 8,000 miles away.

“Our jaws dropped,” said Douglas MacAyeal of the University of Chicago. “We looked in the Pacific Ocean and there, 8,000 miles away, six days earlier, was the winter season’s first really big, nasty storm that developed and lasted about a day and a half in the Gulf of Alaska.”

Writing in the journal Geophysical Research Letters, MacAyeal said his discovery shows how weather in one region of the world can affect events far away.

Another worldwide weather maker is taking shape in the rapidly warming waters of the equatorial Pacific Ocean. On September 13, the Climate Prediction Center reported that region’s sea surface temperatures were 0.5 degree centigrade above normal. Since then, the water temperature has continued to increase leading scientists to an unmistakable conclusion: El Niño is back.

El Niño is the nickname for the periodic warming of the Pacific Ocean near the equator. For reasons still not clearly understood, every three to five years the ocean waters, from the coast of Peru stretching thousands of miles to the middle of the Pacific Ocean, heat up.

The extra heat and moisture released into the atmosphere produces striking differences in the temperature between the equator and areas to the north. That, in turn, leads to a much stronger jet stream (a river of air five to seven miles above the Earth) and a shift in weather patterns around the world.

In 1982, one of the strongest El Niño’s in history was blamed for some 2,000 deaths and estimated losses totaling $13 billion worldwide. Peru had its worst rainfall in history; 11 feet of rain in locations which usually got six inches. Droughts, dust storms and forest fires swept through Australia, Indonesia and Africa. There was a warm, wet spring in the east coast of the United States; shark attacks off the Oregon coast and a rise in bubonic plague cases in New Mexico.

No one knows the strength of this year’s El Niño but El Nino’s in the past have had a huge impact on Florida’s winter weather.

“During strong El Niño events Florida experiences more frequent and stronger low-pressure systems from late fall through early spring,” said Bart Hagemeyer of the National Weather Service Melbourne office. “This increased storminess brings slightly cooler than normal temperatures, a greater chance of heavy rain and flooding, and severe weather such as tornadoes and damaging wind storms.”

In other words, a dry season that’s not so dry, but not necessarily very cold. While Hagemeyer expects more frequent cold fronts this winter the threat of a freeze is quite low.

“This is because during El Niño conditions warmer than normal temperatures are found over western Canada and the U.S. Northern plains which are the source for cold arctic air outbreaks in Florida,” said Hagemeyer. “During an El Niño season the average jet stream track is further south so it's not pulling down arctic air from the far north to Florida.”

If El Nino sticks around through next summer, that southern jet stream track will play a huge role during the hurricane season. El Nino is the hurricane terminator.

In 1997, part of the current cycle of increased hurricane activity, another El Nino had a huge impact on that year’s hurricane season. Eight named storms formed that summer and fall but only one, Hurricane Erika, developed in the deep tropics and only Hurricane Danny impacted the United States.

The Climate Prediction Center continues to monitor this year’s developing El Nino and will issue an update soon.

Posted at 12:18 PM

What Hurricane Season?

It was unanimous.

There was no disagreement.

The top experts in the hurricane prediction business all agreed: the 2006 season was going to be another incredible year.

Dr. William Gray and his young protégée, Philip Klozbach, forecast 17 named storms would form this year, the most storms ever predicted in the 22-year history of the Colorado State University program.

Meteorologists at NOAA and the National Hurricane Center agreed, predicting 13 to 16 storms. Adding that the “potential for hurricanes striking the United States is high.”

NASA hurricane expert Dr. David Adamec said conditions in the Atlantic Ocean were “hurricane friendly”.

Yet with two months to go, the 2006 hurricane season, especially in comparison to the last two years, has been a walk in the park. There have been nine named storms this year with five becoming hurricanes and three intensifying into major hurricanes.

Two of the most powerful storms this season (Gordon and Helene) were fish storms sliding through the middle of the Atlantic Ocean. The season’s other major storm, Gordon, made a direct impact on Bermuda knocking out power to more than half of the island residents but causing no serious damage.

The only storm to impact South Florida was wimpy Ernesto, a tropical storm that swept across the state just over month ago. The storm made landfall in mainland Monroe County on August 30 with winds of 45 mph. Compared to Frances, Jeanne and Wilma, Tropical Storm Ernesto was little more than a few dark clouds and a spit of rain.

So, what happened? How could so many incredibly smart people be so wrong?

Scientists say four unforeseen weather factors have contributed to this year’s relatively quiet year: increased wind shear, an abundance of dry air, the location of the Bermuda High and an increase in troughs along the east coast of the United States.

Wind shear (gusty winds at different levels of the atmosphere) was and continues to be a common feature this hurricane season. Throughout the year, upper level low-pressure systems rotated through the tropical breeding grounds of the Atlantic Basin ripping apart any tropical system that was trying to form.

On many occasions, those upper lows were accompanied by vast areas of dry air (often full of Saharan dust) making it more difficult for a tropical system to take shape. And, even if a storm did develop, it had a tough time intensifying in such a hostile environment.

The Bermuda High, a large high-pressure system often located over the island of Bermuda during the peak of the hurricane season, was instead much further east this year. That meant that any storm that did form was steered into the open waters of the Atlantic Ocean, well away from the east coast of the United States.

Finally, in recent weeks, a series of troughs of low pressure have formed along the east coast of the country blocking any tropical system from approaching the United States.

Still, the hurricane season is far from over. October is historically a fairly active month (Wilma hit on October 24) and tropical systems can still form during November. But, it is safe to say that the predictions made by all of the experts back in May and early June will not pan out. This hurricane season will bear no resemblance to last year’s incredible season.

Posted at 12:00 PM