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The Saharan Air Layer and Hurricanes

posted on Tuesday, September 29th, 2009 at 5:52 pm
Saharan Dust, Image Credit: NASA

Saharan Dust, Image Credit: NASA

Meteorologists studying hurricanes are still trying to understand the Saharan Air Layer, or SAL, and its affect on hurricanes. On the one hand, scientists believe the SAL triggers the formation of easterly waves, and they know hurricanes form from easterly waves. But, on the other hand, the SAL is composed of hot, dry air. Hurricanes don’t like dry air. Studies have shown this. (Chris Landsea, NOAA) So, after the wave forms, the SAL can help kill the developing storm by injecting dry air into it.

Now, there’s a new question: What is the effect of the dust outbreaks that often occur and travel with the Saharan Air Layer? (The image above captures a dust outbreak in February 2000.)

After the record-breaking 2005 hurricane season, an international group of scientists came together to study these easterly waves coming off the coast of Africa. During the 2006 season, research aircraft based on the Cape Verde Islands, flew into these waves to study their development into tropical cyclones. As I tell you this, you may remember that the 2006 year was a bust for hurricane forecasters-it was an uneventful season. Many blamed a late developing El Nino. These researchers are blaming the SAL and the associated dust outbreak.

In the new issue of the Bulletin of the American Meteorological Society, authors Zipser et al. investigate, “The Saharan Air Layer and the Fate of African Easterly Waves”.

The SAL is composed of at least 3 layers:

1. The top layer: hot, dry air from the Saharan Desert 2. The mid-bottom layer: cool, moist air from the Atlantic coast. 3. Underneath this:  cool, dense marine air near the ocean surface.

Dust is not on my list. That’s because the SAL and the associated windy conditions can cause a dust outbreak, and then the dust will travel with the SAL. But, this is not always the case. The SAL is also associated with two other important features of hurricane forecasting; the easterly wave and the easterly jet. (The jet is an area of strong winds that is shown to negatively impact tropical storm development. – Chris Landsea, NOAA)

The majority of hurricanes in the Atlantic Ocean come from African easterly waves. (Keep in mind that about 60 waves form each year, and only a small number become tropical storms. – Chris Landsea, NOAA) In the article, Zipser et al. writes, African easterly waves “… originate over the African continent, with their initial growth fueled by the temperature contrast between the hot air over the Sahara desert and the relatively cooler, humid air to its south.” This difference in temperatures creates instability or rising air all summer long.  The rising, unstable air creates showers and thunderstorms. The thunderstorms then move off the coast of Africa toward the Atlantic Ocean, and then westward toward the United States, because the predominant wind flow in this area is east to west.

African Easterly Wave, Image Credit: NASA

African Easterly Wave, Image Credit: NASA

So, you have an easterly wave, simply put; an area of  showers and thunderstorms that persists as it is carried along with the predominant wind flow from east to west across the Atlantic Ocean. These are the tropical waves or areas of disturbed weather that TV forecasters are always pointing out on the screen saying, “This is an area of potential tropical development.” National Hurricane Center forecasters usually give these waves a low (less than 30%), medium (30-50%), or high (50% or greater) chance of developing.

In the images to your left, an easterly wave develops on the African continent, begins moving westward off the coast, and then starts to rotate.

In the first image, the big area of white clouds tells you there is a mass of unstable air that is forming showers and thunderstorms. (Really white images on satellite tell you you’re looking at high cloud tops, so most likely you are seeing thunderstorms.)

In the second image (6 hours later), the showers and thunderstorms begin to move westward toward the coast.

In the third image, the area of showers and thunderstorms, or African easterly wave begins to rotate. This tropical wave would later become Hurricane Irene in 2005. It did not affect any land areas.

Now that we’ve talked about what an easterly wave is, and how it is the starting point for a majority of hurricanes, let’s get back to the SAL and it’s impact on easterly waves.

In the year 2006, the year these scientists launched their research, the Saharan Air Layer filled the Atlantic. (It can grow as big as the continental United States.)

The images of the Saharan Air Layer in this blog are similar to what it looked like it 2006. At times, it stretched from the African Coast to the Caribbean Sea. In their research, Zipser, found a couple of items of note. First, global computer models underestimate how dry the air is in the mid-levels in the SAL. Computers estimated the relative humidity is about 40-50% higher than it actually is. Remember, hurricanes do not like dry air. That data supported other papers that show the SAL can inhibit the development of a tropical storm or hurricane.

But, what was more interesting was what they witnessed from the dust. While flying through an easterly wave, instruments showed a, “… deep dust layer up to 4 km altitude.” (p. 1148) Researchers noted that this area of high dust concentration was “highly electrically active”, and it made them question whether the dust actually aided in the development of thunderstorms.

Unfortunately, the article has no conclusion regarding the impact of the dust and the development of tropical cyclones.  The authors hope that their data will aide researchers in the future who hope to answer the question conclusively.

-Dawn Brown

Saharan Air Layer, 2001, Image Credit: NOAA

Saharan Air Layer, 2001, Image Credit: NOAA

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