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In reading about Hurricanes Harvey and Irma last week, I was surprised to find that they originated in West Africa.

So have 85% of other US hurricanes.

I wondered why I missed that in my primary and secondary school science classes in the late 1960s through the mid-1970s. Surely, I would have remembered such a strange fact.

If you are like me and in this same age group, the discovery was made after our time.

How hurricanes develop

Researchers did not discover the African origin until sometime in the 1970s. It probably took another several years for it to be verified and included in the science textbooks.

NASA states that several factors must be in place for tropical storms and hurricanes to occur:

For hurricanes to develop, specific environmental conditions must be present: warm ocean water, high humidity and favorable atmospheric and upward spiraling wind patterns off the ocean surface. Atlantic hurricanes usually start as weak tropical disturbances off the West African coast and intensify into rotating storms with weak winds, called tropical depressions. If the depressions reach wind speeds of at least 63 kilometers (39 miles) per hour, they are classified as tropical storms. Hurricanes have winds greater than 117 kilometers (73 miles) per hour.

The African connection

More recently, over the past decade, research has continued into West African storms. This helps not only Africans but also people from countries in the western Atlantic affected by these deadly events as they travel across the ocean.

In 2002, the international African Monsoon Multidisciplinary Analysis (AMMA) project began. In 2006, NASA announced their research in West Africa between August and September that year, part of the AMMA effort:

in the Cape Verde Islands, 563 kilometers (350 miles) off the coast of Senegal in West Africa. This campaign is a component of a much broader international project, called the African Monsoon Multidisciplinary Analyses, aimed at improving the knowledge and understanding of the West African Monsoon.

Researchers will use satellite data, weather station information, computer models and aircraft to provide scientists with better insight into all the conditions that enhance the development of tropical cyclones, the general name given to tropical depressions, storms and hurricanes. This research will help hurricane forecasters better understand the behavior of these deadly storms.

“Scientists recognize the hurricane development process when they see it, but our skill in forecasting which weak system will intensify into a major cyclone is not great,” said Dr. Edward Zipser, mission chief scientist, of the University of Utah, Salt Lake City. “That is why NASA and its partners place a high priority on obtaining high-quality data for weak disturbances, as well as those already showing signs of intensification.”

In August 2007, the Herald Tribune followed up on the research:

“We’re still in that exploratory phase but I think it’s a gold mine,” said Jeffrey Halverson, an associate science professor at the University of Maryland.

About 70 “waves” are born off the coast of Africa every season, but only about 10 to 15 of them become tropical storms or hurricanes. Among them were hurricanes Andrew and Katrina.

“One of the questions that has haunted meteorologists is why some of these waves turn into hurricanes and others don’t,” said Halverson, who worked with NASA on an Africa project last fall.

“It’s one of nature’s last little secrets with regards to hurricanes. We were given lots of clues.”

The Herald Tribune explains more about NASA’s activities (emphases mine):

NASA set up weather stations on Cape Verde, just off the coast of western Africa, and followed the storms in airplanes across the Atlantic Ocean, using new instruments to gauge moisture and air flow.

The team worked with scientists from the National Hurricane Center, who used airplanes to trace the effect of African dust clouds on storms. The dust clouds are thought to stop storm formation but are still not factored into forecasts.

Another group of scientists launched into the air hundreds of tiny balloons, each filled with a sack of weather meters engineered to drop over the storm twice a day and take readings over more days than ever before.

The experiments, completed last fall, uncovered data that scientists are just beginning to mine for conclusions. The first discoveries are expected to be published in the next six months, but a preliminary look is promising.

The balloons, called “driftsondes,” proved for the first time a deeper layer of moisture in the storms. It was something scientists had a hunch about, but did not know for sure until now. Knowing the information is reliable will allow them to make a more accurate forecast.

“The middle of the ocean has been an area where we haven’t been able to check models, so sometimes there’s a high false alarm rate,” said David Parsons, a scientist from the National Center for Atmospheric Research who worked on the project. “In this case we found out that the model was pretty good.”

Remember driftsondes, because they have been showing up in tweets and forecasts about Irma.

This research is just as important to Africa as it is North America. West Africa has suffered its share of hurricanes over the years. Wikipedia has a full list, divided into 20-year periods.

Interestingly, the number of the storms was approximately the same in every period, which should (but won’t) put paid to climate change hysteria.

Some of the storms that affect West Africa, like Hurricane Beryl (1982), also cross the Atlantic.

The Sahara — sand and dust

Along with the moist climate of West Africa is the Sahara Desert factor. The combination of the moisture and dryness can affect hurricanes.

Before I get to that, did you know that African sand can travel across the Atlantic and create sandstorms in Florida? Talk about a small world!

As for dust, the 2006 NASA announcement stated:

During the field campaign, scientists hope to get a better understanding of the role of the Saharan Air Layer and how its dry air, strong embedded winds and dust influence cyclone development. The layer is a mass of very dry, often dusty air that forms over the Sahara Desert during the late spring, summer, and early fall and usually moves out over the tropical Atlantic Ocean.

As part of looking at the Saharan Air Layer, scientists want to better understand dust’s effect on clouds. Some evidence indicates that dust makes it more difficult for rain to form. Cloud models need to account for any such effect, so measurements of cloud-droplet concentrations and size in clean ocean air and dusty air from the Sahara need to be made.

The Herald Tribune spoke with:

Jason Dunion, a meteorologist at the National Hurricane Center who is studying the effects of African dust on storms, calls it “learning what makes hurricanes tick.”

In 2014, the NOAA (National Oceanic and Atmospheric Administration) explained how the Sahara Desert contributes to storm formation.

It sounds paradoxical to connect a desert with intense rainfall, but:

The role the Sahara Desert plays in hurricane development is related to the easterly winds (coming from the east) generated from the differences between the hot, dry desert in north Africa and the cooler, wetter, and forested coastal environment directly south and surrounding the Gulf of Guinea in west Africa. The result is a strong area of high altitude winds commonly called the African Easterly Jet. If these winds were constant, we would also experience fewer hurricanes.

However, the African Easterly Jet is unstable, resulting in undulations in a north-south direction, often forming a corresponding north to south trough, or wave, that moves westward off the West African Coast. When these waves of air have enough moisture, lift, and instability, they readily form clusters of thunderstorms, sometimes becoming correlated with a center of air circulation. When this happens, a tropical cyclone may form as the areas of disturbed weather move westward across the Atlantic.

The NOAA says that the waves from the African Easterly Jet occur all year round at two- to three-day intervals, however, it is only in the late summer and early autumn when they turn into cyclones. That is the time frame for hurricane season.

Furthermore:

Not all hurricanes that form in the Atlantic originate near Cape Verde, but this has been the case for most of the major hurricanes that have impacted the continental United States.

The Weather Channel site has an article from 2014 accompanied by illustrations of the African Easterly Jet and tropical waves. The article says:

During summer there is a persistent temperature contrast between the deep warmth over the Sahara and the cooler atmosphere over the forest and ocean to the south along the Gulf of Guinea Coast.

The result is a mid-level jet stream, strongest at about 10,000 feet, blowing from east to west toward the tropical eastern Atlantic Ocean.

Some hurricanes originating in Africa can reach the Pacific:

Hurricane Iselle, which hit the Big Island of Hawaii on August 8, 2014, was likely part of a wave that formed more than 8,000 miles away off of the West Coast of Africa and an example of the far-reaching influence the Sahara Desert has on our planet’s weather.

Please check out the NOAA map of the reach of these hurricanes starting off of Cape Verde.

The NOAA article concludes:

When it comes to hurricanes and hurricane preparedness, it’s interesting to know how a desert half a world away can influence the formation of severe weather on our coasts—and even parts of the Pacific Ocean.

A 2015 article from Nature World News has more:

Hurricanes form from three main ingredients: moisture, warm ocean temperatures, and the rotation of Earth. But what causes a hurricane to go from a few storm cells and atmospheric disturbances to a full blown storm? According to new findings published in the journal Geophysical Research Letters, intense thunderstorms in Western Africa are actually in part to blame.

“85 percent of the most intense hurricanes affecting the US and Canada start off as disturbances in the atmosphere over Western Africa,” researcher Colin Price from Tel Aviv University said in a statement. “We found that the larger the area covered by the disturbances, the higher the chance they would develop into hurricanes only one to two weeks later.”

Focusing on hurricane season (June through November), Price and his team analyzed images taken by geostationary satellites, which orbit Earth at the precise speed of Earth’s rotation and take pictures of cloud cover every 15 minutes. Cloud cover is a good indicator if a hurricane is going to occur – the more clouds in an area, the larger the atmospheric disturbance.

Of course, not all of these weather disturbances turn into hurricanes:

only 10 percent of the 60 disturbances originating in Africa every year turn into hurricanes. And while there are around 90 hurricanes globally every year, only 10 develop in the Atlantic Ocean.

Pacific cyclones also have African origins

I was further amazed to read the Weather Channel article which says that nearly all Pacific cyclones originate in West Africa.

Incredibly, these waves pick up steam as they travel across Central America:

Not all tropical waves lead to development soon after they depart Africa. In fact, nearly all eastern Pacific tropical cyclones originate, at least in part, from African easterly waves. Many of these waves traverse the entire Atlantic Ocean and Caribbean Sea without causing development until after crossing Central America.

You would think that crossing land would break their power, but this can increase it.

Why the Pacific coast of North America has fewer hurricanes

The Pacific coast of North America has very few hurricanes. Whilst there are exceptions, such as the aforementioned Iselle — which originated in Africa — the conditions for those originating off the Pacific coast are largely absent.

First, the climate is cooler. Secondly, the wind direction — east to west — obviates such an event. Thirdly, the water temperatures off North America’s Pacific coast are much cooler than those in the Atlantic.

The Scientific American asked NOAA researcher Chris W Landsea (appropriate name) about this. He replied:

Hurricanes form both in the Atlantic basin, to the east of the continental U.S. (that is, in the Atlantic Ocean, the Gulf of Mexico and the Caribbean Sea), and in the Northeast Pacific basin, to the west of the U.S. The hurricanes in the Northeast Pacific almost never hit the U.S., however, whereas the ones in the Atlantic basin strike the U.S. mainland just less than twice a year on average.

There are two main reasons for this disparity. The first is that hurricanes in the northern hemisphere form at tropical and subtropical latitudes and then tend to move toward the west-northwest. In the Atlantic, such a motion often brings the hurricane into the vicinity of the East Coast of the U.S. In the Northeast Pacific, the same west-northwest track carries hurricanes farther offshore, well away from the U.S. West Coast.

The second factor is the difference in water temperatures along the U.S. East and West coasts. Along the East Coast, the Gulf Stream provides a source of warm (above 80 degrees Fahrenheit, or 26.5 degrees Celsius) waters, which helps to maintain the hurricane. Along the West Coast, however, ocean-surface temperatures rarely rise above the lower 70s F. (the low 20s C.), even in the middle of summer. Such relatively cool temperatures do not provide enough thermal energy to sustain a hurricane’s strength. So the occasional Northeast Pacific hurricane that does track back toward the U.S. encounters the cooler waters of the Pacific, which can quickly reduce the storm’s strength.

The magazine asked Kerry Emanuel from MIT’s Center for Meteorology and Physical Oceanography, who added this information about central Mexico:

In the eastern Pacific region, one has to go all the way down to the central Mexico coastline to find water warm enough to sustain hurricanes. This warm water lies well within the belt of easterly winds, so almost all the storms that form there move away from the coast, toward the west. By the time those storms recurve, they are usually many thousands of kilometers west of the coast of North America.

A few storms recurve right next to the coast. Some of these make it as far north as Baja California and can strike land with hurricane-force winds. But to make it all the way to the U.S. West Coast, the storms have to traverse a long stretch of ocean water that is far too cold to sustain hurricanes. Occasionally, tropical storms do strike coastal southern California. By the time they do, they have lost their hurricane-force winds, although they may still bring with them very heavy rainfall that can cause extensive flooding.

More on hurricanes tomorrow from a 2017 article.

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