Over past 15 years or so, you may have noticed that a great deal of attention has been given to the Sahel region in Africa. Especially, if you look at the seasonal outlooks by both Dr. William Gray of Colorado State, and NOAA. Both these seasonal forecasts use rainfall in the Sahel region as a key component in their outlooks. In addition, this particular site also mentions the region on occasion, especially when it comes to our Tracking the Tropics segment. Another area in Africa responsible for tropical waves and moisture is the Horn of Africa, which has disturbances come in from the Arabian Sea or Indian Ocean.
The Sahel has also garnered attention recently due to the food crisis in Niger and even neighboring Mauritania and Mali, which have been hit hard by desert locust outbreaks the past several years. While many in the media have stated that the situation in Niger is a famine, many in that country's government including the president insist that it is not a famine because it is not widespread, and it is under control. Organizations such as Africares are in agreement with this assessment.
For those who don't know, the Sahel region is a region in Central Africa that is south of the Sahara Desert, and comprises of parts, or all of the following countries: Senegal, Guinea, Guinea-Bisseau, Mali, Mauritania, Burkina Faso, Ivory Coast, Liberia, Sierra Leone, Togo, Ghana, Benin, Nigeria, Cameroon, and Chad. Rainfall from this region comes from mesoscale convective systems, or strong clusters of showers and thunderstorms that produce tropical rainfalls. More importantly though, these MCS eventually move into the waters of the Eastern Atlantic, and become the one hundred or so tropical waves that develop in the Atlantic Basin each Hurricane Season.
Of these hundred or so tropical waves, about an average of ten of these become named storms, and only about six become hurricanes with two becoming major hurricanes. That average is the 55 year average from 1950-2004, and in the last ten years, we have seen nine seasons go by with numbers well above those averages. Now, according to Dr. William Gray of Colorado State University, and the folks at NOAA, that is because one of the factors, rainfall in the Sahel region of Africa has been normal or above normal.
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Although yearly precipitation for the region for the 20 year period from 1979 to 2000 was below normal, the rainy season in the region produced near normal rainfall. The 2005 season in particular though has been very moist. If you have been reading some of the Tracking the Tropics segments we have been putting up so far this season, you will have noticed several times at least that we have mentioned the abundant moisture in Africa as well as much of the Tropical Atlantic. In 2003, there was above average rainfall during the rainy season in the Sahel, which runs from June to September.
Last year, there was near average rainfall in the Sahel as a near-normal monsoonal pattern in the Western portion of Africa was primarily responsible. Now, what is a monsoonal pattern you ask? Well, it is not a rainstorm like many are led to believe. It is a circulation pattern that results from winds blowing onshore or offshore depending upon the location of high and low pressure systems. For instance, let's take a look at the area most commonly associated with monsoons, India.
During the cold weather months in India, high pressure over Siberia is dominant, which produces a clockwise circulation, which results in an offshore flow and dry conditions. However, as we move from winter to summer, a transition takes place, where a thermal low develops over the Arabian Peninsula. It is a thermal low because of the extremely hot temperatures in the Arabian desert, and the inverse relation that pressure and temperature have. Well, basically, this thermal low creates a counterclockwise flow that results in an onshore pattern. The moist air in this onshore pattern gets lifted over the very high terrain of the Himalayan mountain chain, which causes condensation, and that in turn produces clouds and rain.
Places such as Cherrapunji, India, the rainfall capital of the world, receive most of their yearly rainfall during this period. Monsoon patterns aren't restricted to just India. They can even occur in the Desert Southwest of the United States. During the month of August, the shift in winds causes increased precipitation in that region, which is the bulk of rainfall the Desert Southwest gets during the year. West Africa has a similar type of circulation during its rainy season. In addition to the West African monsoon circulation, above normal sea surface temperatures in the North Atlantic also contributed to the rainfall totals in the Sahel for 2004.
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Another area for rainfall across Africa that results into the eventual emergence of tropical waves and subsequent development of tropical storms and hurricanes is the Horn of Africa region, which consists of Ethiopia, Somalia, Eritrea, the Sudan, Djibouti, Northern Congo, and the Central African Republic. Thunderstorm complexes usually push westward into the Sahel from this region. The start of the rainy season here is usually dictated by the position of the ITCZ during the course of the year.
For those, who aren't familiar with some of the details of climate and global weather, the ITCZ stands for the Intertropical Convergence Zone. This is an area where the trade winds converge, and is a common location for many tropical waves that are starting out. The ITCZ is part of the overall global circulation pattern, which fluctuates during the course of the year according to how much sunlight is reaching particular parts of the earth. Similarly, subtropical ridges, polar easterlies, westerlies (jet stream) also change position, which is how we transition from summer to winter, and back to summer again.
While the overall climate of the region is complicated, the rainy season in the Northern sector of the Horn of Africa, which consists of the countries we mentioned earlier, usually occurs between the months of June and September, but there can be a second rainy season from March to May. So, with this information, people should now have a better idea of what we are talking about when it comes to the Sahel region as well as the rest of Central Africa.
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