In the world of weather, we’ve all heard of the “butterfly effect,” that is, if a butterfly flaps its wings in China, it will affect the weather in New York a few weeks later. While this may be an exaggeration, a new study offers more evidence of how seasonal weather patterns can impact conditions thousands of miles away.
Image to right: This photograph of heavy rainfall in Ghana was taken August 19, 2001 during an ongoing NASA/USAID project. The West African monsoon season typically lasts from May-October, with the largest totals observed during June-September. Click on image to enlarge. Credit: Distinguished Professor, Dr. James W. Jones, University of Florida
One cyclic weather pattern is the monsoon — a seasonal reversal in wind direction — caused by the fact that land and ocean heat up at different rates. Land quickly responds to heating, while oceans, with a much larger heat capacity, take longer to react. The difference in temperature leads to a difference in pressure, ultimately creating a wind shift which changes rainfall patterns. The monsoon system in West Africa is especially important, as it brings vital rainfall to support life and maintain local economies.
Now, new research by NASA and University of Maryland scientists suggests that these naturally occurring annual swings in temperature and pressure create two distinct monsoons in Africa, one in late spring and another in mid-summer. Using data from satellites, including NASA’s Tropical Rainfall Measuring Mission (TRMM), researchers were able to detect a distinct northward “jump” in the monsoon from late spring to mid-summer.
In the first episode, large differences in sea surface temperatures between coastal waters and water further out to sea enhances rainfall on the West African Coast. By July, a second monsoon appears. This downpour is related to African Easterly Waves, low-pressure systems that form on the African continent during the summer months. On average, about 60 waves are generated over North Africa each year, and previous research suggests they may kick off small circulations that develop into hurricanes over the North Atlantic Ocean.
Image to left: The Moderate Resolution Imaging Spectroradiometer (MODIS), aboard NASA’s Aqua satellite, captured this true-color image of Saharan Desert dust blowing southwestward off the coasts of Morocco and Mauritania in West Africa. The light brown plume can be seen wending its way over the Cape Verde Islands in the Eastern Atlantic Ocean. Click on image to enlarge. Credit: NASA/GSFC
Between May and October, about two Easterly Waves per week travel from the African continent onto the cool Eastern Atlantic. These waves generally fall apart, but their remnants do survive and head towards the Western Atlantic and Caribbean, where they regenerate. Only about one in ten Easterly Waves survive to develop into gale-force tropical storms or full-fledged hurricanes. But, almost 85 percent of major hurricanes (wind speeds 111 mph or greater) began as Easterly Waves. Some studies have also suggested that many of the tropical cyclones in the Eastern Pacific Ocean can also be traced back to Africa.
While not all Easterly Waves will form into hurricanes, Florida, like Africa, receives a large chunk of its annual rainfall from these storm systems. But other waves that originate to the north of Ghana are dry and pick up debris and dust from Africa’s Sahara Desert. A strong high-pressure system over the Atlantic Ocean, known as the Bermuda High, helps steer these waves toward the Florida coast. Easterly Waves tend to be drier in El NiÃ±o years, which means they’re less likely to produce hurricanes, but more likely to bring pollutants and dust toward the United States.