Some people have asked me why we bother talking about tropical waves when they are thousands of miles away from South Florida. We don’t want to over focus on these disturbances, but it is important to understand that tropical cyclones don’t develop out of nowhere. A tropical cyclone develops from a pre-existing disturbance such as a frontal trough, an upper-level low that eventually works its way down to the surface, or from a tropical wave.
About 60% of Atlantic tropical storms and Category 1 and 2 hurricanes originate from tropical waves, and nearly 85% of major hurricanes (Category 3, 4 and 5) develop from tropical waves.
Tropical waves are generated by an instability of the African easterly jet. The jet develops as a result of the temperature gradient over western and central North Africa due to extremely warm temperatures over the Saharan Desert and the cooler temperatures along the Gulf of Guinea coast. The waves typically emerge from the coast of Africa every 3 to 4 days, are most pronounced in the low to mid levels of the atmosphere, and move generally westward across the Atlantic Ocean. The waves are most easily seen starting in April or May and continue through the hurricane season. On average, about 60 waves are tracked every year. As far as we know, there is no relationship between the number of waves tracked and total Atlantic tropical cyclone activity for the year.
Although major hurricanes comprise only 24% of U.S. landfalling tropical cyclones, they account for about 85% of the total damage. Most major hurricanes hit the U.S. in August, September and October. Given that the large majority of major hurricanes develop from tropical waves, it makes sense to track these waves as carefully as possible.