Tropical waves, also known as easterly waves, are a common phenomenon in the Atlantic Ocean. The NHC (National Hurricane Center) is tracking Tropical Storm Gabrielle and two tropical waves behind it.
A tropical wave is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. A tropical wave normally follows an area of sinking, intensely dry air, blowing from the northeast. After the passage of the trough line, the wind veers southeast, the humidity abruptly rises, and the atmosphere destabilizes. This yields widespread showers and thunderstorms, sometimes severe.
These waves play a significant role in the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins.
Tropical Wave Formation
Formation and Characteristics
Tropical waves form in the easterly flow along the equatorial side of the subtropical ridge or belt of high air pressure which lies north and south of the Intertropical Convergence Zone (ITCZ). These waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator.
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Tropical waves in the Atlantic basin develop from low-pressure disturbances, which develop as far east as Sudan in east Africa and drift across the continent into the Atlantic Ocean. These are generated or enhanced by the African Easterly Jet.
Hovmöller Diagram
One tropical wave, near the Cabo Verde Islands and expected to continue on a westward path, is given a 10% chance of formation in the next two days and only a 20% for the next seven days. Another tropical wave, already exhibiting great thunderstorm action as it traverses the western half of Africa, is on a course for splashdown in the next few days. It is forecast to move off the coast of Africa and has a low chance of development (20%) over the next seven days.
Computer models are beginning to look a little more interesting as they adjust to the passage of our Madden Julian Oscillation. In its most basic definition, the MJO is a wave that moves west to east across the tropics or along the equator over time. As it moves through, it helps to generate lift, moisture and tropical rains. It tends to revolve through its phases on a basis of 30 to sometimes 60 days, or one to two months. This should help to kickstart a few things out there across the Atlantic, sparking development over the coming weeks.
However, these waves will have to fight off an onslaught of dry air and wind shear.
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Sometimes, a surge of dry air called the Saharan Air Layer (SAL) follows a tropical wave, leaving cloudless skies, as convection is capped by the dry layer inversion.
It’s far out in time, but there are some solutions that suggest both highlighted tropical waves could take a track toward the Caribbean. While Tropical Storm Gabrielle spins away from major landmasses altogether, these upcoming tropical waves will bear some monitoring. Right now, we’re not tracking any immediate threat to our area of Central Florida, and there’s plenty of time to monitor the progression of everything out in the tropics.
Here's a summary of the development chances for the tropical waves:
| Tropical Wave Location | 2-Day Formation Chance | 7-Day Formation Chance |
|---|---|---|
| Near Cabo Verde Islands | 10% | 20% |
| Traversing Western Africa | N/A | 20% |
Formation Of A Tropical Cyclone
If a tropical wave is moving quickly or is organized enough, it can have winds of a strength in excess of tropical storm force, but it is not considered a tropical storm unless it has a closed low-level circulation.
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