Easterly Waves

Long waves occur in bands of geostrophic wind flowing above the friction layer. Long waves may flow toward the west or toward the east depending on which of the major global wind belts they occur in. Easterly waves are "long waves" that occur within the trade wind belt, start over north western Africa, and propagate toward the west in the lower tropospheric tradewind flow across the Atlantic Ocean. They are first seen usually in April or May and continue until October or November. They occur between 5-15 degrees N. They have a wavelength of about 2000 to 2500 km, a period of ~3-4 days, and move at approximately 18 - 36 km/h. Approximately two easterly waves per week travel from Africa to North America during hurricane season. Passing from the African continent onto the cool Eastern Atlantic, the waves generally decay, but remnants mostly survive to the Western Atlantic and Caribbean where they regenerate. Only 9 out of 100 easterly waves survive to develop into gale-force tropical storms, or full-fledged hurricanes.

About 60% of the Atlantic tropical storms and minor hurricanes (Saffir-Simpson Scale categories 1 and 2) originate from easterly waves. However, nearly 85% of the intense (or major) hurricanes have their origins as easterly waves. The majority of synoptic scale systems from Africa propagate beyond the Caribbean and the Central American Isthmus into the Eastern Pacific, where some intensify into Tropical Storms. It has been suggested that nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can also be traced back to Africa. Many Typhoons in the Western Pacific are also believed to develop from Easterly Waves, although more work is needed on the relationship of Easterly Waves in the Western and Eastern Pacific.

Fig.1. Approximate location, amplitude and wavelength of easterly waves.

At first, an easterly wave has a small amplitude, and produces mild rain showers. Powerful thunderstorms and the force of high-altitude winds amplify the wave when atmospheric conditions are favourable. Several severe thunderstorms begin to form, and eventually a tropical storm may develop.

Fig. 2. The development of easterly waves off the west coast of Africa.

Troughs and Ridges in Easterly Waves

In order to understand why an easterly wave generates convection, we need to understand what ridges and troughs are in longwaves, and how they affect the behavior of geostrophic wind. In the figure below, we see the formation of a curve in the geostrophic wind, concave toward lower pressure. This is termed a trough in the wave. As the wind crosses a reference latitude toward the south, we observe a curve in the wind concave toward high pressure. This is called a ridge in the wave.

Fig. 3. Relationship between troughs, ridges and atmospheric pressure in easterly waves.

Regions of Convergence and Divergence in Easterly Waves.

Easterly waves influence the movement and pressure of air in the tradewind flow. This is because at certain locations in the long wave, wind speeds up or slows down. These changes cause stretching (divergence), or piling up (convergence), respectively, of the air parcel in the wave. Let’s start by seeing why air changes speed as it moves through a long wave.

Remember that gradient wind is geostrophic wind that flows parallel to curved isobars, and occurs in the absence of friction. When gradient wind is moving through a curve, centrifugal force acts on the parcel of air toward the outside of the curve (see black arrow below). When the wind is curving around low pressure (i.e. moving through a trough), the centrifugal force is acting opposite to the pressure gradient force, or PGF (green arrow below). If the Coriolis force (red arrow), and the wind speed (yellow arrow), were to remain the same, there would now be an imbalance of force acting contrary to the PGF. In order to balance the forces, and maintain an unaccelerated wind, the wind slows down. This automatically reduces the Coriolis force, and rebalances the gradient wind. Because the wind has temporarily slowed down, we call this subgeostrophic wind.

Fig. 4. Forces acting to produce subgeostrophic wind moving through a trough.

Conversely, when the wind is curving around high pressure (i.e. moving through a ridge), the centrifugal force is acting in the same direction as the PGF. If the Coriolis force, and the wind speed, were to remain the same, there would now be an imbalance of force acting in concert with the PGF. In order to balance the forces, and maintain an unaccelerated wind, the wind speeds up in the ridge. This automatically increases the Coriolis force, and rebalances the gradient wind. Because the wind has sped up (with respect to geostrophic wind along straight isobars), we call this supergeostrophic wind.

Fig. 5. Forces acting to produce supergeostrophic wind moving through a ridge.

Completing the picture of a long wave, we see in the figure below that an air parcel moving along through a series of troughs and ridges will alternate between subgeostrophic (slower) gradient wind in troughs, and supergeostrophic (faster) gradient wind in ridges. When wind slows down during its approach to a trough, the air "piles up", causing convergence. When wind speeds up during its approach to a ridge, the air parcel "stretches", causing divergence.

Fig. 6. Regions of convergence and divergence in an easterly wave.

Ahead of a trough, where the air in the wave is slowing down and converging, some air gets "pushed up" away from the surface, producing lower pressure near the surface. Conversely, ahead of an upper level ridge, where the air is speeding up and diverging, air gets "sucked down" into the long wave, producing subsidence and higher pressure near the surface. In this way, regions of subsidence and ascent at the surface are related to the position of troughs and ridges in the easterly wave. Lowerlayer divergence, subsidence, and fair weather are found ahead (upwind, or to the west) of the trough axis. Convergence, ascending motion and heavy weather (showers and towering cumulus) are concentrated to its rear (to the east).

Fig. 7. Locations of ascent and subsidence in an easterly wave in relation to the trough axis.

Fig. 8. Location of convergence and divergence in an easterly wave in relation to the trough axis.

The horizontal structure of an easterly wave is clearest between 700 and 500 mb, and the wave seldom affects the air above the 100 mb level. The figure below shows how the easterly wave (best seen at the 700 mb level) is causing cyclonic circulation at 850 mb, and convergence at the surface (SFC).

Fig. 9. Streamlines at different heights through an easterly wave. Streamlines and
wind barbs of total flow field for composite African wave at (A) surface, (B) 850 mb,
(C) 700 mb, and (D) 200 mb.

(Adapted from Hastenrath, 1991)

The convection and cloud formation associated with easterly waves is often observed from satellites as an inverted V pattern in clouds over the Atlantic Ocean.