What role the Sahara plays in the tropics

TALLAHASSEE, Fla. (WTXL) — Tropical storms and hurricanes rely on high amounts of moisture in the atmosphere, among other things, to develop and maintain their strength. But there are times in hurricane season when nature spoils the moisture and activity remains low.

Airborne dust from the vast deserts of the African continent often contributes to sedentary periods in the Atlantic Ocean and other nearby waters, limiting the growth of tropical systems.

“How does desert dust get into the air?” you may ask. Tropical waves – clusters of moisture around a low-pressure system – form over land near the Sahara region of Africa. The wind forces of the disturbances lift the dust higher into the atmosphere, and winds at mid- and high-altitudes keep the mineral-rich mass aloft. This dry air source originates in the Sahara Desert during warmer seasons and spreads across parts of the Atlantic Basin, following natural air currents in the atmosphere. The layer of dry, dusty air is usually about one to two miles thick and sits about a mile above the Earth’s surface.

Large amounts of dusty air are common during the early months of the Atlantic hurricane season. Dust levels can vary from light to dense, but layers usually reach their peak between June and mid-August and can move across the vast Atlantic basin during these months.

The combination of lower humidity, stronger upper-level winds and warm air reduces the formation of tropical storms in the open Atlantic. The dust cloud can reach the American mainland and thus also hamper formation efforts in the Gulf of Mexico.

When the Saharan air layer reaches land, including Big Bend and South Georgia, the Saharan dust can give the daytime sky a hazy appearance and add red hues to the rising and setting sun. People with sensitive respiratory systems or related medical conditions may experience difficulty breathing when the dust layer is particularly dense and thick.

Tropical systems need this deep source of moisture to maintain and increase their strength. A disturbance embedded in the dust zone will suck this drier air into its circulation, effectively decreasing the available moisture in its core and causing the disturbance to weaken. This was the case before the July 4 holiday weekend, when a tropical wave in the lower North Atlantic had a reasonable chance of continuing to develop, but it got caught in the Saharan air layer and lost structure as it reached the Caribbean Sea.

There are cases – such as the recent Hurricane Beryl – where tropical waves and the deep-level moisture they carry can penetrate just outside the periphery of a Saharan dust zone. The upper-level wind pattern at the edge can actually support a storm system that wants to develop without including the actual dry air in its circulation.