Factors Affecting Hurricane Development in the Atlantic Ocean
What conditions are necessary for hurricanes to form in the Atlantic Ocean?
How do El Niño and La Niña affect hurricane activity in the Atlantic?
What impact does Saharan dust have on hurricane development?
Tropical cyclones the world over begin as areas of loosely organized cumulonimbus clouds over warm water, but what is it that transforms rather mundane thunderstorms into one of the largest and most destructive atmospheric phenomena? It turns out that the creation of a tropical cyclone (which is typically called a hurricane in the Atlantic Ocean) relies on the presence of a number of elements in both the atmosphere and the ocean—as well as the reduced influence or outright absence of other phenomena that, if they were strong enough, could keep these monstrous storms from getting started in the first place.
Critical elements
Several conditions must be met before hurricanes can form. They must originate over water in areas characterized by low atmospheric pressure and warm surface water; they cannot form over land. In general, the waters of the North Atlantic warm between March and August each year, which prompt national weather bureaus in the U.S. and elsewhere to mark the interval between June 1 to November 30 as “hurricane season,” that is, the period in which hurricanes are most likely to develop and intensify.
Many North Atlantic hurricanes begin as tropical depressions that form from easterly waves (or tropical waves, which are long troughs characterized by low atmospheric pressure) in the tropical Atlantic off the coast of Africa. Within these easterly waves, the air is unstable, meaning that moisture-laden warm air near the surface has the ability to rise and cool rapidly as it ascends, leading to the development of thunderstorms. To produce a hurricane, however, the temperature of these surface waters must be at least 26.5 °C (79.7 °F), with even warmer water, in some cases catalyzed by the effects of global warming, leading to stronger hurricanes and hurricanes that rapidly intensify. Essentially, warm water is the energy source that fuels evaporation at the surface and the upward movement of warm, moist air that drives hurricane development.
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Another key ingredient to hurricane formation involves the ability of this air to rise without any serious disruption or turbulence. During this phase of development, wind shear (the rapid change in wind velocity or direction) becomes important. While there may be little or no turbulence at the surface, a volume of rising air can be broken apart easily by winds that suddenly accelerate or change direction aloft. Strong wind shear disrupts the storm’s rotation and keeps it from becoming organized. However, if wind shear is weak (that is, if the wind blows at no more than 10 meters [33 feet] per second between the surface and an altitude of about 10,000 meters [33,000 feet]), the rising volume of air travels uninterrupted. If this rising air cools rapidly as it climbs, deep, billowing, convective clouds form—the thunderstorms produced can increase in size and begin to rotate. This process also helps existing storms to intensify quickly, allowing them to grow into tropical storms and strong hurricanes.
Geography is also important. A hurricane’s characteristic counterclockwise rotation in the North Atlantic is driven by the Coriolis force, and the developing system must be at least 500 km (300 miles) away, roughly 5° of latitude north of the Equator for the Coriolis effect to act upon it.
Climate change may also be affecting the frequency of hurricane development. An analysis of hurricane records from 1980 to 2024 reveals that tropical cyclone activity is increasing in the North Atlantic basin, and that the number of hurricanes, including the number of major hurricanes (that is, category 3 and above, see Saffir-Simpson hurricane scale) developing per year is trending upward. This is consistent with warming trends in both the oceans and the atmosphere over the same period (see also ocean warming), and research continues to suggest that a warming atmosphere and ocean makes hurricanes more and more likely and increases the likelihood that the storms that do form will be more intense.
Inhibiting factors
Hurricane development in the North Atlantic is also inhibited by other large-scale atmospheric phenomena. During strong El Niño episodes, which are characterized by higher-than-average sea-surface temperatures in the eastern equatorial Pacific Ocean, fewer hurricanes develop in the North Atlantic, and those that do form make landfall less often. In addition, the chances of a hurricane intensifying to become a major hurricane (that is, category 3 and above) are reduced due to stronger wind shear and reduced atmospheric instability (see also Saffir-Simpson hurricane scale). Conversely, during La Niña episodes, in which sea-surface temperatures in the Equatorial Pacific are lower than average, wind shear in the North Atlantic is weaker, and thus conditions favor hurricane development.
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In addition, dust sent aloft by the harmattan (a cool dry wind that blows westward from the western Sahara), which often crosses the Atlantic Ocean on the trade winds to shroud the Caribbean and parts of the North American mainland, can suppress hurricane development. The dust reflects a portion of the incoming solar radiation, preventing some fraction of the Sun’s energy from reaching the ocean’s surface and warming the water. As a result, the ocean underneath the dust plume is cooler than it would be otherwise, which brings about lower air temperatures and lower humidity at the surface and greater atmospheric stability, which keeps air from rising or from rising rapidly. Saharan dust is also associated with greater wind shear.