Formation

In the Atlantic Ocean, tropical cyclones can form from a variety of different disturbances. For any storm to develop, there must be some sort of precursor disturbance, usually tropical in nature, for which the storm to spawn off of. There exist multiple factors that can enhance intensification, but also many factors that can disrupt the system.

Where do Hurricanes Spawn From?

The majority of storms develop from African Easterly Waves (AEWs). These systems, sometimes simply referred to as ‘tropical waves’, are surface troughs that emerge off the coast of Africa from the late spring through the middle of autumn during the monsoon season. About 60% of all Atlantic TCs and 85% of major hurricanes form from tropical waves in the Main Development Region, the region of the tropical Atlantic between the Lesser Antilles and west coast of Africa. Development from AEWs tends to occur during the peak months of the hurricane season, but also happens during the less active months on occasion. AEWs form over eastern equatorial Africa and are steered west by the African Easterly Jet from the subtropical ridge, also known as the Azores High. If conditions for development are favorable once they emerge into the Atlantic, then a tropical cyclone can develop in the MDR. If conditions remain favorable, then they can intensify into powerful hurricanes as they progress westward, potentially threatening the Caribbean or North America. These intense, long-tracked hurricanes are typically referred to as “Cabo Verde-type hurricanes”, as they tend to form in the vicinity of the Cabo Verde Islands.

A large tropical wave in the southern MDR in late September 2016, which would eventually develop into Hurricane Matthew 4 days later. (Suomi NPP/VIIRS)

The remaining 40% of Atlantic systems develop from other types of disturbances, but this tends to occur during the early and late parts of the season. Tropical processes may include monsoonal gyres or Pacific-crossover disturbances. But most storms in this leftover category develop from non-tropical processes. These normally come from decaying frontal boundaries or non-tropical lows. In the case of frontal boundaries, storms can spawn off of them if a disturbance is able to cut off from the frontal zone, and given adequate conditions, can develop into a namable storm. Non-tropical lows, such as upper-level lows, can develop surface lows if barotropic forcing is high enough and convection is sustained; these surface lows can develop into namable storms as well. When a system forms from non-tropical processes, they may start out as subtropical storms, but then later acquire tropical characteristics and become a bonafide tropical cyclone.

A disturbance detaching from a decaying frontal boundary in early October 2017, over the open north Atlantic. This would develop a surface low and become Hurricane Ophelia 5 days later. (MODIS)
An upper-level low north of the Leeward Islands in mid-September 2015, which would later gain surface rotation and widespread thunderstorm activity, becoming Hurricane Joaquin about a week later. (MODIS)

How do they Strengthen?

Apart from requiring an incipient disturbance to be present, tropical cyclogenesis requires multiple different factors, both atmospheric and oceanic, to be favorable for development. The primary fuel source of TC formation and intensification is oceanic heat. Sea surface temperatures and oceanic heat content, which refers to the depth of the heat, are the main contributors to this. Temperatures of at least 26C are required for the enhancement of a disturbance’s convection, but temperatures up to 28C are needed for pure barotropic intensification. These temperatures also need to extend deep into the ocean, up to 150m in some cases. Hurricanes are a nearly ideal heat engine powered by sea surface temperature and tropopause temperature differences. Heat is transferred through the system, some of which is transformed into wind energy as convection grows.

A sea surface temperature map from Sep. 4, 2018, with anything denoted in orange or red being capable of tropical cyclogenesis, assuming atmospheric factors are not a hinderance. (SSEC)

In addition to the oceanic energy, atmospheric factors need to be adequate. Vertical wind shear, which refers to the change in wind speed and direction with height, needs to be minimal, as high wind shear will disrupt the convective structure of the system. Speed shear, denoted by a change of wind speed with altitude, is usually less detrimental than directional shear, which involves the change of direction and possibly speed of winds with altitude, as directional shear is more effective at disrupting updrafts. High levels of moisture, particularly within the middle portions of the troposphere between the 400-700mb levels, are necessary to keep convection alive. Without it, thunderstorms would disintegrate due to less available moisture for use. Rapid cooling with height is another factor, which results in instability and latent heat release within the storm. The precursor disturbance also needs to be at least 5 degrees north of the equator, where the Coriolis forcing is strong enough to initiate surface rotation.

Graphic displaying the differences between speed and directional wind shear. (NWS)

How Fast can they Intensify?

Generally, if at least two thirds of these developmental factors are in place, then a storm will be allowed to intensify gradually, but if all are in place, then a TC can intensify rapidly. It is still not entirely understood why some hurricanes rapidly intensify while others don’t given sufficient conditions for which to do so. Rapid intensification is defined as a 24mb decrease in central pressure over 24 hours, or 1mb per hour, and explosive intensification is defined as a 60mb decrease in central pressure over 24 hours, or 2.5mb per hour. When such a requirement is met, then a TC is able to reach high intensities, as virtually all TCs that have attained category 4 or 5 status have undergone at least one period of rapid intensification. Storms that gradually intensify normally run into hinderances before they can become extremely powerful. The fastest intensification rates known can be in excess of 7kt wind increases per hour and have pressure drop rates near 4mb per hour, but such intensification rates rarely last more than 6 hours or so, as the inner core typically needs to undergo some sort of an adjustment, which can stagnate or even reverse intensification.

Hurricane Maria on Sept. 18, 2017 at 0015 UTC as a category 1 (left) and Maria 24 hours later as a category 5 (right), undergoing a 55mb decrease and 70kt increase in between images. (NRL Navy Imagery)