How tropical cyclones form and decay

Fluid dynamics drives the formation of tropical cyclones, specifically the interaction of warm sea temperatures with atmospheric air masses.

The formation of tropical cyclones is linked to the Sea Surface Temperature (SST) distribution, as the ocean is the primary source of energy for tropical cyclones. For tropical cyclones to develop, the upper ocean heat content down to a depth of about 60 metres must exceed a temperature of about 26°C. Regions where the SST exceeds 26°C are the most favourable areas for tropical cyclone genesis.

You can see that the SST distribution is not uniform in the tropics. In certain regions, such as the Southern Eastern Atlantic and the Pacific, the upwelling of cold ocean water leads to cooler SSTs. In these regions, where sea surface temperatures are lower, tropical cyclones do not form.

The distribution of tropical cyclone tracks, along with their intensity, is categorised according to the Saffir-Simpson scale.

Global climatology of tropical cyclones between 1851 and 2006. “TD” stands for tropical depression, “TS” for tropical storm, and 1-5 refer to the categories of the Saffir Simpson scale. A full-text description is available in the Downloads.

By comparing sea surface temperatures and cyclone tracks, you can see that tropical cyclones originate in regions with warm SSTs, and no storms form in the Eastern Southern Pacific and the South Atlantic. Another important observation is that the Northwest Pacific experiences both the largest number of tropical cyclones and the highest number of the most intense storms. This region, particularly the Philippines, is frequently impacted by extremely intense tropical cyclones. 

A significant number of tropical cyclones also form in the Northeast Pacific and in the Atlantic. If you look closely at the tracks in the Atlantic, you can see that many originate off the west coast of Africa, move across the Atlantic Ocean, and either make landfall in the Caribbean or the USA, or their tracks recurve and move into the mid-latitudes. Some tracks extend all the way to the United Kingdom or other parts of Europe. 

When tropical cyclones interact with mid-latitude weather systems, they can affect the weather and its predictability in these regions. You may have heard news reports that a named hurricane is bringing heavy rainfall to the United Kingdom. Despite being a tropical weather system, tropical systems do influence weather in the mid-latitudes as they can bring heavy precipitation and strong winds to regions like the United Kingdom.

Formation and decay of tropical cyclones

The necessary conditions for tropical cyclone formation were outlined in the statistical analysis by Gray (1979). This study highlights that cyclogenesis (the process by which a tropical cyclone develops in the atmosphere) does not occur within 5°N-5°S of the equator because the Coriolis force (the background rotation of the Earth) is too weak in this region. 

The Coriolis parameter, which partly determines the strength of this force, is given by f=2Ωsinϕ, where f is the rate of rotation of the Earth, Ω is the angular velocity of Earth’s rotation, and ϕ is the latitude. Near the equator, where ϕ is small, f becomes negligible, preventing cyclogenesis.

Conditions for cyclogensis

• Ocean temperatures must exceed 26°C to a depth of 60 metres to provide the energy required for tropical cyclone development.
• Vertical wind shear should be small.
• The atmosphere needs to be potentially unstable, which can be shown by a decrease in equivalent potential temperature ((theta e)) with height, allowing deep moist convection to occur.
• The mid-troposphere also needs to be moist
• A finite-amplitude wave or vortex in the lower troposphere — often originating from an “African easterly wave” — creates favourable conditions for tropical cyclone formation.

Whilst all these conditions are necessary, they are not sufficient. It is an active area of research to understand why some cloud clusters develop into tropical cyclones and others don’t.

It is also important to consider the processes that lead to the weakening of tropical cyclones. 

These include: 

• Loss of energy sources when the storm moves over colder waters or makes landfall, cutting off sensible and latent heat (types of heat energy that affect temperature.)
• Incorporation of cooler, drier air into the tropical cyclone’s circulation.
• Increased surface friction during landfall often results in very gusty winds.
• Enhanced vertical wind shear can disrupt the storm’s structure by separating parts of the vortex.
• Upper-tropospheric convergence behind an extratropical trough can suppress the tropical cyclone’s development.

If you would like to explore the formation and decay of tropical cyclones in more detail, Juliane has provided some additional resources in the See Also section.