Tropical Cyclones: The Basics

One of the biggest threats to coastal communities and marine interests is the tropical cyclone. With the potential to generate damaging winds and tornadoes, heavy seas, flooding rains, and storm surge (see our article here), meteorologists must closely monitor these storms to provide accurate and timely warnings to prevent fatalities and minimize damage. Depending on where you are in the world, tropical cyclones can also be called hurricanes, typhoons or cyclones. However, they are all the same atmospheric phenomenon and are referred to generically as tropical cyclones. Hurricanes develop in the North Atlantic and East Pacific, typhoons are in the West Pacific, and cyclones form in the Indian Ocean and South Pacific.??

A tropical cyclone is a low-pressure area that forms over warm tropical and subtropical waters, with strong winds and clusters of thunderstorms circling around a central point usually where the heaviest convection is occurring. Tropical cyclones have no fronts (meaning no boundaries between warm and cold air masses - the air is all warm), which is distinct from low-pressure systems outside the tropics. Several ingredients are needed for a tropical cyclone to form:

1. The sea surface temperatures must be at least 26°C / 79°F, with the warm water being at least 50m / 160ft deep. Deep, warm water supplies heat, which is the main source of energy for tropical cyclones.
2. A low wind shear environment, with winds being of similar speed and direction from the surface into the upper troposphere (~10km / 35,000ft). If shear is too high, the structure of the developing tropical cyclone will be interrupted, weakening the system.
3. A disturbance which provides convergence at the surface and induces rising motion for the initial formation of an area of low pressure and thunderstorms. The Intertropical Convergence Zone (ITCZ), a nearly continuous band of clouds and occasional thunderstorms oscillating seasonally within 22° north and south of the equator, is a frequent source of these disturbances.
4. Initial formation must be at least 5° latitude away from the equator (approximately 560 km / 350 miles) so that wind flowing toward low pressure and associated thunderstorm convection enhances the rising motion, which pulls in more surrounding air, which then begins to spin counterclockwise (in the northern hemisphere) due to the Coriolis force.
5. Ample moisture in the mid-levels of the atmosphere which maximizes the energy for storm intensification.

These ingredients are available in a limited number of areas in the world, as indicated in the graphic above. Note that the South Atlantic does not have many tropical or subtropical cyclones. In the tropical South Atlantic, the water is simply too cold to sustain a tropical cyclone, and wind shear is usually too strong. Very occasional subtropical storms (systems with mixed tropical and extratropical characteristics) will form off the coast of Brazil, but the area has only one tropical cyclone on record (Catarina in 2004).

Stages of Development

Tropical Depression: Every tropical cyclone starts as a tropical depression, a spinning area of thunderstorms in the tropics with sustained surface winds speeds up to 33 knots / 38 mph. They usually move east to west in association with tropical waves imbedded in the trade winds, or are “steered” by the large scale weather patterns in the mid-latitudes. At this stage, the storms are numbered based on the basin in which they form, and do not have a proper name.

Tropical Storm: If the system remains in favorable conditions, it can strengthen into a tropical storm, with sustained wind speeds of 34 knots / 39 mph to 63 knots / 73 mph. Winds are strongest near the center of the circulation, and weaken farther out. At this stage, storms are given proper names. Meteorologists use these names to more easily communicate with the public about tropical cyclones, especially when there is more than one active cyclone in an ocean basin. Even though tropical storms may not have reached their maximum intensity, they can still pack a punch, especially with torrential rains and flooding, so it is important to not underestimate the impact of these storms.

Hurricane/Typhoon/Cyclone: Once a system attains sustained wind speeds of 64 knots / 74 mph or greater, it becomes a cyclone (or hurricane, or typhoon). Cyclones are on average about 482 km (300 miles) across, although they can vary widely in size. They have several distinct characteristics (see graphic below). Winds blow radially inward toward the center as inflow, providing a steady stream of warm, moist air. The strongest surface winds are located in the eye wall, an intense, stormy ring of convection around the center of the cyclone. The eye wall encircles an area of light winds and sinking motion which frequently forms a clear area in the center of the circulation. This is the eye of the storm.??

Thunderstorms become arranged in rain bands - lines of thunderstorms that spiral around the center of rotation. The updrafts of the convective bands bring warm, moist air from the surface to higher altitudes, which provides energy for the cyclone. Eventually the air in the updrafts cools and can no longer rise. At elevation, the winds spread out away from the tropical cyclone as the outflow, which is usually seen as a cloud shield of feathery, fuzzy cirrus. Tropical cyclones also take vast quantities of energy and moisture from the ocean and transport them poleward as their track usually has a poleward component. This helps to cool the tropical regions while keeping polar regions warmer than they would be otherwise.??

Above is a close-up view of Super Typhoon Haiyan on 07 November, 2013, which is an excellent view of storm structure. The cirrus clouds aloft show strong outflow. Note the eye clear of cirrus clouds. At peak strength, Haiyan had winds of 170 knots / 196 mph, gusting 205 knots / 236 mph.

A tropical cyclone will weaken based on several factors. For example, if the cyclone enters a region of dry air, this will cause the clouds and thunderstorms to evaporate, decreasing the overall energy in the cyclone. When a tropical cyclone moves over cooler waters, it is unable to draw as much heat from the ocean, weakening thunderstorms and winds. Similarly, when a tropical cyclone moves over land, the storm is cut off from its source of energy (the ocean), while also encountering increased friction from land, vegetation, etc. These circulations can also be weakened in areas of strong wind shear, when winds increase with height, which interferes with the upward motion that the storm uses to make energy to maintain the high wind speeds.

Tropical cyclones typically move east to west in the trade winds. However, they can be drawn poleward by other weather systems such as low-pressure troughs. When this happens, the cyclone tends to recurve and accelerate towards the poles, and will either dissipate over cooler waters, or transition into an extratropical cyclone. In the latter case, the strongest winds spread out over a broader area, and fronts develop. Sometimes these storms maintain their wind speeds, causing damage in areas far from the tropics.

Saffir-Simpson Wind Scale

When communicating with the public, it is helpful to have a way to describe how destructive a tropical cyclone can be. In the North Atlantic and East Pacific, the Saffir-Simpson Hurricane Wind Scale is used to describe a cyclone’s ability to cause property damage based on wind speeds, but it does not take into consideration other hazards like rainfall, flooding, tornadoes, or storm surge.

Tropical cyclones generate some of the strongest winds on Earth and are particularly dangerous for ships at sea. The strong, circling winds cause heavy, confused seas that make steering difficult. Fortunately, detection with satellites and forecasting for tropical cyclones has greatly improved over the several few decades. Combined with modern communication methods, we can assist Masters in avoiding these deadly storms.