Tropical cyclone: the story on HearLore

Tropical cyclone

The eye of a tropical cyclone is a deceptively calm zone, often just 30 to 50 kilometers wide, where the sky clears and the wind dies down, only to be replaced by a wall of destruction seconds later. This phenomenon, known as the eyewall, is where the most violent winds and heaviest rain occur, creating a structure that resembles an arena football stadium when viewed from above. For a sufficiently strong storm, air sinks within the eye, suppressing cloud formation and creating a clear window to the ocean below, which may be churning with violent waves despite the calm air. The eyewall expands outward with height, and in intense storms, this structure can undergo eyewall replacement cycles where an outer ring of thunderstorms moves inward to rob the primary eyewall of moisture and angular momentum. When the primary eyewall weakens, the storm temporarily loses intensity, but the outer eyewall eventually replaces it, allowing the system to return to its original ferocity. This internal complexity is what makes the storm so difficult to predict and so devastating when it strikes land, as the heaviest wind damage occurs precisely where the eyewall passes over populated areas.

Fueling The Fire

Tropical cyclones derive their energy from the evaporation of water from the ocean surface, a process that requires sea surface temperatures of at least 26.5 degrees Celsius to initiate and sustain the storm. This heat energy acts as the accelerator for the cyclone, drawing in air from a large area and concentrating the water content of that air into precipitation over a much smaller area. The storm's formation is a delicate balance of factors, including low vertical wind shear, high humidity in the lower to middle levels of the troposphere, and a pre-existing low-level focus or disturbance. When a tropical cyclone passes over the ocean, it causes the upper layers of the ocean to cool substantially through a process known as upwelling, which can negatively influence subsequent cyclone development. This cooling is primarily caused by wind-driven mixing of cold water from deeper in the ocean with the warm surface waters, creating a negative feedback process that can inhibit further development or lead to weakening. However, high ocean heat content, also known as Tropical Cyclone Heat Potential, allows storms to achieve a higher intensity, and faster-moving systems are able to intensify to higher intensities with lower ocean heat content values. The passage of a tropical cyclone over the ocean causes the upper layers of the ocean to cool substantially, a process known as upwelling, which can negatively influence subsequent cyclone development. This cooling is primarily caused by wind-driven mixing of cold water from deeper in the ocean with the warm surface waters, creating a negative feedback process that can inhibit further development or lead to weakening. However, high ocean heat content, also known as Tropical Cyclone Heat Potential, allows storms to achieve a higher intensity, and faster-moving systems are able to intensify to higher intensities with lower ocean heat content values.

Common questions

What is the eye of a tropical cyclone and how wide is it?

The eye of a tropical cyclone is a deceptively calm zone often just 30 to 50 kilometers wide where the sky clears and the wind dies down. This area features sinking air that suppresses cloud formation and creates a clear window to the ocean below despite churning waves.

How does a tropical cyclone derive its energy and what temperature is required?

Tropical cyclones derive their energy from the evaporation of water from the ocean surface which requires sea surface temperatures of at least 26.5 degrees Celsius to initiate and sustain the storm. This heat energy acts as an accelerator that draws in air from a large area and concentrates water content into precipitation over a much smaller area.

Who named the first tropical cyclones and when did this practice begin?

The credit for the first usage of personal names for tropical cyclones generally goes to the Queensland Government Meteorologist Clement Wragge who named systems between 1887 and 1907. The naming of tropical cyclones dates back to the late 1800s and early 1900s to provide positive identification of severe weather systems in a form readily understood by the public.

Why are names of significant tropical cyclones retired from lists?

Names of significant tropical cyclones in the North Atlantic Ocean, Pacific Ocean, and Australian region are retired from the naming lists and replaced with another name. This practice helps reduce confusion about what storm is being described when systems last a week or longer or when more than one storm occurs in the same basin.

How has climate change affected the frequency and intensity of tropical cyclones between 1979 and 2017?

Between 1979 and 2017 there was a global increase in the proportion of tropical cyclones of Category 3 and higher on the Saffir-Simpson scale with the trend being most clear in the North Atlantic and in the Southern Indian Ocean. A 2019 study indicates that climate change has been driving the observed trend of rapid intensification of tropical cyclones in the Atlantic basin.

What happens to ocean temperatures when a tropical cyclone passes over the water?

When a tropical cyclone passes over the ocean it causes the upper layers of the ocean to cool substantially through a process known as upwelling. This cooling is primarily caused by wind-driven mixing of cold water from deeper in the ocean with the warm surface waters creating a negative feedback process that can inhibit further development or lead to weakening.