Tropical Cyclones: What's in a Name?

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Up to this point, you've seen the generic phrase "tropical cyclone" used to describe the low-pressure systems that form over warm tropical seas. However, as you're about to find out, naming and classifying tropical cyclones is somewhat complicated. Before we get into how tropical cyclones are named, let's look at the areas where tropical cyclones tend to form. Do they form just anywhere in the tropics? Not really. As you can see from the image below, the breeding grounds and regions where tropical cyclones typically track can be boiled down to seven areas:

1. Atlantic Basin (the northern Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea)
2. Northeast Pacific Basin (from Mexico to the International Dateline)
3. Northwest Pacific Basin (from the International Dateline to Asia, including the South China Sea)
4. North Indian Basin (includes the Bay of Bengal and the Arabian Sea)
5. Southwest Indian Basin (from Africa to about 100 degrees east longitude)
6. Southeast Indian/Australian Basin (100 degrees east longitude to 142 degrees east longitude)
7. Australian/Southwest Pacific Basin (142 degrees longitude to about 120 degrees west longitude

The typical breeding grounds for tropical cyclones and the regions through which they typically track. Note that tropical storms do not form along the equator (more on this topic later in the course).

Credit: David Babb

Of these seven areas, the Northwest Pacific and Northeast Pacific basins tend to be the busiest, as this frequency plot for tropical cyclones suggests. It shows the average number of occurrences (from 1972 to 2001) that the center of a tropical cyclone occupied an area with dimensions of one degree latitude by one degree longitude (based on each storm's best track). The dark red color indicates the highest average of such occurrences and thus marks the core of these breeding basins for tropical cyclones. Meanwhile, some areas in the tropics are nearly entirely free of tropical cyclones. While tropical cyclones can form outside of the seven areas listed above, it happens relatively infrequently. For example, the southern Atlantic Ocean (south of the equator) is rarely home to tropical cyclones.

With tropical cyclones in any basin possibly impacting multiple countries, how do forecasters keep tabs on all of them? The World Meteorology Organization created a branch called the Tropical Cyclone Programme (TCP) to ensure that all countries bordering and within each basin are adequately prepared for the threat posed by tropical cyclones. To accomplish this goal, the TCP's primary responsibility is to establish a nationally and regionally coordinated network of forecasting centers. To define areas of responsibility, they partitioned the tropical-cyclone basins and assigned Regional Climate Centres (RCCs), which issue the official forecasts and advisories for their respective basins (see figure below). If you wish, you can check out the WMO Web Page that lists the links to the Tropical Cyclone RCCs where you can access all the current RCC advisories.

Although each RCC is responsible for issuing the official forecasts and advisories for their jurisdiction, there can be multiple cities which issue warnings for various countries under the umbrella of a single RCC. For example, the official advisories and forecasts for the Atlantic Basin come from: the National Hurricane Center in Miami, but the Canadian Hurricane Centre in Dartmouth, Nova Scotia issues warnings when tropical cyclones threaten Canada, using the National Hurricane Center's products as their basis. Keep in mind that Atlantic hurricanes and tropical storms moving northward along the East Coast can pose a significant threat to the Canadian Maritimes (the provinces of New Foundland, Nova Scotia, New Brunswick, and Prince Edward Island).

A map of the Regional Specialized Meteorological Centers (RSMCs--cities marked with orange dots) and Tropical Cyclone Warning Centers (TCWCs--cities marked with gray dots), and their regions of responsibility.

Credit: The World Meteorological Organization

In addition to the Regional Specialized Meteorological Centers and Tropical Cyclone Warning Centers on the map above, the Joint Typhoon Warning Center (JTWC) is an additional warning center and serves as a joint effort between the United States Navy and Air Force. JTWC was founded in 1959 in Guam, but has since moved to Pearl Harbor, Hawaii. While JTWC does not issue official public forecasts, they keep tabs on tropical cyclones globally for U.S. Department of Defense interests.

Now that we know who's keeping track of tropical cyclones around the globe, we can delve into how they keep track of them. That's where things can get a bit complicated because standards differ across the globe. For starters, forecasters often have their eyes on clusters of showers and thunderstorms across the tropics (often called "tropical disturbances"). Tropical disturbances do not have closed circulations and are not formally tropical cyclones; however, by convention in the U.S., tropical disturbances that have the potential to develop into tropical cyclones are dubbed "invests." Each invest is tagged with a number from 90-99 along with a capital letter, which corresponds to the tropical basin where it's located (see the table below for the letters that correspond to each basin). Forecasters start with the number 90 and sequentially progress to 99, and then start over again at 90. So, for example, Invest 99L in the Atlantic Basin would be followed by Invest 90L, and so on.

If the tropical disturbance with organized convection develops a closed cyclonic circulation in its surface wind field, it becomes a tropical depression as long as its maximum sustained wind speeds are less than 34 knots (39 miles per hour). Note that different definitions of "sustained" (as described in the link) can lead to different storm classifications in different basins. Tropical depressions are formally considered tropical cyclones, and at this point The Joint Typhoon Warning Center, Central Pacific Hurricane Center, and the National Hurricane Center routinely assign a new number to go along with the letter referring to the basin of origin. For each season in each basin, the digits start at "01" and then increase by one for each successive depression that forms. Since the capital letter refers to the basin of origin, the letters are not changed if tropical cyclones cross 140 or 180 degrees longitude. I should note, however, that depressions in the Atlantic Basin are an exception. When a depression is classified in the Atlantic, the National Hurricane Center simply refers to it by its number ("One", "Two", etc.) and the letter "L" gets dropped from the designation.

Once a tropical cyclone reaches sustained wind speeds of at least 34 knots (39 miles per hour), it becomes a tropical storm and receives a name (more on the naming of tropical cyclones in a bit). Tropical cyclones retain their tropical storm status as long as their maximum sustained winds remain between 34 knots and 63 knots. Once a tropical cyclone reaches maximum sustained winds of at least 64 knots (74 miles per hour), it loses its "tropical storm" label, and earns the classification of hurricane, typhoon, severe cyclonic storm, tropical cyclone or severe tropical cyclone, depending on the basin in which the storm is located (see the table below for what words are used in each basin. At times, I may generically refer to "hurricanes" in the text, but keep in mind that such references also include strong tropical cyclones that go by various labels in basins around the world.

Of course, all "strong" tropical cyclones (hurricanes, typhoons, etc.) are not created equal. Some are much more intense than others. In the Atlantic and Northeast Pacific basins, forecasters use the Saffir-Simpson Hurricane Wind Scale to further classify a given hurricane. Hurricanes classified as "Cat 3", "Cat 4" or "Cat 5" (all hurricanes with maximum sustained winds of at least 96 knots, or 111 mph) qualify as major hurricanes. Although major hurricanes make-up only 21% of the hurricanes that hit the United States, these fierce storms account for over 83% of all the damage from landfalling hurricanes. For the record, Australian forecasters, rank tropical cyclones a bit differently.

Other basins also have different descriptors for extremely intense tropical cyclones. In the Northwest Pacific Basin, for example, the particularly descriptive classification of "super typhoon" is used once a typhoon's maximum sustained wind speed reaches at least 130 knots (more than twice the minimum typhoon wind speed). For some interesting tidbits on some memorable super typhoons, check out the Explore Further section below. In the North Indian Ocean, meanwhile, severe tropical cyclones that attain maximum sustained winds of at least 130 knots graduate to super cyclonic storm.

The Name Game

There's a checkerboard history behind the naming of tropical cyclones in the various basins around the world. If you're interested in the history of naming tropical cyclones, I encourage you to check out the corresponding section within Explore Further below. The reason why tropical cyclones get named, however, is pretty straightforward. The practice of naming tropical cyclones ensures clear, unambiguous communication between forecasters and the general public when forecasts, watches, and warnings are issued. At any given time across the globe (or even within a single tropical basin) there can be multiple tropical cyclones present at any one time. For example, the satellite image from September 2, 2008 (below) shows a whopping four named storms present in the Atlantic Basin!

This satellite image from September 2, 2008 shows four named storms present in the Atlantic Basin. Gustav had made landfall and was present over Texas, Hanna was located in the Caribbean, and Ike and Josephine were located over open waters.

Credit: NASA / NOAA GOES Project

Without the practice of naming tropical storms, deciphering forecasts with four active storms in the basin could have been a real mess -- sifting through coordinates or other technical descriptions of a storm's location. In the end, using names is much simpler for the general public, so let's get to the (not so simple) business of how storms are named. In the Atlantic and eastern Pacific, the World Meteorological Organization and National Weather Service (NWS) have used lists of alternating male and female names in alphabetical order to christen storms since 1979 (check out the lists currently in use, if you wish).

I should point out that any year that the alphabetical list of male and female names is not long enough to accommodate all the named storms in a season, the National Hurricane Center turns to a supplemental list of names; however, prior to 2021, the standard was to use letters of the Greek Alphabet (Alpha, Beta, Gamma, Delta, etc.) to name storms once the original list of names had been exhausted. Use of the Greek Alphabet to name storms only occurred twice (2005 and 2020).

For Central Pacific storms, the Central Pacific Hurricane Center uses its own list of names of Hawaiian origin. Because not many tropical cyclones form in the Central Pacific, they don't restart the list from the beginning each year, and instead they just keep using the same list until all the names have been used. When they reach the end of one list, they simply begin with the first name on the next list.

Meanwhile, in the northwest Pacific Basin, since the year 2000, the World Meteorological Organization has used names which are, for the most part, not male or female names. Instead, most names on the list refer to flowers, animals, birds, trees, or even foods, etc. Others are simply descriptive adjectives. Each name on the list is contributed by a participating nation within the basin. The names are not used in alphabetical order like in the Atlantic and eastern Pacific, however. Instead, the contributing nations are listed in alphabetical order and this ranking determines the order that the names are assigned.

It's important to note, however, that the established lists from the World Meteorological Organization are not universally used for storms in the northwest Pacific. The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) assigns Filipino words as storm names when storms threaten the Philippines so that locals can easily remember them and communicate about the storm. For example, in November 2013 Super Typhoon Haiyan made landfall in the Philippines as one of the strongest tropical cyclones on record at the time of landfall. But, in the Philippines, Haiyan (which is from the Chinese for "petrel" -- a type of seabird) was known as "Yolanda." So, be aware that you may come across two names for some storms in the northwest Pacific Basin.

Finally, in the North Indian Ocean, tropical cyclones weren't named from a traditional list until 2004. Prior to that year, conventions for identifying storms and keeping historical records were somewhat awkward (more in Explore Further). With regard to name selection, eight countries belonging to the WMO Tropical Cyclone panel for the North Indian basin contributed eight names each, which were tabulated into eight columns. In each column, one name from each country appeared, with the names listed in the order determined by the alphabetized contributing nations (the same convention as in the Northwest Pacific basin). You can see the lists of names for tropical cyclones in the North Indian Ocean and all other tropical basins (including basins not covered in-depth here) on the World Meteorological Organization's page of tropical cyclone names.

An average of approximately 80 strong tropical cyclones form each year worldwide -- a small number compared to the hundreds and hundreds of cyclones that parade across the middle and high latitudes each year. Yet, the attention that meteorologists focus on tropical cyclones sometimes seems disproportionately great. That's because strong tropical cyclones can cause staggering losses of life and property. Next up, we'll compare tropical cyclones with mid-latitude cyclones. Not surprisingly, there's a world of difference between them!