Tropical cyclones form over warm ocean waters in a band roughly between 5° and 30° latitude on both sides of the equator. They occur in every major ocean basin except the South Atlantic and the far southeastern Pacific, where conditions rarely support their development. Globally, about 80 to 90 tropical cyclones spin up each year, with the vast majority concentrated in just a few highly active regions.
The Major Ocean Basins
Tropical cyclones develop in seven recognized basins, each monitored by different forecasting agencies. The western North Pacific is the most active basin on Earth, producing roughly a third of all tropical cyclones worldwide. Storms here are called typhoons, and they can form year-round, though activity peaks between July and November. This basin stretches from the coast of East Asia eastward to the International Date Line.
The eastern North Pacific, extending from the western coast of Mexico and Central America out to 140°W longitude, is the second most active basin. Most of these storms curve out to sea and never make landfall, but they can still bring dangerous surf and moisture to coastal areas. Between 140°W and the Date Line sits the central North Pacific basin, home to far fewer storms but still capable of threatening Hawaii.
The Atlantic basin covers the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. Its official hurricane season runs from June 1 through November 30, with peak activity in August and September. The northern Indian Ocean splits into two sub-basins: the Bay of Bengal and the Arabian Sea. The Bay of Bengal is significantly more active, generating storms that frequently strike India, Bangladesh, and Myanmar. In the Southern Hemisphere, tropical cyclones form in the southwestern Indian Ocean (affecting Madagascar, Mozambique, and the island nations east of Africa) and in the South Pacific and Australian region (threatening northern Australia, Fiji, Vanuatu, and other Pacific island nations). Southern Hemisphere seasons run roughly from November through April.
Why They Form Where They Do
Tropical cyclones need a specific set of ingredients, and the geographic zones where those ingredients overlap determine where storms appear. The most fundamental requirement is warm ocean water, generally at least 26.5°C (about 80°F), extending to a depth of roughly 50 meters. That warmth fuels the evaporation that powers a cyclone’s engine.
Equally important is the Coriolis effect, the spin imparted by Earth’s rotation. Within about 5° of the equator, the Coriolis force is too weak to set a storm spinning, which is why cyclones almost never form in that narrow equatorial belt. One striking exception was Typhoon Vamei, which formed near Singapore in December 2001 with its center at just 1.5°N latitude, placing its circulation on both sides of the equator. Before that, the lowest-latitude tropical cyclone on record was Typhoon Sarah in 1956 at 3.3°N.
The atmosphere matters as much as the ocean. Weak vertical wind shear, meaning the winds at 5,000 feet and 38,000 feet are blowing in roughly the same direction and speed, lets a storm build vertically without getting torn apart. As a general rule, shear below about 20 knots across that layer is favorable for development. Strong shear is one of the main reasons certain ocean areas with warm water, like the South Atlantic, almost never produce cyclones.
Countries Hit Most Often
China records the highest rate of tropical cyclone landfalls of any country in the world. The Philippines ranks second, and Japan third. In total, eleven countries average at least one tropical cyclone landfall per year. Other frequently affected nations include Vietnam, Mexico, the United States, Australia, India, Bangladesh, Madagascar, and Mozambique.
The western North Pacific’s sheer volume of storms, combined with the densely populated coastlines of East and Southeast Asia, makes that region the global epicenter of tropical cyclone impacts. In the Atlantic, the Caribbean islands, the Gulf Coast of the United States, and parts of Central America bear the brunt of hurricane landfalls.
Different Names, Same Storm
The storms themselves are identical in structure, but they go by different names depending on location. In the Atlantic and eastern North Pacific, they are called hurricanes. In the western North Pacific, they are typhoons. In the Indian Ocean and South Pacific, they are simply called cyclones (or, in official forecasts from India’s meteorological agency, “severe cyclonic storms”). All of these are tropical cyclones that have sustained winds of at least 74 mph (119 km/h).
How Climate Patterns Shift Cyclone Activity
The El Niño-Southern Oscillation cycle has a pronounced effect on where tropical cyclones cluster in any given year. During El Niño years, increased wind shear over the Atlantic suppresses storm formation there, while the central and eastern Pacific tend to see more activity. La Niña flips the pattern: the Atlantic produces roughly 36% more tropical cyclones than during El Niño years, and those storms tend to be about 6% stronger on average. More of them also track through the Caribbean Sea and toward the Gulf Coast.
In the western North Pacific, the relationship is more complex. During years when La Niña is fading, the large-scale circulation pattern shifts, pushing typhoon formation farther northwest. These climate oscillations don’t create or eliminate basins, but they meaningfully redistribute activity within and between them, which is why some hurricane seasons feel relentless while others are quiet.
Where Tropical Cyclones Almost Never Form
The South Atlantic is the most notable “dead zone” for tropical cyclones. Cool sea surface temperatures, persistent wind shear, and unfavorable atmospheric patterns make development extraordinarily rare. The most famous exception was Hurricane Catarina, which struck southern Brazil in March 2004 and shocked forecasters because the basin had essentially no modern precedent for such a storm. The southeastern Pacific, off the coast of South America, is similarly inhospitable due to cold upwelling waters from the Humboldt Current and strong shear aloft.
The Mediterranean Sea has occasionally produced storms with tropical characteristics, sometimes called “medicanes,” but these are rare, typically small, and far weaker than their open-ocean counterparts. They form when unusual pockets of warm water and low shear briefly coincide, conditions the Mediterranean doesn’t reliably provide.