Basins

There exist multiple basins across which tropical cyclones can develop. The tropics are where most tropical cyclones form, but some can form farther north from non-tropical processes. The majority of tropical cyclones, about 70% on average, form in the Northern Hemisphere. The remaining 30% comes from the Southern Hemisphere, although these values can vary. Tropical cyclones are also referred to by different names depending on their location when they achieve winds of 64kts or greater. There is no difference in the class of storm, only the name.

Areas of the globe’s oceans where tropical cyclones typically develop or track, though they are not constrained to exist within these locations.

The majority of tropical cyclones, about 70% on average, form in the Northern Hemisphere. This is despite the fact that there exists less ocean area than there is in the Southern Hemisphere. Here, TCs spin counter-clockwise like all low pressure systems. The Northern Hemisphere consists of the North Atlantic Ocean, the Eastern, Central, and Western Pacific, and North Indian Ocean basins.

Typhoon Mangkhut in the Western Pacific during September 2018. (Himiwari-8)

The North Atlantic Basin (ATL) is a moderately active basin under the jurisdiction of the National Hurricane Center in Miami, FL. The basin consists of the North Atlantic Ocean, the Caribbean Sea, and Gulf of Mexico. On average, the basin sees 12 named storms, 6 hurricanes, and 3 major hurricanes. ACE values usually total near 100 units. TCs typically track from east to west, forming from tropical waves emerging off the coast of Africa. The season runs from June 1 to November 30, but as with any basin, this does not mean storms can form outside these boundaries. Countries and regions frequently impacted by ATL tropical cyclones include the United States, Mexico, Central America, The Bahamas, the Greater Antilles, the Lesser Antilles, Cabo Verde, and sometimes Maritime Canada, northern South America, and the Azores. Extratropical remnants of recurving TCs can occasionally impact Europe and Iceland. The costliest TCs ever recorded are normally found in the Atlantic because high levels of infrastructure and property expenses are located in hurricane-prone areas.

The Eastern Pacific Basin (EPAC), also under the control of the NHC, consists of waters east of 140W and north of the equator. The basin is normally slightly more active than the Atlantic, with 15 named storms, 8 hurricanes, and 4 majors, totaling in around 130 units of ACE. The season runs from May 15 to November 30, but unlike most other basins, TCs forming outside these boundaries are extremely rare, and usually form within 10 days of these delineations. EPAC TCs also follow climatology to a tee, in that almost all of them follow a straight east-to-west track out to sea, fizzling in less conducive environments to their north. Mexico is the primary country impacted by EPAC TCs, but some may form as far east as Central America at times.

The Central Pacific (CPAC) consists of waters around the Hawaiian Islands, specifically between the International Dateline and 140W. The CPAC falls under the control of the Central Pacific Hurricane Center in Honolulu, HI; the season runs from June 1 to November 30. The basin is considerably less active than the EPAC, with an average of 5 TCs existing in the basin, 2 of which become hurricanes and only 1 becomes a major. Most storms track into the basin from the EPAC, but some storms do form in the basin on occasion. Sometimes, EPAC TCs will track through the entire basin and move into the Western Pacific. Likewise, CPAC TCs can also track west into the neighboring basin. Hawaii is the only landmass within the basin, but the island chain rarely sees direct impacts from storms.

The Western Pacific (WPAC) is the most active basin on Earth, consisting of Pacific waters west of the International Dateline and north of the equator. The basin is controlled primarily by the Japanese Meteorological Agency in Tokyo, Japan. TCs in this basin are referred to as typhoons rather than hurricanes, but there are no differences between the two apart from the location. The basin sees a whopping 26 named storms on average, with 15 typhoons, 8 major typhoons and 4 supertyphoons (winds of 130kts or greater). ACE produced in the basin is normally around 300 units, which is about 40% of the annual ACE output globally. The season lasts throughout the entire year, but TCs are most likely to form between the months of May and December. The basin is so active because it contains the world’s largest supply of oceanic heat without nearby landmasses to disrupt cyclogenesis, resulting in high levels of moisture and instability. As a result, the majority of the world’s most powerful TCs on record were in the WPAC. Typhoons frequently impact countries throughout Southeast and East Asia, including China, the Philippines, Japan, Taiwan, Vietnam, South Korea, and the Northern Mariana and Ryukyu island chains. The first two typically receive multiple impacts in a single year.

The North Indian Ocean (NIND) encompasses the Bay of Bengal and the Arabian Sea. It is under the jurisdiction of the Indian Meteorological Department in New Delhi. The basin is markedly less active than its neighbor, only generating about 4 TCs a year. 2 of these go on to become cyclones, their equivalent to hurricanes and typhoons, and 1 of them becomes a major cyclone. Interestingly, the season’s boundaries are not uniform like the aforementioned basins, but instead, activity peaks in spring and autumn, with off-seasons during the winter and summer. The autumn peak is normally more active than the spring one. TCs within the NIND typically impact the Indian Subcontinent, but Bay of Bengal TCs may also impact Sri Lanka, Bangladesh, Myanmar, or even Thailand. Arabian Sea TCs normally dissipate before they impact land due to the arid conditions around them, but they have been known to impact Somalia, Yemen, Oman, Iran, and Pakistan. Storms in this basin have been some of the deadliest in recorded history because coastal impoverished communities tend to reside in flood-prone areas, causing extreme levels of destruction when one of these storms comes onshore.

Moving south of the equator, TCs are less common than they are within the Northern Hemisphere, comprising of about 30% of the world’s TCs each year. Storms within the Southern Hemisphere rotate clockwise because of Coriolis forcing. All TCs above hurricane-equivalent strength are referred to as cyclones within the primary basins. These basins include the Southwest Indian Ocean, the Australian region, and Southwestern Pacific Ocean. On occasion, the Australian region is considered to be split between the South Indian and Southwest Pacific Oceans.

Cyclone Eunice in the Indian Ocean during January 2015. (Terra/MODIS)