During 2015, tropical cyclones formed in seven major bodies of water, commonly known as tropical cyclone basins. Tropical cyclones will be assigned names by various weather agencies if they attain maximum sustained winds of . During the year, 140 systems have formed and 93 were named. The most intense storm of the year was Hurricane Patricia, with maximum 1-minute sustained wind speeds of and a minimum pressure of . The deadliest tropical cyclone was Cyclone Komen, which caused 280 fatalities in Southeast India and Bangladesh, while the costliest was Typhoon Mujigae, which caused an estimated $4.25 billion USD in damage after striking China. A record-high 40 Category 3 tropical cyclones formed, including nine Category 5 tropical cyclones in the year. The accumulated cyclone energy (ACE) index for the 2015 (seven basins combined), as calculated by Colorado State University (CSU) was 1,047 units.
The most active basin in the year was the Eastern Pacific which documented 26 named systems, was its basin's most active since 1992, despite only amounting to 25 named systems in the Western Pacific. Conversely, both the North Atlantic hurricane and North Indian Ocean cyclone seasons experienced a below average season numbering 11 and 4, respectively. Activity across the southern hemisphere's three basins–South-West Indian, Australian, and South Pacific–was spread evenly, with each region recording seven named storms apiece. That hemisphere's strongest tropical cyclone was Cyclone Pam, which bottomed out with a barometric pressure of 896 mbar (hPa; ) in the South-Pacific Ocean.
Tropical cyclones are primarily monitored by a group of ten warning centers, which have been designated as a Regional Specialized Meteorological Centre (RSMC) or a Tropical Cyclone Warning Center (TCWC) by the World Meteorological Organization. These are the United States National Hurricane Center (NHC) and Central Pacific Hurricane Center, the Japan Meteorological Agency (JMA), the Indian Meteorological Department (IMD), Météo-France (MFR), Indonesia's Badan Meteorologi, Klimatologi, dan Geofisika, the Australian Bureau of Meteorology (BOM), Papua New Guinea's National Weather Service, the Fiji Meteorological Service (FMS) as well as New Zealand's MetService. Other notable warning centers include the Philippine Atmospheric, Geophysical, and Astronomical Services Administration (PAGASA), the United States Joint Typhoon Warning Center (JTWC), and the Brazilian Navy Hydrographic Center (BNHC).
Global atmospheric and hydrological conditions
Background
By January 2015, westerly wind burst activity picked up again. The first Kelvin wave developed around March and another formed around May. In addition, another strong westerly wind burst event took place around July as a result of twin tropical cyclones straddling the equator. An even stronger event in October, and an unusually stronger event during late December 2015 into January 2016, also resulted from twin cyclones on opposite sides of the equator. In May 2015, the Australian Bureau of Meteorology respectively confirmed the arrival of weak El Niño conditions. El Niño conditions were forecast in July to intensify into strong conditions by fall and winter of 2015. In addition to the warmer than normal waters generated by the El Niño conditions, the Pacific Decadal Oscillation was also creating persistently higher than normal sea surface temperatures in the northeastern Pacific. In August, the NOAA CPC predicted that the 2015 El Niño "could be among the strongest in the historical record dating back to 1950." In mid November, NOAA reported that the temperature anomaly in the Niño 3.4 region for the 3-month average from August to October 2015 was the 2nd warmest on record with only 1997 warmer.
After Typhoon Higos developed during February 2015, a new forecast scenario opened: El Niño might strengthen and persist through 2015. This scenario was supported by the same climate features that had predicted the weak El Niño developing during 2014. During their March 2015 diagnostic discussion, NOAA's CPC and the IRI reported that El Niño conditions had been observed during February 2015, after the above average sea surface temperatures had become weakly coupled with the tropical atmosphere.
During November and December 2015, values within NOAA's Oceanic Niño Index peaked at , which surpassed December 1997 value of . NOAA subsequently reported that the 3-month average from November 2015 to January 2016 of the ONI had peaked at , which meant that the 2014–16 event was tied with the 1997–98 event for the strongest values on record. However, overall the event was considered to be one of the three strongest El Nino events since 1950, since there was a number of different ways to measure the strength of an event. The event subsequently started to weaken with sea surface temperature anomalies across the equatorial pacific decreasing, while predictions about a possible La Niña event taking place during 2016 started to be made.
Effects
The 2014–16 El Niño event influenced tropical cyclone activity around the world, where it contributed to record breaking seasons in the Central Pacific and Eastern Pacific tropical cyclone basins. By contrast, it limited Atlantic hurricane activity, producing strong vertical wind shear, increased atmospheric stability, stronger sinking motion and drier air across the tropical Atlantic. The Central Pacific basin saw its most active tropical cyclone season on record with 16 tropical cyclones recorded during 2015. Within the Southern Hemisphere, the El Niño pushed tropical cyclone activity in the South Pacific Ocean eastwards, with activity flourishing near Vanuatu, Fiji and Tonga. As a result of this displacement and other factors such as a positive Indian Ocean Dipole, the 2015–16 Australian region cyclone season was the least active since reliable records started during the 1950s, with only three named tropical cyclones developing in the region compared to an average of eleven.
The event also contributed to six systems forming outside of the season boundaries, within the North Atlantic, Eastern and Southern Pacific basins. These systems included Tropical Cyclone Raquel, which was considered by some to be a part of both the 2014–15 and 2015–16 seasons, but was later deemed to only be a part of the 2014–15 season. Tropical Depressions 01F and 02F developed in the South Pacific during July and October 2015, which affected Fiji, Vanuatu and the Solomon Islands. Tropical Depression Nine-C subsequently formed in the Central Pacific on 31 December, whose remnants in turn contributed to the development of Hurricane Pali on 7 January. This also caused the latest end and earliest start to the 2015 and 2016 Pacific hurricane seasons, respectively.
Other significant tropical cyclones during the event included: Cyclone Pam, which became the second most intense tropical cyclone in the South Pacific in terms of wind speed and devastated Vanuatu; Cyclone Winston, was the most intense tropical cyclone in the Southern Hemisphere and devastated Fiji; Cyclone Fantala, which was the strongest storm in terms of 1-minute and 10-minute sustained winds in the South Indian Ocean; and Hurricane Patricia, which was the second-most-intense tropical cyclone on record globally in terms of barometric pressure, and the strongest in terms of 1-minute sustained winds.
Summary
North Atlantic Ocean
The season was the last of three consecutive below-average Atlantic hurricane seasons. It was a slightly below average season in which twelve tropical cyclones formed. Eleven of the twelve designated cyclones attained tropical storm status. Of the eight tropical storms, four reached at least Category 1 hurricane intensity. The 2015 season extended the period without major hurricane landfalls in the United States to ten years, with the last such system being Hurricane Wilma in 2005. The lack of activity was primarily attributed to an atmospheric circulation that favored dry, sinking air over low latitudes to the west of 40°W and westerly wind shear enhanced by El Niño. A few notable events occurred during the season. Ana was the first tropical cyclone to form in the off-season since Beryl in 2012. Erika became only the second storm in the satellite era to be retired without reaching hurricane strength (Tropical Storm Allison was the first) and only the third to be retired without having made landfall (Hurricanes Klaus and Fabian were the first and second, respectively). Fred was one of the easternmost tropical storms recorded and made landfall in Cape Verde as a Category 1 hurricane, becoming the first hurricane to strike that country since 1892. Joaquin was the most intense storm of non-tropical origin in the satellite era and the strongest to affect the Bahamas in October since 1866. The tropical cyclones of this season caused 89 deaths and at least $731.8 million in damage.
The Atlantic hurricane season officially ended on November 30, 2015. Tropical cyclogenesis began early, with Ana developing on May 8, over three weeks prior to the official beginning of the season and far ahead of the long-term climatological average of July 9. The month of June featured one tropical cyclone, Bill, which formed on June 16. Claudette, the only system in the month of July, developed on July 13. The rate of tropical cyclogenesis increased in August, though no storms developed in the first half of the month. Danny, the season's first major hurricane, formed on August 16, followed by Erika on August 24, and Fred on August 30. September, which is the climatological peak of hurricane season, featured five additional tropical cyclones – Grace, Henri, Tropical Depression Nine, Ida, and Joaquin. The most intense storm of the season was Joaquin, which peaked as a strong Category 4 hurricane with maximum sustained winds of , just short of Category 5 on the Saffir–Simpson hurricane wind scale. Joaquin lasted into October, though no other systems developed that month. The season's final cyclone, Kate, developed on November 8 and became extratropical on November 11. The season's activity was reflected with an Accumulated Cyclone Energy (ACE) rating of 63, which was well below the 1981–2010 median of 92.
Eastern & Central Pacific Oceans
Overall, 31 tropical cyclones formed, of which 26 reached tropical storm intensity and were named. A total of 16 of these storms became hurricanes, and a record-breaking 11 became major hurricanes. These values make the 2015 season the second-most active on record. The Central Pacific, meanwhile, had its most active year on record, with 16 tropical cyclones forming in or entering the basin, easily surpassing the old record of 11 set in 1992 and 1994. The season officially began on May 15 in the Eastern Pacific, and on June 1 in the Central Pacific; the season ended officially on November 30, though a tropical depression formed well after that on December 31. The well-above-average activity levels were largely attributed to the strong 2014–16 El Niño event, which brought anomalously high sea surface temperatures and low vertical wind shear to western parts of the basin. In fact, for the region between the 116th meridian west and the International Date Line, sea surface temperatures from July to October averaged , the highest value on record since reliable data records began in 1979. Around Hawaii, sea surface temperatures were higher than at any point during the past 60 years. Wind shear in the region was also at its lowest on record during that period. The existence of a large region of rising air over the basin during much of the season, a feature typical of El Niño events, further facilitated the development and intensification of the season's numerous tropical cyclones.