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, one hundred thirty-four systems have formed and ninety-two 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. So far, forty Category 3 tropical cyclones formed, including nine Category 5 tropical cyclones in the year.
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 the fewest cyclones reaching tropical storm intensity in recorded history, 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.