During 2017 tropical cyclones formed within seven different tropical cyclone basins, located within various parts of the Atlantic, Pacific and Indian Oceans. During the year, a total of 146 tropical cyclones had formed. 88 tropical cyclones had been named by either a Regional Specialized Meteorological Center (RSMC) or a Tropical Cyclone Warning Center (TCWC). The strongest and deadliest tropical cyclone was Hurricane Maria with a minimum barometric pressure of 908 mbar (hPa; ) and killing 3,000 people in Puerto Rico and Dominica. The costliest tropical cyclone of the year was Hurricane Harvey in the Atlantic, which struck Houston metropolitan area in August causing US$125 billion in damage, tying with Hurricane Katrina as the costliest tropical cyclone worldwide.
The most active basin in the year was the Western Pacific, which documented 27 named systems. However, the season had slightly below-average activity, and was the first season since 1977 to feature zero Category 5-equivalent super typhoons forming in the basin. The Eastern Pacific, despite amounting to 18 named systems, which constitutes above-average activity, was also significantly less active than the previous three Pacific hurricane seasons (2014, 2015 and 2016). However, the North Atlantic featured 17 named storms, and was the costliest tropical cyclone season on record. It also had the fifth-most named storms since reliable records began in 1851 – tied with 1936 – and the most major hurricanes since 2005. 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, and with the most powerful cyclone, Cyclone Ernie, explosively intensifying to a Category 5 tropical cyclone. Two other Category 5 tropical cyclones formed in the North Atlantic, totalling to three Category 5 tropical cyclones that formed during the year. The accumulated cyclone energy (ACE) index for the 2017 (seven basins combined), as calculated by Colorado State University (CSU) was 621.1 units, which was below the 1981-2010 mean of 770.2 units.
Global atmospheric and hydrological conditions
For the majority of the year, the tropics were dominated by neutral El Niño–Southern Oscillation (ENSO) conditions, before La Niña conditions set in later in the year. As the year opened, sea surface temperatures anomalies across the central and east-central equatorial Pacific were cooler than average, while the impacts of La Niña lingered within the atmospheric circulation, following an abrupt end to the 2016 La Niña episode. Over the next few months, these anomalies warmed and nearly reached the thresholds needed for an El Niño event to be declared, however, they subsequently cooled throughout the rest of the year and the 2017–18 La Niña event was declared to be underway.
Scientists pointed out that the increase of emissions contributes to warmer ocean waters and more moist air for rain. Because of sea level rise it is assumed that the storm surge of Hurricane Irma and other storms will cause greater flooding to vulnerable areas. Data collected by NASA showed that ocean surface temperatures in the path of Irma were above 30 °C (86 °F), capable of sustaining a Category 5 hurricane. Prior to affecting the U.S. mainland, Miami's mayor Tomás Regalado noted on Hurricane Irma, "This is the time to talk about climate change. This is the time that the president and the EPA and whoever makes decisions needs to talk about climate change." A day later the head of the EPA, Scott Pruitt said, "..to discuss the cause and effect of these storms, there's the… place (and time) to do that, it's not now." Following Irma's landfall, Donald Trump was asked about the connection between hurricanes and climate change, and stated that "We’ve had bigger storms than this." Richard Branson who was directly impacted by hurricane Irma noted, "..hurricanes are the start of things to come. Look, climate change is real. Ninety-nine per cent of scientists know it's real. The whole world knows it's real except for maybe one person in the White House."
United Nations secretary general António Guterres citing the devastation from hurricanes noted in September, "The catastrophic Atlantic hurricane season has been made worse by climate change. Cutting carbon emissions must clearly be part of our response to the disaster. The rise in the surface temperature of the ocean has had an impact on weather patterns and we must do everything possible to bring it down."
The Associated Press looked at the yearly average accumulated cyclone energy (ACE), which accounts for wind speed and storm duration to assess hurricane power of the past 30 years and found it to be 41 percent higher than the previous 30 years. They asked several experts about their opinion, James Kossin from NOAA "There's no question that the storms are stronger than they were 30 years ago." Climate scientist Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, "The only caveat being that the increase might be exaggerated somewhat because of undercounting early storms." Meteorologist Philip Klotzbach noted, "What's happening with hurricanes — the frequency, the duration, and the energy — is probably a combination of factors caused by both nature and man, a mish-mosh of everything." Kerry Emanuel who studies hurricanes, told the BBC, "The warming of the climate has increased the underlying probabilities of very heavy rain events like happened in Harvey and very high category hurricanes like Irma. It is just not sensible to say either storm was caused by climate change, but the underlying probabilities are going up."
Summary
North Atlantic Ocean
The Atlantic basin was hyperactive with 17 named storms, 10 consecutive hurricanes, and 6 major hurricanes. It was also the second consecutive above-average season since 2016 and the costliest season on record. Most of the damage was caused by hurricanes Harvey, Irma, and Maria. Hurricane Nate was considered the worst natural disaster in Costa Rican history. Due to the damage that the four hurricanes made, the names Harvey, Irma, Maria, and Nate were retired by the WMO after the season.
The first named storm, Arlene, formed on April 19 as a subtropical depression. When Arlene was named on April 20, it became only the second named storm to form in April in the Atlantic, the other being Ana in 2003 and the strongest tropical cyclone on record in April. No storms formed in May.
Throughout June and July, activity increased with Tropical Storms Bret, Cindy, Don, and Emily and Tropical Depression Four. Tropical Storm Bret was the earliest named storm to form in the Main Development Region, affecting Venezuela and Trinidad and Tobago. Tropical Storm Cindy also existed throughout mid-June, affecting Louisiana. Together, both caused at least $28 million in damage and five deaths within the areas they affected. In July, tropical depression Four and tropical storms Don and Emily existed with the latter causing widespread floods throughout central Florida. Despite this activity within the first two months, most storms were weak or short-lived and resulted in the lowest ACE for the first five named storms on record, surpassing 1988.
In August, Hurricanes Franklin, Gert, Harvey, and Irma formed with the latter two causing catastrophic damage to the contiguous United States. Hurricane Harvey caused catastrophic flooding and was tied the costliest hurricane on record along with Hurricane Katrina in 2005. Harvey was also the wettest tropical cyclone ever recorded in the United States with a rainfall total of 60.58 inches in Nederland, Texas. Hurricane Irma caused a trail of damage from the Leeward Islands to Florida, causing $77.2 billion in damage, and became the first Category 5 hurricane to make landfall in the Leeward Islands.
In September, copious activity occurred with Hurricanes Jose, Katia, Lee, and Maria, with the latter becoming the tenth most intense hurricane on record. Along with the four systems, Irma also persisted through early September, making it and Maria the first Category 5 hurricanes to occur within the same month. Hurricane Maria caused catastrophic damage to Puerto Rico and Dominica with over 3,000 fatalities and $91.606 billion in damage. Accumulated cyclone energy also increased rapidly during this month, resulting in three hurricanes achieving an ACE over 40, the first on record to do so, September 8 becoming the most active day on record based on ACE, and Hurricane Irma achieving the third-highest ACE on record in the Atlantic with 64.9 units.
By October, activity still persisted although the potential for a La Niña would begin peaking by November. Hurricane Nate became the worst natural disaster in Costa Rican history as a precursor, while Ophelia became the worst tropical cyclone to affect Ireland in 50 years. Philippe was a short-lived system that contributed to a bomb cyclone shortly after its dissipation.
Finally, Tropical Storm Rina became the last storm to form in this basin, dissipating on November 9. The extratropical remnants of this system later contributed to Cyclone Numa (or Storm Zenon), which affected Greece, Italy, France, Tunisia, and Turkey and caused the worst weather event in Greece since 1977. The season ended on November 30.
Eastern and Central Pacific Oceans
The 2017 Pacific hurricane season was a moderately active season with eighteen named storms, nine hurricanes, and four major hurricanes. Unlike the three previous hyperactive seasons (2014, 2015, and 2016), the season was significantly less active in terms of ACE and most storms that existed through this basin were either weak or short-lived.
Early May saw the formation of Tropical Storm Adrian, which was the earliest named tropical cyclone to exist east of 140°W at the time; it was surpassed by Tropical Storm Andres in 2021. Throughout June and July, the basin saw some considerable activity, with the latter month having the fifth-highest ACE in the Eastern Pacific. Storms that existed during this time include Hurricanes Dora, Eugene, Fernanda, Hilary, Irwin, Tropical Storms Beatriz, Calvin, and Greg, and Tropical Depression Eight-E, which contributed to the ACE count throughout the earlier parts of the season. Throughout August and September, activity decreased significantly, but stagnated in terms of named storms with some being short-lived. For example, in September, only four named storms formed (Otis, Max, Norma and Pilar) and October featured tropical storms Ramon and Selma, in which none attained hurricane status. Selma became the first named storm to make landfall in El Salvador and also contributed to the formation of Philippe in the Atlantic. No storms have formed in November since 2010.
In the Central Pacific, no storms have formed since 2011. The season began on June 1 and both seasons ended on November 30, 2017.
Western Pacific Ocean
The 2017 Pacific typhoon season was a below-average season in terms of accumulated cyclone energy and the number of typhoons and super typhoons, and the first since the 1977 season to not produce a Category 5-equivalent typhoon on the Saffir–Simpson scale.