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. Recent studies simulating continued anthropogenic climate change provide evidence that extratropically transitioning tropical cyclones (TCs) will become more frequent and will hit western Europe more often (Baatsen et al. 2015; Haarsma et al. 2013). Mokhov et al. (2014) asserted, “Under the tendency towards global warming, we can expect an increase in the number of intensive cyclones in the warmer and more humid troposphere.” We saw Hurricane Gonzalo of 2014 as an occasion to assess if these aforementioned properties of extratropically transitioned storms—frequency, intensity, and tracks—have changed. East of the Leeward Islands a tropical depression formed on 12 October 2014. On its way it passed through the northern Leeward Islands and intensified to a category 4 hurricane (Saffir-Simpson hurricane wind scale) on 16 October, known as “Gonzalo”. After changing its direction to northeast, Gonzalo weakened and crossed Bermuda with gusts of more than 200 km h−1 and heavy rains of about 70 mm within 24 hours. On 19 October, the storm transitioned to an extratropical cyclone off the coast of Newfoundland (Brown 2015). While continuing its path across the North Atlantic towards northwestern Europe, the cyclone was absorbed by a cold front and strengthened again. Afterwards, it hit the northern part of the United Kingdom on 21 Octber. It crossed the North Sea and then central parts of Europe, and went down to the Balkans. On 23 October ex-Gonzalo merged with another low pressure system that led to heavy precipitation for several days in this region. Maximum wind gusts between 100 and 180 km h−1, causing North Sea storm surges, were reported from several countries1,2,3. In addition, ex-Gonzalo triggered regional precipitation amounts of 50–100 mm in 24 hours, while the advection of cold air led to a sudden temperature drop with snowfall in some areas. Gonzalo and its remnants caused several fatalities, storm surges, structural damage, and power outages on both sides of the Atlantic4,5. Gonzalo attracted strong media attention as it affected many countries along its path6. There is no general definition of extratropical transition (ET) of TCs (Malmquist 1999). Basically, it is a gradual transformation of a TC into a system with extratropical characteristics while moving poleward into a more baroclinic environment with higher wind shear, a larger Coriolis parameter, and lower sea surface temperatures (Jones et al. 2003). The ET storm may interact with upper-level troughs or extratropical low pressure systems. Evans and Hart (2003) describe ET as the transition of a warm-core TC that interacts with a baroclinic midlatitude environment and then develops a cold core. Forty-six percent of the Atlantic TCs transitioned into extratropical cyclones between 1950 and 1996 (Hart and Evans 2001). This result was supported by Jones et al. (2003) for 1970–99 and Mokhov et al. (2014) for 1970–2012 who found 45% of North Atlantic TCs underwent ET. But only very few

Hurricane Gonzalo and its Extratropical Transition to a Strong European Storm