Science Week: Three ex-hurricanes hit Ireland in recent weeks. What's going on?
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Three ex-hurricanes hit Ireland this September. The remnants of ex-tropical storm Ophelia hit Ireland as storm Agnes on September 27 with winds up to 80km/h, leaving people without power and causing flooding in many parts of the southwest and Co Cork. Only days earlier ex-hurricane Nigel brought rain and strong winds to Ireland, and that followed the wet weather from ex-hurricane Lee on September 19. Why are so many ex-hurricanes bringing storms to Ireland this year?
In September several hurricane tracks have curved northwards, including hurricanes Lee, Nigel and Ophelia. As the storms travelled northwards over the Atlantic each of these hurricanes transitioned into an ex-tropical storm and hit Ireland a few days later. Hurricanes, also known as tropical cyclones, form in the tropics, where the water is warm enough to provide the energy to fuel the storm. Typically, the waters need to be over 26C.
In the tropical Atlantic, hurricanes form off the west coast of Africa and travel westwards in line with the steering currents. When a hurricane turns toward the north, the steering wind changes and the hurricanes can get caught up in the westerlies and travel towards Europe. As the tropical cyclone moves northwards over cooler waters, it transitions into a typical storm formed by cold and warm air masses colliding, but its origins remain in the tropics, so it is called an ex-tropical cyclone. The energy generated from the collision of cold and warm air masses helps to fuel the ex-tropical cyclone as it travels towards Europe.
A recent study led by Dr Samantha Hallam found that tropical cyclones were more likely to curve northwards when the tropical Atlantic is particularly warm, which is the case in 2023 and was also the case in 2010 and 2017. There are two reasons for this. When the tropical Atlantic is particularly warm it can act to weaken the Azores high, an area of high atmospheric pressure near the Azores in the Atlantic. A weaker Azores high results in a lower atmospheric pressure off the east coast of America enabling tropical cyclone tracks to curve northwards. In addition, tropical cyclones form further east in the Atlantic closer to the west coast of Africa, which increases the likelihood of hitting Europe when they curve northwards.
Why do they tend to come mostly in September?
Hurricanes intensify by extracting energy from the warm ocean surface via air-sea heat fluxes, so a warmer ocean can lead to more intense hurricanes. The ocean temperatures in North Atlantic are at their warmest in September, which is one of the reasons why that's the peak of the Atlantic hurricane season. In addition, tropical Atlantic wind shear tends to reach a minimum in September. Wind shear is the variation in the wind speed and direction with height in the atmosphere. If there is a strong wind shear it can prevent hurricanes from forming. So, the warmest ocean temperatures and weak wind shear together are conducive to most hurricanes forming in the Atlantic in September, which in turn means ex-hurricanes are most likely to hit Ireland in that month.
Are the changes due to a changing climate?
Research undertaken by Randy Aird found that since 1968 there has been a significant increase in the number of ex-tropical cyclones found in the latitude band 50-60 degrees north, in the Atlantic. Ireland lies between 51 and 55 degrees north. The study also found that warmer sea surface temperatures contributed to the increase in ex-tropical cyclone frequency further north. The study suggests there has been an increasing trend over the last 55 years. Other studies also suggest an increase in ex-tropical cyclones over Europe in a warming climate.
The Atlantic has also been particularly warm this year which has helped to fuel the tropical storms for longer, enabling them to travel further. In October 2017, the ocean temperatures in the northeast Atlantic were also particularly warm around latitude 45N which helped sustain Hurricane Ophelia, enabling it to hit the shores of Ireland that year.
The latest IPCC report highlights that the oceans have warmed by 0.88C since the 1900s and rising ocean temperatures are expected to continue under global warming. This has the potential to bring more ex-hurricanes to the shores of Ireland in the future and Ireland will need to be prepared.
Research into tropical cyclones is funded by project TOPIM (tropical cyclone ocean-coupled potential intensity model) as part of the Shared Ocean Programme, bringing together Irish and Caribbean researchers and funded by the Marine Institute and Irish Aid.
Dr Samantha Hallam is an Irish-based post-doctoral researcher, based at the Irish Climate Analysis Research Units (ICARUS), Maynooth University. Randy Aird is a Jamaican-based master's Climate Change student, who recently completed his thesis (Are Atlantic tropical cyclones moving northwards?) at Maynooth University.