Unusually, three ex-hurricanes hit Ireland during September. First was ex-hurricane Lee on September 19th, then Nigel, followed by the remnants of ex-tropical storm Ophelia which hit Ireland as storm Agnes on September 27th, with winds of 80km/h. (It’s just a coincidence that tropical storm Ophelia 2023 has the same name as ex-hurricane Ophelia which brought significant damage to Ireland in 2017 - annual tropical storm names originate from lists produced by the national hurricane centre and rotate every 6 years.)
Agnes caused flooding in many parts of the southwest and Co Cork, where dozens of homes were left without power.
Hurricanes, also known as tropical cyclones, form in the tropics off the west coast of Africa where the warm waters provide the energy to fuel the storm. Initially hurricanes travel westwards in line with the steering currents; when they turn towards the north as Lee, Nigel and Ophelia did, the steering flow changes and the hurricanes get caught up in the westerlies and travel towards Europe. As they track northwards, the tropical cyclone transitions into a typical storm formed by cold and warm air masses colliding. At this point, it is referred to as an ex-tropical cyclone, as its origins remain in the tropics.
In our recent study on the role of air-sea heat flux and wind stress, we 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.
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There are two reasons for this. When the tropical Atlantic is 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. Tropical cyclones form farther east when the Atlantic is warmer close to the west coast of Africa, which increases the likelihood of the storms hitting Europe when they curve northwards.
The reason September is when these storms mostly hit Ireland also has to do with warmer oceans. Hurricanes intensify by extracting energy from the warm ocean surface via air-sea heat fluxes, so a warmer ocean can lead to stronger hurricanes. The ocean temperatures in the North Atlantic are at their warmest in September, which is one of the reasons why the peak of the Atlantic hurricane season happens during that month. Tropical Atlantic wind shear also subsides to its lowest levels in September – wind shear is the variation in the wind speed and direction with height in the atmosphere. A strong wind shear can prevent hurricanes from forming. So, the combination of warmest ocean temperatures and weak wind shear means most hurricanes form in the Atlantic in September, which in turn means ex-hurricanes are most likely to hit Ireland at the same time.
Storms Babet and Ciarán which also hit Ireland in recent days were not, however, ex-tropical storms. They were storms driven by an unusually strong jet stream - or band of strong wind at ten thousand meters above the earth’s surface. A stronger jet stream - associated with the difference in temperature between cold polar air and warm sub-tropical air - can make storms more powerful. The anomalously warm Atlantic Ocean and cold conditions over the south-eastern US have increased the strength of the jet stream this autumn. Our research suggests there has been a 10mph increase in the jet stream speed over the Atlantic in the last 140 years.
So, are the tropical cyclone changes due to a changing climate? Research undertaken by Maynooth University climate change master’s student Randy Aird, who is based in Jamaica, 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 farther north. Other studies also suggest an increase in ex-tropical cyclones over Europe in a warming climate.
The Atlantic experienced a marine heatwave this year, which has helped to fuel the tropical storms for longer, enabling them to travel farther. In October 2017, the ocean temperatures in the northeast Atlantic were also particularly warm around latitude 45 degrees north which helped sustain Hurricane Ophelia, enabling it to hit the shores of Ireland where it caused significant flooding and storm damage.
The latest Intergovernmental Panel on Climate Change report finds that the oceans have warmed by 0.88 degrees since the 1900s, and rising ocean temperatures are expected to continue under global warming. All of this suggests that, unfortunately, we are unlikely to have seen the last of the ex-hurricanes hitting Ireland, particularly during September.
Dr Samantha Hallam is a post-doctoral researcher at the Irish Climate Analysis Research Unit at Maynooth University. Research into tropical cyclones at the university is funded by the Marine Institute and Irish Aid as part of the Shared Ocean Programme, bringing together Irish and Caribbean researchers