This week the powerful Super Typhoon Bolaven developed in the western Pacific Ocean. It began forming on Saturday and gradually strengthened as it tracked north-east, passing between Guam and the Northern Mariana Islands on Tuesday as a category 1 equivalent storm.
Bolaven subsequently underwent a process known as rapid intensification, with sustained wind speeds increasing from 90mph to 160mph (145km/h to 255km/h)– the equivalent of a category 5 hurricane – in just 12 hours. It continued to strengthen into Wednesday, reaching a peak intensity of sustained winds of 180mph, making it the second strongest storm of 2023, just shy of the 185mph achieved by Super Typhoon Mawar in May.
The process of rapid intensification is defined as an increase in sustained winds of 35mph within 24 hours, a mark Bolaven easily surpassed, and it was not the only tropical storm to do so this week. Hurricane Lidia formed in the eastern Pacific Ocean last week and, despite an initially slow development, on Tuesday sustained winds increased by 65 mph in 24 hours. Lidia then ploughed into the west coast of Mexico as a category 4 storm, bringing sustained winds of 140mph, alongside heavy rain that resulted in landslides and some flash flooding.
The phenomenon of rapid intensification is caused in part by very warm sea surface temperatures, with those in the western Pacific now trending around 1C-3C above average. As our climate continues to warm, we are likely to see more frequent examples of rapid intensification of tropical storms in future.
While Lidia quickly dissipated as it moved inland, Bolaven is slowly weakening as it tracks north-west towards North America, where its remnants are expected to affect the weather next week as it interacts with the jet stream.
Meanwhile, September was officially confirmed as the warmest on record by the Copernicus Climate Change Service. The month was 1.8C warmer than pre-industrial levels, smashing the previous record by an astonishing 0.5C. El Niño in the equatorial Pacific is partly responsible for the extreme temperatures, but the UK Met Office said they would be “virtually impossible” without the influence of human-induced climate change. Now 2023 is on course to be the warmest year in recorded history.
Many European countries had their hottest Septembers, including France, Germany and Poland. France experienced an average temperature of 21.5C, making it the hottest September since records began in 1900, while Germany had average temperatures almost 4C higher than the 1961-1990 baseline. The Russian capital, Moscow, also had its hottest September since 1847, the third warmest since records began.