Hurricane Irma Caribbean Landfall likely influenced by both human-driven climate change and natural variability

Press Summary (First published 2023/10/10, Updated on 2023/11/22)

• Cyclones landfalling in the Caribbean similar to hurricane Irma are up to 13 mm/day wetter in the present than they have been in the past.

• Hurricane Irma was a very uncommon event.

• We mostly ascribe the heavy precipitation of hurricane Irma Caribbean landfall to human driven climate change, and natural climate variability likely played a modest role.

Event Description

Hurricane Irma was the tenth tropical cyclone of the 2017 hurricane season in the North Atlantic Ocean. It became a major hurricane, reaching Category 5 on the Saffir-Simpson scale, following Hurricane Harvey, which reached Category 4 just a week earlier. Irma maintained its Category 5 status for an unusually long period. It caused catastrophic damage in several areas, including the islands of Barbuda, Saint-Barthélemy, Saint-Martin, Anguilla, the British Virgin Islands, Cuba, and forced the evacuation of over six million residents in Florida. Irma was one of the most powerful hurricanes ever recorded in the North Atlantic, with sustained winds of up to 287 km/h (178 mph). After devastating the Caribbean, it continued its path towards the United States, leading to evacuation orders in Florida and other states. Eventually, Irma weakened to a Category 4 hurricane as it made landfall in Florida, causing significant but somewhat reduced damage compared to earlier forecasts. It later weakened further to a tropical storm and continued its path through Georgia and Alabama before dissipating on September 12, 2017.

Hurricane Irma had varying impacts on different regions. In Guadeloupe, the hurricane caused a significant marine swell and coastal flooding, but the island largely avoided major material or human damage. However, around 8,000 households lost power, and some roads were blocked. Basic services were swiftly restored within 48 hours, and schools and government offices reopened promptly. On the other hand, Saint-Barthélemy and Saint-Martin, directly in Irma's path, faced extreme devastation. They were placed on the highest alert level (violet), resulting in the confinement of the population. The hurricane's gusts exceeded 320 km/h for several hours, leading to widespread destruction, including sand-covered streets, uprooted trees, damaged vehicles, and houses blown away. Many solid buildings were also severely affected, including luxury hotels in Saint-Barthélemy and the collapse of a Swedish church tower in Gustavia. Additionally, water and electricity were disrupted, with severe flooding and communication challenges. In the Dutch part of the island, authorities reported two deaths and numerous injuries, with initial port and airport closures. In March 2018, the cost of Hurricane Irma's impact on Saint-Martin and Saint-Barthélemy alone was estimated at 3 billion euros. Over the past century, Irma ranks as one of the three most powerful hurricanes to hit these two French islands, alongside Donna in 1960 and Luis in 1995. The hurricane passed about 100 kilometers off the coast of the Dominican Republic, avoiding catastrophic damage. However, the Dominican Republic experienced heavy rains, flash floods, and coastal flooding, primarily in the province of Puerto Plata. This resulted in material damage to tourist resorts in Cabarete and Sosúa, leading to the evacuation of 5,000 people. In Haiti, the northern part of the country, particularly the Northeast Department, was affected by significant rainfall. Due to a sudden rise in water levels, one of the two bridges over the Massacre River, connecting Ouanaminthe in Haiti to Dajabón in the Dominican Republic, was destroyed. These bridges were the second most important crossing points and trade links between the two countries. Some injuries were reported, along with widespread power outages in the northern part of Haiti. Humanitarian workers on the ground reported that flooding was the primary challenge faced by the northern part of the country. In the short term, there was a high likelihood of a resurgence of the cholera epidemic that has plagued Haiti since 2010. Additionally, while the human toll was relatively spared, the floods and winds caused significant damage to fields, food crops, and stocks in five affected regions, stretching from Môle-Saint-Nicolas in the west to Ouanaminthe in the east. This raised concerns about a potential food crisis in the medium term, in a country already vulnerable in this regard, according to Action Against Hunger.

The Surface Pressure Anomalies reveal a narrow band of low pressure anomalies which reflect the track of hurricane Irma during the days analysed. Temperature Data show no anomalies for this event. Precipitation Data indicate that most of the area covered by the analysis experienced extreme precipitation reaching up to 100mm per day of analysis in most of the domain analyzed. Windspeed Data also show that the cyclone produced sustained winds over the northern part of the domain analysed.

Climate and Data Background for the Analysis

The IPCC AR6 WG1 report states that it is likely that the global proportion of Category 3–5 tropical cyclone has increased over the past four decades and the global frequency of TC rapid intensification events has likely increased over the past four decades. None of these changes can be explained by natural variability alone (medium confidence). Moreover the proportion of intense TCs, average peak TC wind speeds, and peak wind speeds of the most intense TCs will increase on the global scale with increasing global warming (high confidence). At the regional scale there is limited evidence of current trends in observed wind speed and wind storms in Central America, nevertheless in the Souther Central America region climate projections indicate a decrease in frequency of tropical cyclones accompanied with an increased frequency of intense cyclones, and an increase in mean wind speed (medium confidence).

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For this event, we have medium-low confidence in the robustness of our approach given the available climate data, as the event is unusual in the data record.

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to the low pressure system leading to hurricane Irma Caribbean lanfdfall have changed in the present (2001–2022) compared to what they would have looked like if they had occurred in the past (1979–2000) in the region [-75°E -65°E 13°N 23°N]. The Surface Pressure Changes show that cyclones have not significantly changed their intensity compared to the past except in some limited areas. Precipitation Changes show that similar events produce heavier (0-13mm/day) precipitation in the present than in the past but results are significant only on limited area of the region considered. Windspeed Changes show that similar events produce faster (4-8 km/h) winds. Considering the affected urban areas, San Juan and Santo Domingo see an increase in precipitation and wind in the present while the signal is small for Port-au-Prince. We also find that Similar Past Events are more frequent In November, while in the past they were mostly occurring in October.

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation, may have influenced the event.  This means that the changes we see in the event compared to the past may be mostly due to human driven climate change.

Conclusion

Based on the above, we conclude that cyclones following similar tracks as hurricane Irma during the Carribean landfall have become 0-13 mm/day wetter in the present than in the past. We interpret hurricane Irma Caribbean landfall as an unusual event whose characteristics can mostly be ascribed to human driven climate change.

Contact Authors

Davide Faranda, IPSL-CNRS, France  📨davide.faranda@lsce.ipsl.fr  🗣️French, Italian, English

Erika Coppola, ICTP, Italy  📨coppolae@ictp.it  🗣️Italian, English

Flavio Pons, IPSL-CNRS, France  📨flavio.pons@lsce.ipsl.fr  🗣️French, Italian, English