Heavy rain in Hurricane Otis mostly strengthened by human-driven climate change

Press Summary (First Published 2023/10/26, Updated 2023/11/22)

• Cyclones affecting southern Mexico are wetter  (10 to 25 mm/day)  in the present than they have been in the past in the Acapulco area.

• Hurricane Otis was a largely unique event. 

• While natural variability partly explains the high precipitation, human-driven climate change contributed to make the rainfall more intense.

Event Description

Tropical Cyclone Otis made an unexpected landfall in south Mexico, close to Acapulco, as a Category 5 hurricane resulting in severe infrastructure damage on October 25th, 2023. Within about 18 hours, Otis rapidly intensified from a Tropical Storm to a Category 5 hurricane. It is the strongest ever known hurricane making landfall in Mexico and one of the most rapidly intensifying cyclones in the world, at least in the satellite era (since 1979).  

According to numerical model guidance one day before the landfall, no signs of a rapid intensification of Otis were evident, and thus, warnings were issued only a few hours before the Hurricane hit Acapulco (Mexico), one of the most touristic city  in the country. Otis hit Acapulco with 265 km/h sustained winds and wind gusts up to 322 km/h. No rain gauge measurements or radar-based rainfall estimations are available at the moment of this analysis, but severe flooding have been reported in Acapulco and surrounding cities due to torrential rainfall. Storm surge was also significant according to footage. 

Otis originated from a depression in Eastern Pacific (9°N, 96°E) on October 20th, 2023, and slowly moved northward approaching Mexico, intensifying to a Tropical Storm on October 23th, 2023, with an estimated minimum sea-level pressure of about 1000 hPa. In the second half of October 24th, 2023, a rapid intensification phase led to a drop of the sea-level pressure at the center of Otis, to approximately 923 hPa within 18 hours according to the National Hurricane Center (NHC) in USA. This central pressure fall can only be compared to Hurricane Patricia in 2015, which is considered to be the strongest hurricane in the Western Hemisphere so far. At 6 am local time on October 25th, 2023, Otis was about 100 km north-west from Acapulco rapidly weakening, but it was still producing wind gusts up to 177 km/h according to NHC. Prior to Otis there had been no Category 5 landfalls in the East Pacific since Hurricane Patricia struck as a Category 4 hurricane with wind speeds reaching 240 km/h.

In Acapulco, Mexico, Hurricane Otis, described as a "nightmare scenario," caused devastation as it made landfall. It struck Acapulco with winds reaching 265 km/h, stripping palm trees and damaging buildings, including a hospital and a shopping center. Landslides due to heavy rain blocked roads, leaving residents without power. Although Otis weakened to a Category 2 hurricane as it moved into Guerrero state, high winds, torrential rain, and flooding remained threats. Precise estimates on the extent of the damages by Otis are not available at this stage, because communication remains cut off, and approximately 500,000 people are without electricity. The damage caused by the hurricane appears to be substantial. The destructive winds swept palm trees and hotel lobbies, trapping thousands of tourists who had come for the All Saints' holiday. At present, authorities have no information on the well-being of the population due to the lack of communication. Mexican President Andrés Manuel Lopez Obrador acknowledged the communication blackout and the striking images of the devastation as daylight breaks. 

The Surface Pressure Anomalies reveal a narrow band of low pressure anomalies which reflect the track of hurricane Otis during the analysed days. Precipitation Data indicate that the city of Acapulco experienced extreme precipitation reaching up to 400mm/day total precipitation. At this date, we do not report results concerning Wind Speed Data, as the dataset shows wind speed values only up to 50 km/h, a largely underestimated value compared to the category of hurricane Otis at landfall.

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 tropical cyclones rapid intensification events has likely increased too. None of these changes can be explained by natural variability alone (medium confidence). Moreover, the proportion of intense tropical cyclones, average peak tropical cyclones wind speeds, and peak wind speeds of the most intense tropical cyclones will increase on the global scale with increasing global warming (high confidence). While at the regional scale there is limited evidence of current trends in observed wind speed and wind storms in Central America, in the Southern-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 hurricane Otis, we have low confidence in the robustness of our approach given the available climate data, as the event is largely unique in the data record.

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to the low pressure system associated with hurricane Otis have changed in the present (2000–2022) compared to what they would have looked like if they had occurred in the past (1979–2000) in the region [-104°E -96°E 12°N 18°N]. The Surface Pressure Changes show that cyclones similar to Otis do not attain stronger intensity in the present than in the past. Precipitation Changes show that similar events produce heavier (10-20 mm/day) precipitation in the present than in the past. Considering the affected urban areas, Acapulco and Chilpancingo see an increase in precipitation and wind in the present while the touristic city of Puerto Escondido in southern Mexico shows opposite trends.  We also find that Similar Past Events appear with about the same frequency as present events.

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation may have influenced the event. This suggests that the changes we see in the event compared to the past may be due to human driven climate change, with a contribution from natural variability. 

Conclusion

Based on the above, we conclude that cyclones similar to hurricane Otis have become 10-20 mm/day wetter in the present than in the past. We interpret hurricane Otis as a largely unique event for which natural climate variability played a role.

Contact Authors

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

Stavros Dafis, Data4Risk & National Observatory of Athens/meteo.gr 📨sdafis@noa.gr  🗣️Greek, English

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

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