Heavy precipitation in New York September 2023 floods locally strengthened by human-driven climate change 

Contact Authors

• Tommaso Alberti, INGV, Italy  📨tommaso.alberti@ingv.it  🗣️Italian, English

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

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

Citation

• Alberti, T., Faranda, D., & Coppola, E. (2024). Heavy precipitation in New York September 2023 floods locally strengthened by human-driven climate change. ClimaMeter, Institut Pierre Simon Laplace, CNRS. https://doi.org/10.5281/zenodo.14164713

Press Summary (First published 2023/10/02, Updated 2024/11/14)

• Low pressure systems similar to that affecting New York are locally 0-3 mm/day wetter in the present than they have been in the past.

• New York floods was a somewhat uncommon event.

• We ascribe the high precipitation associated with New York floods to human driven climate change and natural climate variability likely played a minor role. However, large uncertainty remain in the attribution of this event because of the high variability of the precipitation analogues. 

Event Description

On September 29th, an exceptionally violent storm has struck the East Coast, particularly in the states of New York and New Jersey. The dynamics leading to these torrential rains are primarily linked to a convergence of cold air masses descending from Canada and very warm, humid air rising from the Gulf of Mexico. In New York City, where a state of emergency has been declared, up to 200 mm of rain has fallen mostly on the 29th. Brooklyn and Manhattan have been submerged under one and a half meters of water, with restaurants, shops, and especially subways all flooded. While no fatalities have been reported, scenes of chaos emerged as residents waded through knee-deep water in search for higher ground and drivers abandoned their vehicles on overwhelmed highways. The deluge wreaked havoc on the country's biggest transportation network, as heavy rains inundated certain subway stations in New York City and caused significant disruptions on various regional train lines. JFK Airport experienced about 200 mm of rain, making it the wettest day recorded there since 1948, while LaGuardia Airport was similarly submerged with nearly 120 mm of rainfall, leading to flight cancellations, according to reports. Even the Central Park Zoo found itself underwater, prompting a temporary escape by a sea lion from its overflowing enclosure.

The Surface Pressure Anomalies reveal a weather pattern characterized by a low-pressure system off the coast of North Carolina and a high-pressure system to the north. This unusual atmospheric setup led to a significant influx of rainfall directed toward the city. Precipitation Data indicate that the New York City area experienced rainfall amounts surpassing 100 mm per day, which, while substantial, was less than the measurements recorded at select weather stations where rainfall reached as high as 200 mm per day.

Climate and Data Background for the Analysis

The IPCC AR6 WG1 report states that the frequency and intensity of heavy precipitation events are projected to increase almost everywhere in the USA. In the Eastern North America region observed increase in extreme precipitation events have been already detected in the past and anthropogenically attributed with medium confidence. With global temperature increases due to human-induced climate change, New York may face more frequent and severe heatwaves, heavy precipitation leading to urban flooding, elevated storm surge risks, and chronic flooding from sea-level rise. The extent of these impacts depends on factors such as the global warming, local geography, and regional climate patterns. 

Our analysis approach rests on looking for weather situations similar to those of the event of interest observed in the past. For the New York floods we have medium-high confidence in the robustness of our approach given the available climate data, as the event is similar to other past events in the data record.

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to the low pressure systems leading to the recent New York floods 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 [-80°E -70°E 35°N 45°N]. The Surface Pressure Changes show that low pressure systems have not significantly changed their intensity compared to the past. Precipitation Changes show that similar events produce larger (0-3 mm/day) amounts of precipitation in New York area. However, they also tend to produce the same amount of precipitation amount in other areas, such as Southern part of the region. Considering the affected urban areas, New York, Brooklyn and Hoboken see an increase in precipitation in the present (0-3 mm/day). We also find that Similar Past Events have become more frequent in October, by doubling their frequency of occurrence, and less frequent in August and September. 

Finally, we find that sources of natural climate variability may have partly influenced the event. This suggests that the changes we see in the event compared to the past may be mostly due to human driven climate change, with a secondary contribution from natural variability.

Conclusion

Based on the above, we conclude that low pressure systems leading to New York floods similar to that observed in September 2023 are locally 0-3 mm/day wetter than they would have been. We interpret the New York floods as an event whose characteristics can mostly be ascribed to human driven climate change. However, large uncertainty remains in the attribution of this event because of the high variability of the precipitation analogues.