Low confidence prevents ascribing Cyclone Belal changes in intensity to human-driven climate change

Press Summary (First published 2024/01/25)

• Low pressure systems making landfall in La Reunion, similar to Cyclone Belal, tend to produce similar precipitation and slightly less wind in the present as they have in the past.

• Cyclone Belal was a somewhat uncommon event (only 3 previous direct landfalls since 1980), with the last one on January 19, 1993.

• Natural climate variability likely was responsible for Cyclone Belal's characteristics.

Event Description

Cyclone Belal made landfall on La Reunion island on January 15th 2023.  Alert levels up to purple were activated, meaning that during the approach of the storm, all the population was confined, including emergency services. The cyclone also hit Mauritius, where heavy morning rains surprised residents, causing chaos and damage. Although Cyclone Belal did not cause the feared catastrophe, its impacts were significant, according to Préfect Jérôme Filippini. The island stayed on alert as damages in many areas started to be assessed. Cyclone Belal featured a highly asymmetrical distribution of rain and wind, with the northern and eastern parts of La Reunion receiving the strongest winds, and northwestern and inner parts receiving the highest rainfall accumulations. 

One fatality was reported so far – in Saint-Gilles, on the west side of the island. As Cyclone Belal headed towards Mauritius, heavy morning rains caught the island's nearly 1.3 million residents resulting in chaos and damage. The capital, Port-Louis, experienced flooded streets and stranded vehicles. The alert level was raised to 3 out of 4, with torrential rains and gusts records exceeding 150 km/h in many areas, with up to 200 km/h on mountain Maïdo according to Météo-France.  In Reunion Islands, 150,000 households were left without electricity, and over 130,000 people lacked access to running water for several days. Numerous trees obstructed streets in the west, and secondary roads were flooded. Violent winds partially submerged a large fishing boat in the Saint-Gilles marina. The mayor of Sainte-Marie in northern Reunion requested a natural disaster declaration due to severe damages in the area. 

The southwestern Indian Ocean (SWIO) is a common place for cyclones to form, but landfalls in La Réunion remain rare events. The SWIO cyclonic season spans November to April, and about 10 tropical storms form each season, among which 7 become tropical cyclones. There were, strictly speaking, 3 direct landfalls in La Réunion over the period 1980-2019 (Clotilda in 1987, Firinga in 1989 and Hondo in 2008). However, cyclones whose center passes within about 200 km have the potential to impact the islands. In particular, Cyclone Bejisa in 2014 impacted La Réunion, and Cyclone Freddy in 2023 impacted Mauritius. Over the period 1980-2019, 18 tropical cyclones centers were found within 200 km of La Réunion. 

The Surface Pressure Anomalies fell up to 17 hPa over la Reunion close to landfall, while surface temperature Anomalies had an increase up +2°C. Precipitation Data indicate that the majority of the area experienced extreme precipitation, reaching up to 600 mm/day in La Reunion. Windspeed Data indicate that the cyclone generated sustained winds up to 200 km/h over the landfall surrounding area. These estimates are based on MSWX data .

Climate and Data Background for the Analysis

According to the IPCC report (IPCC AR6 WGI FR - Page 205), anthropogenic climate change has increased observed precipitation, winds, and extreme sea level associated with some tropical cyclones, and there is evidence for an increase in the annual global proportion of Category 4 or 5 tropical cyclones in recent decades. However, the confidence level for these findings varies from medium to low. Regarding the economic damages caused by individual extreme events, the report (IPCC AR6 WGII FR - Page 1990) states that formal attribution to anthropogenic climate change has been limited, but climate change could account for a substantial fraction of the damages.  In summary, the IPCC report suggests that climate change could have an impact on cyclone intensity and associated hazards, but the confidence level for these findings varies. The IPCC report also highlights the potential economic damages caused by individual extreme events, and recent studies have used a variety of approaches for attributing these damages to climate change.

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 low confidence in the robustness of our approach given the available climate data, as there are few cyclones making landfall in a Reunion in the period considered.

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to the low pressure system leading to Cyclone Belal 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 [52°W 60°W 25°S 18°S]. In this particular case, considering the limited number of low pressure systems impacting La Reunion, namely we narrow down the search to the top 12 low pressure systems landfalling in La Reunion for the two periods.  The Surface Pressure Changes show that low pressure systems have slightly weakened (1-2 hPa) their intensity compared to the past except in areas distant from the cyclone eye. There are positive Temperature Changes but they are modest in magnitude (<1°C). Precipitation Changes show that similar events produce similar precipitation in the present than in the past. Windspeed Changes show that similar events produce weaker (4-10 km/h) winds in the Indian Ocean to the South-East of  la Reunion and Mauritius islands. We also observe that Similar Past Events occur more frequently in November and December in the present, whereas in the past, they were more prevalent in February, March, and April. Regarding the affected urban areas, Saint-Denis and Saint-Benoit in La Reunion show no significant changes in precipitation, while Port Louis in Mauritius experiences a decrease in rainfall of up to 20 mm/day (0% to 10%)  during present low pressure systems than in the past. All urban areas exhibit a modest decrease in wind speed, up to 10 km/h (0% to 5%) in present low pressure systems than in the past.

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation and the Pacific Multidecadal Oscillation, may have influenced this event. 

Conclusion

Based on the above, we conclude that low pressure systems similar to Belal and landfalling in La Reunion tend to produce similar precipitation and  slightly less wind in the present as they have in the past. We interpret Cyclone Belal as an event for which natural climate variability played a role in driving the trajectory.

Quotes & Contact Authors

Stella Bourdin,  IPSL-CEA, France  📨stella.bourdin@lsce.ipsl.fr  🗣️French, English  

Suzana J. Camargo, LDEO, Columbia University, USA;   suzana@ldeo.columbia.edu, English, Portuguese 

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