Back in April and May 2024, universities and weather agencies alike predicted an active hurricane season in the Atlantic. Warm ocean waters, they argued, provided ideal conditions for a surge in storms, with predictions ranging from 15 to 25 named storms. However, by mid-September, the usual peak of hurricane season, only seven storms had been named. Where were the “missing” hurricanes?
To understand this unexpected lull, we must look beyond the Atlantic. The key lies in an unlikely culprit: unprecedented rainfall in the Sahara Desert.
The 2024 hurricane season started with a bang. Hurricane Beryl, making landfall on the island of Carriacou in Grenada on July 1st, became the earliest category 5 storm on record. But this early intensity was quickly followed by a lull. A surge of Saharan dust, filling the air over the tropical Atlantic, created an environment too dry to sustain the moist clouds necessary for hurricane formation. While July is generally a less active part of the season, this dry spell was unusual, especially compared to the past few years, when only one storm was named in July from 2021 to 2023.
August saw the arrival of Hurricanes Debby and Ernesto. However, a significant silence fell over the Atlantic between August 13th and September 3rd, with no named storms. This level of inactivity had only occurred once before on those dates, in 1968.
The stark contrast to 2023, a year marked by record-breaking ocean temperatures and 20 named storms (the fourth most active hurricane season on record), was striking. This year, ocean temperatures are as hot as predicted, yet other factors, like the Saharan rainfall, were not anticipated.
The connection between the Sahara and the Atlantic hurricanes can be traced to the African easterly waves. Between June and September, west Africa experiences a monsoon, with moist air from the Gulf of Guinea travelling northward, bringing rain to the Sahel region. This creates a temperature contrast between the wet Sahel and the dry desert, which allows a high-altitude atmospheric jet to form. Smaller “waves” spin off from this jet, known as African easterly waves, and these are associated with large thunderstorms and low pressures.
These easterly waves, which originate in the east and move westward over Africa, can travel off the coast and create areas of low pressure and rotating air in the Atlantic, potentially developing into hurricanes. It’s estimated that around 60% of major Atlantic storms can be traced back to these waves.
However, this year, the unusually warm sea allowed moist air to travel further north over west Africa, leading to record-breaking rainfall in the Sahara. Northern Chad witnessed weeks of torrential rain, while Morocco and Algeria suffered massive floods, with some areas receiving a year’s worth of rain in just two days.
This extreme rainfall pushed the easterly waves further north, emerging into the ocean via Mauritania or Western Sahara instead of Senegal or the Gambia. The cooler ocean waters in this area, combined with the waves bringing in cooler, drier air from the north, reduced the energy needed for wave development into storms. Essentially, the seeds for these storms were planted in the wrong soil, lacking the warmth and moisture required to blossom.
Further contributing to the quiet season were other factors like the Madden-Julian Oscillation, a global weather pattern, which has been in a phase unfavorable for tropical cyclone development. Additionally, the upper atmosphere has been too warm for storm formation, with strong winds hindering the development of potential tropical cyclones.
State-of-the-art climate models suggest that a warming climate may result in a wetter and further north-shifted west African monsoon, potentially leading to similar quiet hurricane seasons in the future. However, rainfall projections in Africa remain uncertain, emphasizing the need for further research to understand these changes, both for extreme weather forecasting in the region and for understanding the downstream impact on hurricane activity.
While the typical season for easterly waves is nearing its end, there’s still a chance for storms to form from disturbances in the Caribbean. The sea surface is expected to continue heating up, providing necessary moisture and heat. Researchers at Colorado State University, known for their accurate hurricane forecasts, predict a 50% chance of a return to normal hurricane activity in the next two weeks.
The 2024 hurricane season may be unusually quiet, but it’s not over yet. With high sea surface temperatures, any developing tropical cyclones could rapidly intensify into high-impact storms. While we’ve enjoyed a temporary reprieve, Hurricane Beryl serves as a stark reminder that even during a quiet season, hurricanes can still cause catastrophic damage.
