The ocean is above average warm, yet the Atlantic hurricane season has so far been unusually quiet
This week marked the notional peak of the Atlantic hurricane season in a climatological sense – September 10 is, on long-term average, the day when hurricanes occur most frequently. Yet this year, not only on September 10 but also for several days prior to that date, no hurricane or tropical storm was present in the Atlantic. The last tropical system was tropical storm Ferdinand, which formed on August 23 (and dissipated five days later). In other words, by Friday, September 12, it will have been 19 days since the last storm formed. While longer stretches without tropical activity do occur from time to time, such a lull during the peak of the season is unusual. Last year, however, a similar situation occurred from August 13 to September 8, the first such period since 1968 to last at least 27 days. Later, though, two exceptionally destructive hurricanes, Helene and Milton, developed.
So far this year, 6 tropical storms have formed, with only one reaching hurricane strength. That was Erin, which intensified to the fifth and strongest category, but fortunately missed land. On average, the third hurricane forms by September 7 and the fourth by September 16.
Both this year’s and last year’s “hurricane calm” are surprising because in both cases sea surface temperatures were, or still are, unusually high, even record-breaking. Warm ocean waters are one of the key conditions supporting hurricane development and are also a major parameter used in seasonal outlooks of hurricane activity – which, incidentally, had predicted an above-average season this year. However, the course of both this year and last suggests that this parameter may be somewhat overvalued in the context of long-term forecasting. In fact, during June and July, sea surface temperatures in the main hurricane development region of the central tropical Atlantic were near or even below average, though they have been above average since late July.
Another factor suppressing tropical cyclone development was stronger wind shear, especially over the Caribbean Sea. In addition, during June and July, surface air pressure over the tropical Atlantic, including its western part, reached record-high levels – another factor acting against hurricane formation. Finally, dry air played a role: initially from Africa (aided by a dry spring and first half of summer in eastern Africa), and in recent weeks more from the peripheral circulation of the Azores High. This dry air often penetrated far west over the Atlantic, sometimes reaching the Caribbean, and reduced mid-tropospheric humidity. That complicates the formation of deep convective storm clouds, which are essential for releasing latent heat that fuels hurricane development. Also important is the dynamics of the upper troposphere, where this year anticyclonic ridges of higher pressure have frequently appeared, suppressing upward motion. Incidentally, another link can be found in the so-called Hadley circulation – a system of air flow with rising motion near the equator and descending motion in the subtropics. This year, due to higher temperatures at mid-latitudes, the Hadley circulation has expanded northward, ultimately leading to less favorable conditions for tropical instability. Weaker instability means weaker upward motion and a more difficult formation of storm clouds.
About a month and a half remain in the official Atlantic hurricane season. According to current models, atmospheric conditions should gradually become more favorable for the development of tropical cyclones. During the second half of next week, a fairly deep tropical cyclone is expected to form in the central Atlantic and move northwestward. Models also suggest the potential for significant intensification into a strong hurricane, though given the forecast range, this expectation should be taken with caution. Similarly, a tropical storm or hurricane may form near Mexico’s Yucatán Peninsula – but uncertainty there is also high. As past years have shown, the closing stages of the hurricane season can still bring surprises and powerful storms.
