At the end of a summer of endless heat waves, record drought, and crazy flood events, be grateful for one mercy: the Atlantic hurricane season is off to a remarkably slow start.
Will we have a tropical-cyclone-free Atlantic in August?
So far in the Atlantic, there have been three named storms and zero hurricanes, with an accumulated cyclone energy (ACE) index just 11% of average for the date. By August 25th, the hurricane season is typically about 20% complete, as measured by ACE, so the start to the 2022 season is the slowest since 1988. And with just two areas of interest on Thursday’s National Hurricane Center Tropical Weather Outlook (see below) – both with only minor chances of development – 2022 has a chance of going 0-for-August for named storms.
From 1950 onward, the only two years with no named Atlantic storm in August (or even tropical depressions, for that matter) are 1961 and 1997. What’s interesting is that 1961 turned out to be a very back-loaded year, with three Cat 4s and two Cat 5s after September 1, so no reason to let our guard down on 2022 quite yet.
It’s also interesting that 1961 had slightly cooler-than-average conditions in the eastern tropical Pacific, though not cool enough to rank as a La Niña event (the Oceanic Niño Index, ONI, for Aug-Sep-Oct was -3.0).
As for 1997, one of the three strongest El Niño events in the post-1950 record was in full swing by hurricane season of 1997, with an ONI of +2.1 in Aug-Sept-Oct. Given the suppressive effects of El Niño on Atlantic hurricane formation, it’s not too shocking that 1997 was a quiet year. However, we now have a strengthening La Niña event in place (NOAA reported on Monday that the most recent weekly ONI was -1.2), so the tranquil Atlantic of August 2022 is a more puzzling turn of events.
A tale of shear and dry air
The lack of activity is certainly not the result of a lack of warm ocean waters. The hurricane main development region (MDR) in the tropical Atlantic is currently about 0.2 degree Celsius above average; the Caribbean and Gulf of Mexico are both about 0.4 degree above average.
The credit for the slow start to the season belongs instead to dry air and high wind shear. The central and eastern Atlantic have had an active series of tropical upper tropospheric troughs (TUTTs), which have created dry, stable air over much of the region where August tropical cyclones usually brew. The year so far also has featured an active summer for dry air coming off the coast of Africa. The two effects have led to a 10-15% drop in relative humidity at mid-levels of the atmosphere in the central tropical Atlantic, compared to average, during the first 22 days of August (Figure 1). It is uncertain why we’ve been seeing so much dry, stable air in the central tropical Atlantic this month.
The Caribbean has largely avoided dry air so far in August. However, the Caribbean has had unusually high levels of wind shear so far this month (Figure 2). This was unexpected given the current La Niña event underway in the eastern Pacific, as the atmospheric circulation during La Niña typically favors lower-than-average wind shear in the Caribbean. It is uncertain why we’ve been seeing higher Caribbean shear. The latest forecast from the GFS model calls for lower Caribbean shear during the last five days of August, then increasing again during the first five days of September.
Tropical wave moving through the Lesser Antilles
A tropical wave was moving westward at 15 mph through the Windward Islands of the Lesser Antilles on Thursday morning. This wave had light wind shear of 5-10 knots and warm sea surface temperatures, near 29 degrees Celsius (84°F). However, dry air to the wave’s north and west were interfering with development, and satellite images showed only a small area of heavy thunderstorms with little rotation. The eastern Caribbean is typically an area hostile to hurricane development, because of the accelerating surface trade wind wind flow that causes dry, sinking air aloft.
By Tuesday, August 30, when the wave will be in the central Caribbean south of Jamaica, the system may find more favorable conditions for development. A number of the members of the latest GFS and European ensemble forecast show the wave developing into a tropical depression or tropical storm by Wednesday, August 31, when it will be in the western Caribbean, near Mexico’s Yucatan Peninsula. In its 2 p.m. EDT Thursday Tropical Weather Outlook, the National Hurricane Center gave the wave 2-day and 5-day odds of development of 0% and 20%, respectively.
Tropical wave near the coast of Africa
A tropical wave recently emerged from the coast of Africa and was headed westward at 15-20 mph on Thursday morning. This wave had moderate wind shear of 10-20 knots and sea surface temperatures a bit cool for development, near 27 degrees Celsius (81°F). Dry air to the wave’s west was interfering with development, and satellite images showed only a small area of heavy thunderstorms associated with the wave. However, the wave had plenty of spin.
By August 30, when the wave will be in the central tropical Atlantic, the system may find more favorable conditions for development. A number of members of the GFS and European ensemble show it developing into a tropical depression or tropical storm by Wednesday, August 31. In its 2 p.m. EDT Thursday Tropical Weather Outlook, the National Hurricane Center gave the wave 2-day and 5-day odds of development of 10% and 20%, respectively. The earliest this wave might affect the Lesser Antilles Islands would be Friday, September 2, according to the 0Z Thursday run of the European model ensemble.
Website visitors can comment on “Eye on the Storm” posts (see comments policy below). Sign up to receive notices of new postings here.