After an unusually quiet few weeks for what was long expected to be a wild Atlantic hurricane season, things are starting to change. The last named system in the Atlantic, Hurricane Ernesto, became extratropical as it moved rapidly east of Newfoundland on August 20, and it is unlikely that we will have the next named storm before Labor Day weekend. However, long-range models are increasingly confident that the deep tropical Atlantic will soon produce a system worth tracking, and there is good reason to believe that a busy September and/or October is just around the corner.
In the Tropical Weather Outlook released at 2 p.m. ET on Tuesday, August 27, the National Hurricane Center forecasts a 20% chance of hurricane development in the western tropical Atlantic over the next seven days, and a 20% chance of hurricane development over the next two days. The possibility is close to zero. There are no worrisome tropical waves bubbling up yet, but models are increasingly suggesting a disturbance in the Intertropical Convergence Zone, a semi-permanent band of low pressure that brings together air and north-south oscillations of showers and thunderstorms ( Convection currents) surround the equator and currently extend from near the coast of West Africa to the Caribbean Islands. If this disturbance does form, it will likely move west-northwest toward the Lesser Antilles around the middle of next week.
Barring any surprises, this means we likely won't have any named storms for the full two weeks from August 20th to September 3rd. Given that we're just a few weeks away from the traditional peak of the Atlantic season on September 10, that's an impressive lull in any year—let alone one that's expected to be hyperactive. In fact, this two-week period has contained at least one Atlantic tropical or subtropical system every year since 1956, according to hurricane expert Michael Lowry.
Why does the Atlantic seem — or actually — so calm lately?
The claim that the 2024 Atlantic season has not lived up to expectations for a hyperactive season that seasonal forecasters set months ago is both true and fictional.
Indeed, there have been only five naming systems to date, compared with an average of six at this time. However, by most other measures, the Atlantic season as of August 27 has actually busier higher than the 1991-2020 average. This includes accumulated cyclone energy (55.1 units vs. average of 28.4), named storm days (24.5 vs. 20.3), hurricanes (3 vs. 2.1), hurricane days (12.0 vs. 5.5), and major hurricane days (4.5 vs. 5.5). ). However, this advantage was entirely due to the record-setting and prolonged duration of Category 5 Hurricane Beryl in late June and early July. Without Beryl, this season will be an average if not below average season. The relatively low number of named storms is a particularly alarming change compared to trends in recent seasons, which have seen a series of weaker named storms in addition to the really serious hurricane threat.
Storm watchers on social media and leading hurricane researchers on email lists are constantly speculating and wondering why the Atlantic has been behaving so poorly in recent days. Here are some of the factors being discussed:
The African swell is farther north than usual. Several systems from West Africa, typically the seedlings of Cape Verde hurricanes, moved off the coast at higher latitudes than usual. As a result, they inhaled large amounts of Saharan dust and dry air, stunting their development, and the waters they sailed in were cooler than usual. These disturbances are further suppressed by unusually strong mid-level northeasterly winds in the eastern Atlantic, associated with the persistent positive North Atlantic Oscillation.
The main development area is too stable. Sea surface temperatures remain at or near record highs across much of the tropical Atlantic Major Developed Region (MDR) from Africa to the Caribbean. However, despite the warm ocean and surface atmosphere, this season's mile-high temperatures have been even more unusually warm. The showers and thunderstorms that trigger hurricane heat engines rely on instabilities: relatively warm, moist air in the lower levels and relatively cold air in the upper levels. Anomalous warmth at levels such as 200 millibars (about 39,000 feet) appears to outweigh the effects of surface warmth, resulting in the most stable MDR conditions in decades.
The east wind shear is too strong. To give an incipient tropical cyclone the best chance of vertical growth, low-level and upper-level winds should be as similar as possible. At times, upper-level winds are westerly, which when combined with the typical easterly trade winds near the surface can produce adverse vertical wind shear. In recent weeks, a more unusual development hurdle has emerged: the upper east stream that crosses the MDR has actually stronger Stronger than the easterly trade winds—in fact, it has a counterproductive effect on hurricane development.
The Atlantic’s version of anti-Nina is taking shape. Just as the equatorial Pacific triggers El Niño and La Niña events, the equatorial Atlantic has its own quasi-periodic oscillations that transport warm or cool water to the eastern end of the basin, causing so-called Atlantic Niño and Atlantic Niña events, concentrated south of the Sahara between Africa and South America. Atlantic hurricanes are not as famous or globally influential as Pacific hurricanes, but they can influence the short-term evolution of the Atlantic hurricane season. This year, the eastern equatorial Atlantic Ocean shifted from a Niña (warm) climate to a Niño (cool) climate at the fastest rate monitored in more than 40 years. This kind of fluctuation typically strengthens the southeasterly trade winds across the eastern MDR and helps trigger hurricanes, but it hasn't done that yet—at least not yet, although that could change. For a detailed explanation, see Franz Philip Tuchen (CIMAS/NOAA/University of Miami) recent article on Climate.gov.
Experts agree that September will bring more typical atmospheric conditions to the region, ushering in the busy peak month of the Atlantic hurricane season.
Typhoon “Shanshan” strengthens to Category 3 When grinding comes to a stop
Meanwhile, in the Pacific, residents of Japan's southernmost main island of Kyushu continue to be on high alert for Typhoon Shanshan, which is expected to arrive on Thursday. At 11 a.m. ET on Tuesday, the Joint Typhoon Warning Center (JTWC) said that “Shanshan” had strengthened into a high-end Category 3 storm with winds of 125 mph and a central pressure of 937 mb, located 313 miles south of Sasebo, Japan. , moving northwest at only 1 mile per hour.
Satellite loops show a solid area of severe thunderstorms in Shanshan surrounded by a large eye that has increased to 30 miles in diameter due to eyewall replacement cycles. Shanshan's intensity may have peaked, as its slow movement will allow the typhoon to rise into cooler waters, causing the typhoon to weaken. The Joint Typhoon Warning Center and the Japan Meteorological Agency predict that “Shanshan” will make landfall in Kyushu around 12:00 on Thursday. However, Shanshan is in an area with weaker steering currents, and the Joint Typhoon Warning Center warned that their track and intensity forecasts are less reliable. Worryingly, Tuesday's 12Z run of the GFS model predicts Shanshan will linger in southern Japan and adjacent waters over the next 12 days, creating multiple landfills.
We help millions of people understand climate change and what to do about it. Help us reach more people like you.
