September: What Makes It the Heart of Hurricane Season?

The Atlantic hurricane season begins on June 1, but an equally important date to mark on your calendar is September 1—the start of the most active month for hurricane activity. Since official record keeping of hurricanes began in 1950, over 60% of all Atlantic named storms have developed in the months of August or September. 

What is it about late August and September that produces a flurry of tropical cyclones within the Atlantic Ocean?

Tropical Disturbances A-Plenty

One of the reasons why cyclone activity climbs is the hyperactive African Easterly Jet (AEJ). The AEJ is an east-to-west oriented wind (much like the jet stream that flows across the US) that flows across Africa into the tropical Atlantic Ocean. It exists thanks to the contrast in temperature between the deep, hot air over the Sahara Desert and the cooler, more humid air over the more forested areas of central Africa and the Gulf of Guinea. (As you may remember, temperature contrasts drive weather, including the flow of wind.)    

The AEJ blows over the tropical Atlantic as a fast flowing stream of air. Since the flow near the AEJ goes faster than that further away in the surrounding air, what happens is that eddies begin to develop due to these differences in speed. When this happens, you get what's called a "tropical wave"—an unstable kink or wave in the main AEJ flow pattern.

(On satellite, these disturbances appear as clusters of thunderstorms and convection originating over North Africa and traveling westward into the tropical Atlantic.) By providing the initial energy and spin needed for a hurricane to develop, tropical waves act like "seedlings" of tropical cyclones.

The more seedlings the AEJ generates, the more chances there are for tropical cyclone development. 

Of course, having a storm seedling is only half of the recipe. A wave won't automatically grow into a tropical storm or hurricane, unless several of the atmosphere's other conditions, including sea surface temperatures (SSTs), are favorable. 

The Atlantic's SSTs are Still in Summer Mode  

While temperatures may be cooling off for us land dwellers as fall begins, SSTs in the tropics are just reaching their peak. Because water has a higher heat capacity than land, it heats more slowly, which means the waters that have spent all summer absorbing the sun's warmth are just reaching their maximum warmth at summer's end.   

Sea surface temperatures must be 82°F or warmer for a tropical cyclone to form and thrive, and in September, temperatures across the tropical Atlantic average 86°F, nearly 5 degrees warmer than this threshold. 

Significance of September 10-11 

When you look at hurricane climatology (the long-term averages of hurricane and tropical storm activity in the Atlantic basin) you'll see a sharp increase in the number of named storms forming between late August into September. This increase typically continues until September 10-11, which is thought of as the season's peak.

"Peak" doesn't necessarily mean multiple storms will form at once or be active across the Atlantic on this particular date, it simply highlights when the bulk of named storms will have occurred by. After this peak date, you can typically expect storm activity to gently decline, with another five named storms, three hurricanes, and one major hurricane occurring on average by the season's November 30 end.

September Holds Record for Most Atlantic Hurricanes at Once

Although the word "peak" doesn't necessarily point to when the greatest number of cyclones will happen at once, there are several occasions when it did. 

The record for most hurricanes to ever occur at the same time in the Atlantic basin occurred in September 1998, when as many as four hurricanes—Georges, Ivan, Jeanne, and Karl—simultaneously spun across the Atlantic.

As for the most tropical cyclones (storms and hurricanes) to ever exist at one time, a maximum of five occurred on September 10-12, 1971.

Hurricane Origin Locations Peak, Too  

Cyclone activity not only heats up in September, but the places where you can expect cyclones to spin up do too. In late summer and early fall, there's generally an increased chance storms will develop in the Caribbean Sea, along the Eastern Atlantic Seaboard, and in the Gulf of Mexico. 

By November, cold fronts and increasing wind shear—two disrupters to tropical development—penetrate into the Gulf of Mexico, Atlantic, and sometimes into the western Caribbean Sea as well, which spells the end of the peak August-October period. 

Resources & Links:

NOAA National Hurricane Center Tropical Cyclone Climatology

NHC Reynolds SST Analysis