NOAA

NOAA/Wikipedia

NOAA

NOAA/Wikipedia

NOAA

NOAA/Wikipedia

NOAA

NOAA/Wikipedia

NOAA

NOAA/Wikipedia

The United States was already on edge about the Atlantic tropics when the month of September began, as Hurricane Harvey had just slogged through Texas and delivered devastating inland flooding to a large stretch of the state, including the fourth largest city in the country. Harvey may end up ranking as the costliest hurricane on US record, although damages haven't been officially determined.

But this Category 4 hurricane was but a prelude for what was to come in September. Five hurricanes—Irma, Jose, Katia, Lee, and Maria—would form. Four would become major hurricanes. Two of the storms, Irma and Maria, would reach Category 5 status and bring widespread devastation, especially in the Caribbean Islands, and to a lesser extent in the mainland United States.

Now, finally, the Atlantic tropics have calmed down. No named storms remain, and there's only a slight chance of another storm forming within the next five days. And so ends the busiest month on record in the Atlantic Ocean.

Super September

September is typically the busiest month for hurricanes in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. This is because the conveyor belt of low-pressure systems moving off the western coast of Africa into the Atlantic reaches its zenith during this time frame, the tropical ocean waters are near peak warmth, and wind shear is typically fairly low across the basin.

So when September 2017 set a "calendar month" record for overall Atlantic activity, it set a record for any month in any given year. (As ever, when it comes to hurricane records, we must have caveats. Although data goes back to the 1850s, and reanalysis projects have attempted to build confidence in earlier records, only with the onset of satellites in the 1960s do we begin to have a complete record of Atlantic hurricane activity.)

With that said, according to Phil Klotzbach of Colorado State University, this September set the following significant records: cumulative days of named storms (53.5 days, besting the mark of 52.25 days in September 2004); hurricane days (40.25 days, besting 34.5 in September, 1926), and major hurricane days (18, besting 17.25 days in September, 1961). Finally, in terms of the overall duration and intensity of storms, as measured by "accumulated cyclone energy," this September's total of 175 units smashed the previous record of 155 in September 2014.

So, climate?

In short, this month's storm activity was unprecedented in recent years. But given the dataset problems described above, it's difficult to say how unprecedented, and for how long. Climate change is thought to make the strongest hurricanes more intense, but there is not yet data or climate modeling to suggest that there will be more tropical storms and hurricanes in a warmer world.

With that said, while the Atlantic Ocean has raged with tropical activity during the last month, much of the rest of the Northern Hemisphere tropics have been relatively quiet. Particularly the Northwest Pacific Ocean basin, which typically is more than twice as busy as the Atlantic during this time of year, has been quiet this summer and early fall (see charts embedded below).

Phil Klotzbach

Phil Klotzbach

Phil Klotzbach

Phil Klotzbach

Phil Klotzbach

Ryan Maue

Ryan Maue

Across the Northern Hemisphere this year, then, tropical activity has more or less evened out. Based on the accumulated cyclone energy statistics kept by Colorado State, the "normal" value for the Northern Hemisphere through October 1 is 403.7 units. This year, it has measured 411.3, a statistically insignificant bump of 2 percent.

Listing image by NOAA