The Atlantic hurricane season has only just recently passed its historical peak (September 10) and there is a lot more time left. But this year is already going into the record books for many reasons. Here are just a few.
First, for the Atlantic Basin …
This season marked the first time since 2010—when Igor, Julia, and Karl were classified as hurricanes—that there have been three hurricanes on the map at the same time: Irma, Jose, and Katia simultaneously occurred in the Atlantic last week. Other years since the beginning of the satellite era are 1967, 1980, 1995, and 1998.
More significantly, it is the first time ever that there have been two Category 4 or higher hurricanes at the same time (Irma and Jose). In 1999, there were two Category 4s on the same day—Floyd and Gert—but not at the same time; Floyd lost that status early in the day while Gert was just acquiring it. It is also the first time ever that there have been two hurricanes with 150+ mph winds (Irma and Jose) at the same time.
And Some Firsts for Irma …
For an Atlantic storm, Irma has the second-highest ACE (accumulated cyclone energy) ever at 67.5; that is close to Ivan’s record (70.4) in 2004, but Irma acquired it in about half the time. To put that number in perspective, Harvey generated an ACE of 11.4. And, Irma, all by itself makes the Atlantic Basin an above-average year in terms of ACE.
Related to that is that Irma is also the most intense (from a sustained wind perspective) tropical cyclone ever in the Atlantic Basin, outside the Caribbean and Gulf of Mexico. Its 185 mph winds for 37 consecutive hours also set a record worldwide for the longest at that intensity, surpassing super typhoon Haiyan’s record of 24 hours.
It is the first time ever that the U.S. has been hit by two major (Category 3 or higher) hurricanes in about a two-week period (the timespan was 15 days: Harvey struck on August 26 and Irma on September 10). This record may be particularly noteworthy considering that the U.S. had not been hit by a major hurricane landfall in 3,937 days (i.e. almost 11 years) before Harvey; that, by the way, was another record.
Why Has the Atlantic Been So Active Since August 1?
The Atlantic Multi-decadal Oscillation (AMO) is still positive. In fact, despite projections over the last 2–3 years that the AMO would become negative, it is still quite positive (0.307 for July, 2017). For perspective, since 1995, when the AMO flipped to its positive phase, July 2017 has the eighth-highest July value. The highest for July was in 2005 with a value of 0.451.
High sea-surface temperature anomalies are consistent with the AMO—in fact they exceed +2°C off the coast of West Africa. This feature by itself could be contributing to storms forming in the first place. Additionally, there is an elongated band of +1°C anomaly extending across the Atlantic between 10° and 20°N.
The El Niño-Southern Oscillation (ENSO) is neutral (the index is actually slightly negative). Early season projections were for an El Niño (an index value greater than 0.5) to develop, but successive projections downplayed the possibility. After peaking at an ENSO 3.4 index of 0.4 early in the season, the index has been slowly decreasing. The current June-July-August average is -0.1. The corresponding low shear provides an environment conducive for storm intensification in the MDR (main development region).
The North Atlantic Oscillation (NAO) has been very negative since the beginning of August. While a negative NAO index is conducive for recurving tropical cyclones paths back into the Atlantic and a positive one to steer them toward the U.S. (and into the Gulf), this by itself does not seem to be playing a role.
However, sea level pressure (SLP) anomalies are slightly higher over the western tropical Atlantic. This implies that the subtropical high is stronger and/or extends farther westward. Coupled with all the other factors, this East-West elongated steering flow could be taking the strong storms that are forming off the western coast of Africa and steering them more westward into an area where sea surface temperatures (SSTs) are climatologically high anyway; there are pockets of +30°C SSTs around Cuba, western Florida, and in the western Gulf just south of Texas. (Ocean temperatures peak in late September.)
An index called the Maximum Potential Intensity (MPI) indicates the strongest a storm can be as a result of several simultaneous conditions. At the time Irma was approaching, the MPI for western Florida yielded a storm with a minimum central pressure in the mid-880s (millibars)!
Many of the above conditions can—and probably will—persist through the remainder of the 2017 Atlantic hurricane season. Given that the second half started just a few days ago, more records may yet be broken.