Why the hurricane season was so active?

 

A little overview of the 2023 Atlantic hurricane season.

The 2023 Atlantic hurricane season, officially ended on Nov. 30. In spite of it being a strong El Nino; The season was very active. In fact, the 2023 Atlantic hurricane season ranks 4th for most-named storms in a year. Looking at the numbers, there were 20 named storms (including an unnamed subtropical storm that formed on January 16), seven of which were hurricanes, three of them became major hurricanes, and the season had an overall Accumulated Cyclone Energy (ACE) of 146. In total there were 3 landfalling tropical cyclones on the U.S. main land. The numbers show 2023 saw the most named storms in the Atlantic basin of any El Nino influenced year in the official record.

For comparison, based on data from 1991 to 2020, an average season typically sees 14 named storms, seven hurricanes, and three major hurricanes.

2023’s 20 named storms, ended up a tie with 1933 which also saw 20 named storms. The three years that surpassed 2023 numbers were, 2021 which had 21, 2005 which saw 28, and 2020 which saw 30.

I released part three of the hurricane outlook back in May, my final numbers were 14-20 named storms, 8-12 hurricanes, 3-5 major hurricanes. The CONUS would see 2-4 landfalls with one of them possibility being major. The ACE would be 100-150.

So overall my outlook verified very well.

Looking at the storm tracks map

We can see the Atlantic itself was exceptionally active, while the Caribbean and Gulf of Mexico saw hardly any activity.  

 Image credit WGNO in New Orleans

 Landfalling Systems:

Three Atlantic storms made landfall on the U.S., Major Hurricane Idalia and two U.S. landfalling Tropical Storms Harold and Ophelia.

The first U.S. landfall of the 2023 Atlantic hurricane season came on August 22nd as Tropical Storm Harold moved ashore Padre Island in South Texas with 50 mph winds, causing minor damage and localized flooding across the southwestern U.S.

The second landfalling system Idalia was the only hurricane to hit the US this year. She started as an area of disturbed weather over the eastern Pacific. She then moved across Central America into the northwestern Caribbean, where she slowly organized, becoming a tropical depression on the 26^th of August; she became a tropical storm on the 27^th.   By early Monday morning on the 28th, Idalia began her northward turn and after crossing the western tip of Cuba Monday night, Idalia would rapidly intensify from a tropical storm into a major hurricane on Tuesday the 29^th over the Gulf of Mexico. She obtained major category 4 hurricane status with sustained winds of 130 mph. Then right before landfall, she underwent an eyewall replacement cycle which allowed her to weaken slightly to a 125-mph category 3 hurricane. She made landfall in Florida’s Big Bend, near Keaton Beach, Fla bringing storm surge flooding up to 7 to 12 feet, on Aug. 30, 2023 at 7:45 a.m.

The third was Tropical Storm Ophelia which made a landfall at Emerald Isle, North Carolina on September 23. With maximum sustained winds of 70 mph, Ophelia’s widespread heavy rainfall, significant river and storm surge flooding across eastern North Carolina. She also brought heavy rainfall to such cities as New York City, Washington, D.C. and Philadelphia.

While Hurricane Lee didn’t make a U.S. landfall it was a close call, Lee formed in early September, soon after he rapidly intensified into a Category 5 hurricane. After transitioning from a hurricane to a post-tropical cyclone, Lee made his landfall in western Nova Scotia, Canada, on September 16. Lee was a large storm with Hurricane-force winds were extending up to 140 miles from the center of Lee and tropical-storm-force winds were extending up to 390 miles from the center. Due to his size, Lee’s swells created high surf and rip currents along the entire U.S. Atlantic coast. Lee’s strong winds, with gusts to over hurricane strength, caused extensive power outages in Maine and parts of Canada.

Why the hurricane season was so active?

 When the tropical eastern-central Pacific Ocean is unusually warm, El Niño can form. During El Niño events, the warm upper-ocean temperatures change the vertical and east-west atmospheric circulation in the tropics. That initiates a teleconnection by affecting the east-west winds in the upper atmosphere throughout the tropics, ultimately resulting in stronger vertical wind shear in the Atlantic basin. That wind shear can tamp down hurricanes.

Typically, during El Nino, the warm upper-ocean temperatures in the equatorial Pacific change the vertical and east-west atmospheric circulation in the tropics, which customarily means sea surface temperatures in the Atlantic Basin tend to be cooler than average. Also, characteristically El Nino features increased atmospheric stability and vertical wind shear in the Atlantic; historically all of these created conditions that hamper storm development in the Atlantic. But as I showed in the tropical activity storm tracks, 2023 has acted anything but a typical year with increased storm activity in the main north Atlantic.

In general, hurricanes are more likely to form and intensify when a tropical low-pressure system encounters an environment with warm upper-ocean temperatures, plenty of moisture in the atmosphere, instability and weak vertical wind shear.

The East Pacific also was very active this year, all this activity created conditions that favored sinking air over the Gulf of Mexico (GOM) and Caribbean. This was one reason the Caribbean and Gulf we’re extremely quiet during the season.

In the Atlantic, there were three major factors that worked together that heightened the environment needed to increase the number of storms. Atlantic sea-surface temperatures, weak wind shear, and the location and strength of the Azores/Bermuda high.  All three of these things are so intertwined that they really can’t be described by themselves.  

1 Warm Atlantic sea-surface temperatures:

Atlantic, sea temperatures this summer were unusually high with record high temperatures which counterbalanced the strong El Nino. This made it easier for storms to form and develop.  The Atlantic went through a series of marine heatwaves. 

The pattern over the US and Atlantic Basin, played a big role.

We had a large heat dome over Central U.S along with a persistent blocking pattern over the Labrador Sea, Greenland and Iceland. This was the reason, for the cooler and wet spring into summer.  This high latitude blocking means that there is higher than average pressure in that region. This also typically leads to lower-than-average pressure near Bermuda and the Azores, which was the case this year.

2 Azores/Bermuda high:

The Azores/Bermuda sub-tropical high is a pressure system that sits over the Atlantic in the summer. It acts as a barrier to hurricanes, and can shift in size and location. Once storms clear the high, they head north.

Over the hurricane season this subtropical high was weaker and farther east than average. Both of these things had major impacts on tropical storm development.

A weaker Bermuda/Azores high, means the easterly trade winds are weaker. Because the trades are weaker, there is much less upwelling of cooler water from below the surface. As a result, the water keeps warming. This occurred in a big way in the Main Development Region" of the tropical Atlantic this season. The tropical Atlantic warmed at a faster rate than ever before. With the trade winds much weaker than average there was no upwelling so the water stayed hot much longer than is typical.

3 Wind Shear:

Wind shear over the open Atlantic and Main Development Region in 2023 was much weaker than is typical during an El Nino.  On the other hand, the shear was a little stronger over the Gulf of Mexico, Caribbean and along the East Coast.

wind shear refers to the variation in wind speed and direction with height in the atmosphere. For the 2023 hurricane season wind shear was generally strong near the East Coast and generally weaker out over the open Atlantic.

For a tropical system to develop and strengthen, wind shear needs to be low. The higher the wind shear, the more difficult it is for a tropical system to organize and strengthen. Generally speaking, you want wind shear to be less than 20 kt for a system to develop. Higher than that the wind shear disrupts the structure of tropical storms. For rapid intensification to occur you want the wind shear to be 10 kt or less.

The weaker easterly winds kept Saharan Dust to a minimum during the season. With there being much less dust than average over the north Atlantic, more of the Sun’s energy reached the surface of the Ocean. All of these factors led to the Atlantic Basin seeing record breaking ocean water temperatures. And allowed the Atlantic to more or less overrule the influence from the strong El Nino in the Pacific.

While Atmospheric and Ocean conditions over the Atlantic Basin allowed for higher amounts of tropical cyclones in spite of El Nino. There was one El Nino influence that impacted the Atlantic Basin. That would be steering currents over the Atlantic Basin.

The placement and weakness of the subtropical high setup and environment allowed storms to turn north into the open Atlantic well before reaching the U.S. or even the Caribbean. So, storms like Category 5 Lee and Category 4 Hurricane Franklin veered away from the U.S. mainland.  If the Bermuda high had been strong and farther this summer, storms would have made it into western Atlantic, with the likely outcome of more U.S. landfalling tropical cyclones.

The arrows on the 500mb show the general direction of the steering flow.

The large heat dome ridge over the Plains and the persistent trough of low pressure in the Northeast was also a player in the storm track pattern that kept storms out of the GOM and away from the East Coast. If there had been more in the way of stronger high pressure over the Northeast into the Midwest, there would have been a better chance for them to be trapped under the high with no escape to recurve away, increasing the odds of landfalling systems.   

Well, that’s a basic rundown on why the 2023 hurricane season went the way it did.