After another particularly busy hurricane season in the Atlantic that exhausted the National Hurricane Center’s list of “conventional” storm names, the 2021 Atlantic hurricane season flatlined the second week of October. While Tropical Storm Wanda did briefly interrupt the quiescent interlude at the start of November, the Atlantic has been silent since Nov. 7.

It’s not just the Atlantic that’s been eerily quiet. All across the world, the tropics have been devoid of significant cyclone activity. Not a single hurricane-strength storm has formed anywhere on the planet since Oct. 29, a calm occupying the Northern and Southern hemispheres. That’s happened only twice before since 1966, according to Phil Klotzbach, a hurricane researcher at Colorado State University.

Perhaps the most bizarre feat has been the lack of major hurricane-strength storms, Category 3 or higher, worldwide since Sept. 25. The absence of storms of this intensity at this time of year hasn’t occurred in at least 65 years. It includes Atlantic and northeast Pacific hurricanes, typhoons in the northwest Pacific and tropical cyclones in the Indian Ocean. These storms are structurally all the same but called different things in different parts of the world.

The sudden dearth of these powerhouse storms comes after a feverish start to the Atlantic hurricane season, which stalled suddenly in October.

2021 in the Atlantic

The Atlantic season peaked in late August as Hurricane Ida made a run at Category 5 strength while lashing southeastern Louisiana with catastrophic wind gusts exceeding 150 mph. The storm laid siege to areas 50 miles south of New Orleans, including Grand Isle and Port Fourchon, which were underwater for a time. Ida eventually weakened farther inland and moved up the East Coast as a remnant rainstorm, dumping record moisture in the Northeast that killed more than 40 as extreme flooding gripped southern New England, New York City and Philadelphia. A high-end tornado outbreak accompanied the rains.

Then major hurricanes Larry and Sam both racked up considerable ACE, or accumulated cyclone energy, a measure of how much atmospheric energy is expended on tropical systems. But those storms remained mostly out at sea during September and the early part of October. Storm activity simmered markedly as October progressed. The season as a whole will wind up about 18 percent ahead of average from an ACE perspective.

Tropical storminess in the rest of the world

Of the various ocean basins supporting tropical storms, the Atlantic is the only one to see above-average activity this year. Ordinarily, the oceans sort of balance out because an increase in hurricane activity and rising air in one basin would probably be tied to sinking motion and reduced storm chances elsewhere. The northeastern Pacific is 29 percent behind average in regards to ACE, with a 38 percent deficit in the northwestern Pacific.

Storm activity in the northern Indian Ocean is way below average, and the Southern Hemisphere is virtually silent. The latter is to be expected though because the Southern Hemisphere doesn’t really awaken until December.

Klotzbach wrote that wind shear, a change in wind speed and/or direction with height, can explain a lot about the global distribution of storminess. Wind shear has a tendency to disrupt fledgling disturbances and prevent them from organizing. But where wind shear is weak, storms tend to flourish.

The presence of La Niña conditions, which developed this fall, reduces shear in the western Caribbean and favors increased activity, Klotzbach wrote. Conversely, it increases shear in the northern Pacific (both the east and west) and limits storm activity.

That explains the quiet Pacific, where there has been an abnormal amount of wind shear since mid- to late October.

What’s trickier to understand is the Atlantic. The initial period of heightened storminess through September was to be expected because of the lack of wind shear. But Klotzbach described the quiet period in recent weeks as a “surprise” in an email.

Last year, which also featured La Niña conditions, Atlantic hurricane season remained very active through October and November. The Caribbean yielded storms Gamma, Delta and Zeta, as well as Category 4 twins Eta and Iota, which both ravaged the same areas in Honduras barely two weeks apart.

Klotzbach said the Caribbean was ripe for development again this year, but the “seeds” that were needed to grow into storms never made it there to begin with.

In addition, Klotzbach pointed to an increase in “outgoing long wave radiation” over the Caribbean detected by satellites. Put simply, satellites in space have spotted more heat radiating out from the region. That means a reduction in cloud cover signifying sinking air unfavorable for storm development.

The lack of recent tropical activity worldwide is a welcome but temporary respite from a slew of destructive storm seasons, intensified by climate change. Despite the hiatus in storm activity, oceans continue to warm from human-caused heating of the planet and are primed to fuel devastating storms before long.