The Atlantic hurricane season this year has been a doozy, with 25 named storms forming at a breakneck record pace and lashing broad swaths of the U.S. coastline. But the Pacific Ocean, normally busier than the Atlantic and featuring stronger storms, has been uncharacteristically tepid. In fact, the planet’s largest ocean basin is a little more than half as active as normal.

Tropical cyclones in the Pacific north of the equator have generated 62 percent of the energy typically observed to date. Meteorologists refer to this quantity as accumulated cyclone energy, or ACE. Tropical cyclones include tropical depressions, tropical storms, hurricanes and typhoons.

Because of its size, meteorologists often analyze storms in the northern Pacific by splitting it into two sectors, the east and west. In a typical eastern Pacific hurricane season, named storms would have churned through 120 ACE units by now. This year, its ACE is at a lowly 74.

In the western Pacific — statistically the most active hot spot for tropical cyclones in the world — the energy deficit has been even more striking. Only 86 ACE units have been logged, two-fifths of the normal 214 units for this point in the season. In the western Pacific, hurricanes are called typhoons, but the storms are structurally no different.

How the season has unfolded

In the eastern Pacific, the season kicked off early, with a premature tropical depression forming April 25, weeks ahead of average. The eastern Pacific’s first major hurricane, rated Category 3 or higher, occurred in July, with Hurricane Douglas peaking at Category 4 status. It remained over the open ocean during its entire lifetime, and its remnants only brought a rainy and breezy day to parts of Hawaii.

Hurricane Genevieve reached Category 4 strength well offshore of western Mexico in mid-August, as did Marie in early October over the open east Pacific. But, as a whole, the eastern Pacific season has produced little in the way of direct land impacts.

In the western Pacific, only Haishen has achieved super typhoon status, a designation given to typhoons with peak winds of at least 150 mph. Super typhoons are equivalent to high-end Category 4 hurricanes. Haishen went on to wallop parts of southern Japan and the Korean Peninsula.

At this time last year, Hagibis, the fourth super typhoon of 2019, had already developed, explosively intensifying from a tropical storm to a Category 5-equivalent super typhoon in just 18 hours’ time, one of the fastest rates recorded.

Seven super typhoons developed in the very active 2018 season.

Why has Pacific activity been so depressed?

The laggard, albeit welcome, respite from storminess in the Pacific is antithetic to the Atlantic’s bustling activity, yet the two are related. As a general rule of thumb, when one ocean basin booms, the other remains quiet.

Part of the reason behind that is a developing La Niña pattern. The first sign of a brewing La Niña is the cooling of waters in the eastern tropical Pacific. That sends processes in motion that can impede Pacific hurricane activity while enhancing it over the Atlantic.

Among those mechanisms is the Walker Circulation. In essence a large overturning circulation that exists in the tropics, it is composed of rising air on one side, and increased sinking, or subsidence, on the other.

During La Niña patterns, warmer waters over the Atlantic favor the Walker Circulation’s broad rising motion. That’s helped exacerbate the hurricane season, while cooler waters on the Pacific side have been less supportive to spurring strong storms.

There is also an increase in wind shear during La Niñas, or a change of wind speed and/or direction with height, over the alley of the central and eastern Pacific where hurricanes are most common. That disruptive wind shear has a tendency to play a game of atmospheric tug of war with systems as they try to get organized and more often than not tears them apart before they even have a chance to develop. That also cuts back on the Pacific’s hurricane statistics.

In the Atlantic, the opposite has been true, with warmer waters and minimal wind shear.

So while the Atlantic and east Pacific may be separate basins, the dynamics governing their hurricane seasons are interrelated and connected. Meteorologists anecdotally rely on a loose fitting balance between the two, but rarely is it as visible as in years like this.

“With the [eastern Pacific] at 62 percent of its average ACE by this date, and the Atlantic at 137 percent, that general model fits really well,” wrote Brian McNoldy, Capital Weather Gang’s tropical weather expert. “It’s nice when nature is well-behaved like that.”

In other words, the two ocean basins — when averaged — are perfectly normal.