Here at the starting point of “the Pineapple Express,” it’s a little hard to imagine being at one end of a 2,500-mile ribbon of moisture that in California fills rivers to overflowing, triggers mudslides and breaches levees.

And yet, it’s right there on satellite maps: sometimes originating right over Hawaii, sometimes as much as hundreds of miles to the north, pointed like a dagger at the West Coast.

Oddly, even as California copes with a seemingly endless parade of “atmospheric rivers,” Hawaii has been unusually calm and dry.

In fact, the contrast is evidence of how the two places are connected meteorologically.

Satellite image shows atmospheric river hitting California.
In October 2021, a powerful storm off the Pacific Northwest coast steered an intense atmospheric river into San Francisco. NASA Worldview

The storm track headed to California in recent weeks pushed the ridge of high pressure that usually stays north of Hawaii directly over the islands. That, in turn, shut off the trade winds. It also suppressed the vertical formation of clouds that could have led to rain.

Wet and windy in California. Dry and calm in Hawaii.

There’s other evidence of the connection. A huge swell on the North Shore, which last week led to a spate of rescues and exhilaration for surfers, resulted from the storm track to the north that eventually slammed into California.

Californians have long used the term Pineapple Express to describe the winter storms that can bring welcome precipitation or, if they’re particularly strong or stall out or come in a series, widespread damage and misery.

In fact, the Pineapple Express is just one example of an atmospheric river.

The story starts in the tropics, where direct sunlight evaporates sea water and easterly winds north and south of the equator converge, creating a belt of moisture. Disturbances in that band cause vast amounts of moisture to break off and move towards the poles, according to a visualization by NASA.

The atmospheric river that hits the West Coast often starts to take shape around Hawaii, hence the pineapple moniker. The bands can be from 250 to 375 miles wide, 2 miles deep and can carry as much as 25 times the amount of water at the mouth of the Mississippi River, according to the National Oceanic and Atmospheric Association.

Low-pressure systems and their associated cold fronts funnel the band of moisture northward. The cold front can cause moisture in the atmospheric river to rise and condense into precipitation. The moisture also rises and condenses when the sky rivers hit the mountains of California and other West Coast states, Canada and Alaska. Atmospheric rivers account for as much as half the annual precipitation on the West Coast, NOAA says.

graphic showing the science behind an atmospheric river

At any given moment, atmospheric rivers are flowing at various points around the globe. They tend to meander like a garden hose flailing around. But it’s not random – the path is heavily influenced by the jet stream, the fast-flowing atmospheric air current. One way the jet stream was discovered was U.S. bomber pilots in World War II missing their targets in Japan because of the fierce wind.

The jet stream, while generally going from west to east, also can meander and at times runs north and south.

But during the current series of storms, the jet stream directing storms to California has remained stubbornly “zonal” – flowing directly from west to east.

“The jet stream is the story here,” said Steven Businger, professor in the Department of Atmospheric Sciences at the University of Hawaii Manoa.

That configuration has forced the high pressure that usually resides north of Hawaii right over the state, and the downward flow of air is suppressing the development of rain-bearing clouds.

This image from 2017 shows the location of atmospheric rivers around the world.
This image from 2017 shows the location of atmospheric rivers around the world. NASA/Goddard Space Flight Center Scientific Visualization Studio

“It creates a cap that limits how high clouds can go,” said John Bravender, warning coordination meteorologist for the National Weather Service in Honolulu. “They can’t develop tall enough to form rain clouds.”

The trade winds that normally buffet the state are caused by the gradient between high pressure north of the state and low pressure at the equator. But when the high pressure is directly overhead, the gradient is not in play in Hawaii and the trade winds die.

One side effect, now that Kilauea is erupting again on the Big Island, is that in the absence of the trade winds, in the right conditions, vog can drift over the other islands. That also happened last week.

The weather in Hawaii may be pleasant right now, but like California, the state needs rain to relieve persistent drought.

In the winter, rain often comes in the form of “Kona lows,” which are especially important on the leeward sides of the islands which, unlike the windward sides and mountains, don’t get much moisture from the trade winds.

That will take a reorientation of the jet stream to north-south. In that configuration, the jet stream can buckle and a low-pressure system can break off, taking on a life of its own, said Kevin Kodama, a NOAA hydrologist.

That could flip the script, leaving California dry and Hawaii soggy.

“It’s all part of the same circulation system,” Kodama said.

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