Science & Nature

Rossby Waves Affect Our Weather and Tides, But What Are They?

Rossby Waves Affect Our Weather and Tides, But What Are They? thumbnail

Carl-Gustaf Arvid Rossby was a Swedish-born meteorologist who left quite a legacy behind. Rossby spent World War II training scientists for the U.S. military and gave prescient warnings about climate change. Though he died in 1957, the man’s name lives on in a phenomenon he discovered during the turbulent 1930s: Rossby waves.

Also called “planetary waves,” these have a huge effect on our atmosphere and oceans. As James R. Holton and Gregory J. Hakim write in their book, “An Introduction to Dynamic Meteorology,” Rossby waves are “[the] wave type that is of the most importance for large-scale meteorological processes.”

They influence everything from high tides to extreme weather patterns. And that’s just what happens on Earth. Let us not ignore the sun, which experiences Rossby waves of its own. So do the atmospheres of Venus and Jupiter.

But what defines these things? What is it that makes a Rossby wave a Rossby wave?

Fluid Science

Rossby waves are something that can occur within fluids.

We’re not only talking about liquids here. Any flowing substance that’s constantly getting deformed by stresses (like friction) that act upon its surface in a parallel direction is considered a fluid. By this metric, the air itself is a fluid, along with our wet and wonderful oceans.

A true Rossby wave needs to meet some specific criteria. For one thing, these waves only occur inside barotropic fluids — a category of fluids whose densities are a function of pressure alone. Also, Rossby waves are natural byproducts of the Coriolis effect.

Putting a Spin on Things

It’s time for a disclaimer. People have long accused the Coriolis effect of tampering with Australian toilets. This is false. Contrary to what the urban legends say, the Coriolis effect has little to do with how water swirls in a newly flushed john.

What it actually does involves the way some objects appear to move when they travel across a rotating body, like Earth or the sun.

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Our home world rotates eastward around its own axis, the invisible line that cuts through the planet, connecting the North and South poles. Also, Earth is a lot thicker around the equator than it is at either pole. So, by necessity, as the planet turns, its equatorial regions spin at a faster speed than the higher latitudes do.

Every time you stand on the equator, you — and the ground beneath your feet — are being spun eastward at almost 1,030 miles per hour (1,670 kilometers per hour). But when you stand in the Arctic city of Utqiaġvik, Alaska, at a latitude of 71 degrees above the equator, your eastward spin speed will be reduced to just 340 miles per hour (or 550 kilometers per hour).

That difference explains why airborne objects traveling in a North-South direction seem to veer off-course instead of moving in a straight line.

“Rossby waves exist because of the Coriolis effect,” says Mausumi Dikpati, a senior scientist, modeler and theoretical solar physicist at the Colorado-based High Altitude Observatory (HAO), in an email. “No rotation, no Rossby waves.”

Sea and Sky

On Earth, Rossby waves occur in our atmosphere and our oceans alike. Scientists have known about atmospheric Rossby waves since 1939, while oceanic Rossby waves were first observed back in 1977.

The two have a lot in common. Dikpati tells us they’re both caused by “the action of Coriolis forces.”

Another contributing factor is the sun. Instead of warming our planet evenly, sunlight heats up the equator at a faster rate than temperate zones and the poles. Hence, Earth has a natural temperature gradient based on latitude.

Then we have the winds, whose strength and direction can be profoundly affected by their height.

“Atmospheric Rossby waves are excited by instabilities of the latitude temperature gradient and vertical wind gradient, while oceanic Rossby waves can be excited by both that and the effects of wind on the ocean surface,” Dikpati explains.

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When these waves take place in the ocean, they’re usually smaller in scale than the Rossby waves in our atmosphere. Moreover, Dikpati says oceanic Rossby waves are “confined to ocean basins, while atmospheric waves propagate all the way around Earth.”

Rising Tides

You can’t really observe marine Rossby waves with the naked eye when you’re standing on the planet’s face. But fortunately for scientists, artificial satellites can monitor their progress.

Calling the waves “slow-moving” would be an understatement. At low latitudes, they might “take months to a year to cross the [Pacific] ocean,” according to the U.S. National Oceanic and Atmospheric Administration (NOAA).

Farther away from the equator, Pacific Rossby waves can travel at a pace that’s even more leisurely, sometimes requiring a decade or more to traverse the world’s biggest ocean.

Despite their slowness, Rossby waves aren’t something that coastal communities should ignore. They can make high tides higher than usual. Not surprisingly, the waves have also been linked to severe floods.

Entire shorelines might be affected; incoming Rossby waves can raise the water level across hundreds of miles (or several hundred kilometers) of ocean-side property for a period of months — wreaking havoc on the local infrastructure.

A 2018 study found that Rossby waves may cause coastal sea levels to go up by over 3.9 inches (10 centimeters), which is nothing to sneeze at, not in a world where almost 40 percent of the population lives within 60 miles (or about 100 kilometers) of the ocean.

Now Streaming

Jet streams are another playground for Rossby Waves.

Formed at the convergent boundaries of warm and cold air masses, jet streams are fast-moving currents. They cruise through our atmosphere roughly 5 to 9 miles (8 to 14.4 kilometers) above the planet’s surface, traveling at an average speed of 110 miles per hour (or 177 kilometers per hour).

Earth has four primary jet streams — with two in the polar regions and a set of “subtropical” streams located on either side of the equator. All four travel from west to east.

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“Waves in the [jet stream], as we see them on weather maps, are closely related to Rossby waves,” says Dikpati. “They interact with the mean east-west winds to produce the wavelike patterns we see as well as the [large-scale] changes in weather we feel. This interaction is at the core of modern numerical weather prediction models.”

Scientists documented “extreme heatwaves” in Europe during the years 2003, 2010 and 2015. Each of those events has been linked to a series of Rossby waves that had meandered through the Northern Hemisphere’s subtropical jet stream.

The twisting waves can stall high or low-pressure weather systems, restricting their movement for long periods. Under the wrong circumstances, that can trigger natural crises like floods and droughts. Back in 2018, Rossby wave activity was implicated in both Japanese flash flooding and North American heatwaves.

Such events can have global ramifications. A 2020 study published in the journal “Nature Climate Change” argued that Rossby waves could pose a major threat to international food security. It’s not hard to imagine heat spells brought on by these waves lowering crop production by as much as 11 percent in regions that’ve become crucial to the world’s food supply. Delightful.

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