What Are Rossby Waves and How Do They Impact Weather and Ocean Tides?

Essential Insights

Carl-Gustaf Arvid Rossby, a Swedish meteorologist, left an enduring legacy. During World War II, he trained scientists for the U.S. military and issued early warnings about climate change. Although he passed away in 1957, his name remains immortalized in the discovery of Rossby waves, a phenomenon he identified during the tumultuous 1930s.

Known as "planetary waves," these atmospheric and oceanic phenomena have a profound impact. As detailed in the book "An Introduction to Dynamic Meteorology" by James R. Holton and Gregory J. Hakim, Rossby waves are described as "the most significant wave type for large-scale meteorological processes."

Rossby waves affect a wide range of natural events, from extreme tides to severe weather conditions. Their influence isn't limited to Earth; they also occur on the sun, as well as in the atmospheres of Venus and Jupiter.

What exactly characterizes these waves? What sets a Rossby wave apart from other atmospheric phenomena?

Fluid Science

Rossby waves are phenomena that can form within fluids.

This term isn't limited to liquids. Any substance that flows and is continuously reshaped by parallel surface stresses, such as friction, qualifies as a fluid. By this definition, both the air we breathe and the vast oceans are considered fluids.

To be classified as a true Rossby wave, certain conditions must be met. These waves exclusively form within barotropic fluids, where density depends solely on pressure. Additionally, Rossby waves arise naturally due to the Coriolis effect.

Putting a Spin on Things

Let's clear up a common misconception. Many believe the Coriolis effect influences how toilets flush in Australia. This is a myth. Despite popular tales, the Coriolis effect has almost no impact on the direction water spins in a toilet bowl.

In reality, the Coriolis effect describes how objects seem to move when observed from a rotating frame of reference, such as Earth or the sun.

Earth rotates eastward along its axis, an imaginary line running between the North and South poles. Additionally, the planet's equatorial region is wider than its polar areas. As a result, during rotation, the equator moves faster than regions at higher latitudes.

When you stand at the equator, you and the ground beneath you are moving eastward at approximately 1,030 miles per hour (1,670 kilometers per hour). However, if you stand in Utqiaġvik, Alaska, located at 71 degrees north of the equator, your eastward speed drops to just 340 miles per hour (550 kilometers per hour).

This speed variation explains why objects moving North-South in the air appear to deviate from their intended path rather than following a straight trajectory.

"Rossby waves owe their existence to the Coriolis effect," explains Mausumi Dikpati, a senior scientist, modeler, and theoretical solar physicist at the High Altitude Observatory (HAO) in Colorado, via email. "Without rotation, Rossby waves wouldn't exist."

Sea and Sky

On Earth, Rossby waves manifest in both the atmosphere and the oceans. Atmospheric Rossby waves were discovered in 1939, while oceanic Rossby waves were first identified in 1977.

Both types share significant similarities. According to Dikpati, they are driven by "the influence of Coriolis forces."

The sun also plays a significant role. Rather than heating Earth uniformly, sunlight warms the equatorial regions more rapidly than temperate areas and the poles. This creates a natural temperature gradient that varies with latitude.

Winds are another key factor, as their intensity and direction can be heavily influenced by altitude.

"Atmospheric Rossby waves are driven by instabilities in the temperature gradient across latitudes and the vertical wind gradient. Oceanic Rossby waves, on the other hand, can be triggered by these factors as well as wind interactions on the ocean's surface," Dikpati clarifies.

Oceanic Rossby waves are typically smaller in scale compared to their atmospheric counterparts. Additionally, Dikpati notes that oceanic waves are "limited to ocean basins, whereas atmospheric waves can travel across the entire globe."

Rising Tides

Marine Rossby waves are invisible to the naked eye from Earth's surface. However, scientists can track their movement using artificial satellites.

Describing these waves as "slow-moving" doesn't do them justice. Near the equator, they can "take months to a year to cross the [Pacific] ocean," as stated by the U.S. National Oceanic and Atmospheric Administration (NOAA).

In regions farther from the equator, Pacific Rossby waves move even more sluggishly, sometimes taking a decade or longer to cross the world's largest ocean.

Although slow, Rossby waves pose significant risks to coastal areas. They can amplify high tides and are often associated with devastating floods.

Entire coastlines can be impacted; incoming Rossby waves may elevate sea levels across hundreds of miles (or kilometers) of coastal land for several months, causing widespread damage to infrastructure.

A 2018 study revealed that Rossby waves can elevate coastal sea levels by more than 3.9 inches (10 centimeters), a significant concern in a world where nearly 40 percent of the population resides within 60 miles (approximately 100 kilometers) of the ocean.

Now Streaming

Jet streams also serve as a key environment for Rossby waves.

Jet streams form where warm and cold air masses meet, creating high-speed air currents. These streams flow 5 to 9 miles (8 to 14.4 kilometers) above Earth's surface, moving at an average speed of 110 miles per hour (177 kilometers per hour).

Earth features four main jet streams: two in the polar regions and two subtropical streams near the equator. All four move in a west-to-east direction.

"Waves in the [jet stream], visible on weather maps, are closely tied to Rossby waves," explains Dikpati. "They interact with the prevailing east-west winds, creating the wavelike patterns we observe and the [large-scale] weather changes we experience. This interaction is fundamental to modern weather prediction models."

Researchers have recorded "extreme heatwaves" in Europe during 2003, 2010, and 2015. These events were connected to Rossby waves that traveled through the Northern Hemisphere's subtropical jet stream.

The meandering waves can cause high or low-pressure systems to stall, locking them in place for extended periods. In extreme cases, this can lead to disasters like floods and droughts. In 2018, Rossby waves were linked to flash floods in Japan and heatwaves across North America.

These events can have worldwide consequences. A 2020 study published in "Nature Climate Change" highlighted that Rossby waves could significantly endanger global food security. Heatwaves triggered by these waves might reduce crop yields by up to 11 percent in key agricultural regions, which are vital to the global food supply.