Robby Waves Introduction
Rossby waves are a type of initial oscillations occurring naturally in rotating fluids. Rossby waves, also known as planetary waves are first observed by Carl-Gustaf Arvid Rossby. The waves are observed in the atmosphere and oceans of planets because of the rotation of planets.
Atmospheric Rossby waves are massive meanders in high altitudes that have a significant impact on weather. These waves are related to the jet stream and pressure system. Oceanic Rossby waiver moves along the boundary between the warm upper layer and the cold deep part of the ocean.
Rossby Waves Definition
Rossby waves, also known as planetary waves are massive meanders in high altitudes that have a major effect on weather. The emergence of Rossby waves is due to shear in the rotating fluid so that the Coriolis force varies along the sheared coordinate. In the Planetary atmosphere, the emergence of the Rossby wave is due to changes in the Coriolis effect with latitude.
Rossby Waves Formation
Rossby waves are formed when tropical air is moving poleward and polar air moves towards the equator. Because of the difference in temperature between the Equator and the poles due to the difference in the amount of solar radiation received, heat tends to flow from low to high latitudes. This is attained in part, by these air movements.
Rossby waves are the principal component of the Ferrel circulation. The heat is transferred to the poleward by the tropical waves and polar air absorbs heat as it moves toward the equator.
The existence of the Rossby waves explains cyclones and anticyclones.
Rossby Number
Rossby number, also known as Kibel number is a dimensionless number used for measuring liquid. It is the ratio of inertial force to Crosiloses force for a given flow of rotating liquid. Rossby number is commonly used in the geographical phenomena in the atmosphere and ocean, where it features the importance of Coriolis acceleration, originating from planetary rotation.
Rossby Number is Defined as the:
Rₒ = \[\frac{U}{Lf}\]
In the above equation, U is the velocity scale, f is the Coriolis parameter, and L is the horizontal length scale.
Types of Rossby Waves
The Different Types of Rossby Waves are:
Atmospheric Rossby Waves
Oceanic Rossby Waves
Topographic Rossby Waves
Atmospheric Rossby Waves
Atmospheric Rossby waves primarily result from the conservation of potential vorticity and are affected by the Coriolis force and pressure gradient. The rotation causes fluid to move towards the right as they move in the Northern hemisphere and to the left as they move in the Southern hemisphere. For example, a fluid moving from the equator toward the north pole will deviate towards the east whereas the fluid movie moving from the north towards the equations will deviate towards the west.
These deviations are generated by Coriolis force and conservation of potential vorticity which gives rise to change in relative vorticity. This is similar to the conservation of angular momentum in the mechanism. In the Planetary atmosphere, including Earth, the Rossby waves are caused due to changes in the Coriolis effect with latitude.
Oceanic Rossby Waves
Oceanic Rossby waves are wide-reaching waves found within an ocean basin. In comparison to the Atmospheric Rossby waves which are in the order of hundreds of kilometers, the oceanic Rossby waves are in the order of centimeters ( at the surface) to meters (at thermocline). The waves may take months to cross an ocean basin.
The Oceanic Rossby waves are so large and massive that they can change the Earth’s climatic conditions. The waves gain momentum from the wind stress at the ocean surface layer and are thought to communicate climate changes because of the variability in forcing and due to both the wind and buoyancy. Satellite observations have approved the emergence of Oceanic Rossby waves.
Jet Stream and Rossby Waves
A Jet stream is defined as a current of fast-moving air that is generally several thousand miles ago and is relatively thin. The jet streams are found in the upper layer of the atmosphere at the tropopause- the boundary between the stratosphere and the troposphere.
Jet streams are important because they provide the worldwide weather pattern and help meteorologists to forecast weather based on their position. Also, they are important to air travel because flying in or out can minimize flight time and fuel consumption.
The meandering or whirl movement of the jet stream is known as Rossby waves. The Rossby waves were first observed by Carl- Gustaf Arvid Rossby.
Did You Know?
Rossby waves, also known as planetary waves, are naturally occurring waves in rotating fluid.
The terms baroclinic and barotropic are used to describe Rossby waves' vertical structure.
Rossby waves in Earth’s atmosphere are easy to observe large-scale meanders of the Jet stream.
Atmospheric Rossby waves, similar to Kelvin waves can occur on any rotating planet with an atmosphere. The Y-shaped characteristics of clouds on Venus are credited to Kelvin and Rossby waves.
FAQs on Rossby Wave
1. What is a Rossby wave in simple terms?
A Rossby wave is a very large, meandering wave pattern in the atmosphere or oceans. Also known as planetary waves, they form as a result of the Earth's rotation. Think of them not as surface waves, but as giant, slow-moving horizontal wiggles in high-altitude winds (like the jet stream) or in ocean currents, which play a crucial role in shaping weather and climate patterns.
2. How are Rossby waves formed?
Rossby waves are formed due to the conservation of potential vorticity on a rotating planet. The key trigger is the variation of the Coriolis effect with latitude. As a parcel of air or water moves poleward, the increasing Coriolis effect causes it to deflect and turn back toward the equator. As it moves equatorward, the decreasing Coriolis effect causes an opposite deflection. This continuous north-south oscillation within a west-to-east flow, like the jet stream, creates the characteristic large-scale wave pattern.
3. What is the main difference between Rossby waves and jet streams?
The main difference is that a jet stream is a fast-flowing current of air, like a river, while a Rossby wave is the large-scale meander or bend *in* that river of air. The jet stream is the physical flow itself, responsible for moving air masses. The Rossby wave is the pattern of that flow, which dictates where the jet stream goes and how it influences weather over large regions.
4. How do Rossby waves influence weather patterns in the mid-latitudes?
Rossby waves are fundamental to mid-latitude weather. Their structure directly transports heat across the globe.
- The troughs (southward dips) of the waves allow cold, polar air to move towards the equator.
- The ridges (northward crests) allow warm, tropical air to move towards the poles.
5. Where do Rossby waves occur in the Earth's system?
Rossby waves are found in two main parts of the Earth's system:
- In the atmosphere: They are most visible as 4-6 large-scale meanders in the polar jet stream, circling the globe at high altitudes.
- In the oceans: Oceanic Rossby waves are massive, slow-moving undulations that travel horizontally across ocean basins. They move much slower than their atmospheric counterparts, taking months or even years to cross an ocean, and they affect sea level and ocean temperature.
6. Why do Rossby waves have a natural westward propagation?
A Rossby wave's inherent motion is always westward relative to the fluid flow it is in. This is because the restoring force that creates the wave—the change in the Coriolis force with latitude (known as the beta effect)—always pushes displaced fluid back toward its original latitude in a way that causes the wave pattern to propagate to the west. However, if the eastward background wind (like the jet stream) is strong enough, it can overcome this westward movement and carry the wave eastward across the globe.
7. What is the difference between Rossby waves and Kelvin waves?
While both are large-scale waves influenced by Earth's rotation, they differ in key ways:
- Mechanism: Rossby waves are generated by the change of the Coriolis effect with latitude. Kelvin waves are a special type of wave where the Coriolis force is balanced against a topographic boundary like a coastline or a dynamic boundary like the equator.
- Propagation: Rossby waves always propagate westward (relative to the mean flow), while equatorial Kelvin waves always propagate eastward.
- Dispersion: Rossby waves are dispersive (their speed depends on their wavelength), whereas Kelvin waves are non-dispersive (all wavelengths travel at the same speed).
8. Are Rossby waves dispersive, and what does that mean?
Yes, Rossby waves are dispersive waves. This means that waves with different wavelengths travel at different speeds. As a result, an initial wave pattern will spread out and change its shape over time, as longer waves outpace shorter ones. This is different from non-dispersive waves, like sound waves, where all components travel together at the same speed.
