# Rossby Wave

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## 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.

1. Are Rossby Waves Dispersive Waves?

Ans: A dispersive wave is a wave whose patterns change as time passes. The group velocity in a dispersive wave is usually different from the phase speed. Yes, Rossby waves are dispersive waves. The waver can appear in the atmosphere only when the atmosphere is stably stratified.

2. Where Most of the Investigation About the Rossby Waves were Made?

Ans: Most of the observations about the Rossby waves were made on the Earth’s atmosphere. On Earth’s atmosphere, Rossby waves can easily observe (4-6) large-scale meanders of jet streams. When these deviations become more noticeable, the mass of cold or warm air detach and become more strong cyclones and anticyclones respectively, and are responsible for frequent weather patterns at mid-latitudes.

3. What are the Two Factors on Which Direction of the Movement of Rossby Waves Depends?

Ans: The two factors on which direction of the movement of Rossby waves depends are :

1. The speed of the westerly flow

2. Several troughs/ridges around a latitude.

Rossby waves will be motionless if there is a westerly flow of 15m/s with three troughs and ridges. The speed lesser than this “critical” speed will result in a westerly movement of the Rossby waves whereas higher speed results in easterly propagation.