Understanding Rossby Waves

In order to know what are Rossby Waves we need to understand Jet Streams. A subtropical jet stream is showed in the below space photo.

• Jet streams are wind streams that reach great speeds in narrow zones at a high altitude.
• They occur where atmospheric pressure gradients are strong.
• The greatest wind speeds occur in the center of the jet stream, with velocities decreasing away from it.
• Each hemisphere normally exhibits westerly polar and subtropical jet streams.
• An easterly jet occurs in summer over Asia and Africa.

Westerly Polar Jet: The most poleward type of jet stream is located along the polar front. It is called the  polar-front jet stream(or simply the “polar jet”) The polar jet is generally located between 35° and 65° latitude in both hemispheres. It follows the boundary between cold polar air and warm subtropical air .

Subtropical Jet Stream: The subtropical jet stream occurs at the tropopause, just above subtropical high-pressure cells in the Northern and southern hemispheres.

Easterly Jet: The tropical easterly jet stream occurs at even lower latitudes. It runs from east to west occurs only in summer and is limited to a northern hemisphere location over Southeast Asia, India, and Africa.

Now Disturbances occurring in these jet streams are called Jet Stream Disturbances or Rossby Waves.

These disturbances are broad wave like undulations.

Many processes can produce disturbances in the eastward flow of the jet stream. One of the most important 

is related to baroclinic instability,which makes the atmosphere unstable to small disturbances in the jet stream and allows these disturbances to grow over time.

Understanding the development of these disturbances

These disturbances are a result of complex interaction between 

1. the upper-air circulations—formed by the high- and low-pressure center
2. global temperature gradient between the warmer, low-latitude air and colder, high-latitude air

First of all slight disturbances arise from pressure differences between one location and another, resulting 

in geostrophic wind variations. In addition, the pressure differences are accompanied by temperature differences, with high pressures aloft found over regions of warm air and low pressures aloft over regions of cool air.

Step 1:  Pressure disturbances in the upper atmosphere are associated with gradients in temperature. Here, 

the warmer air column (red) produces high pressures aloft, while the cooler air columns (blue) produce lower pressures. The pressure patterns produce disturbances in the jet stream, which circulates geostrophically 

around the pressure centers.

Step 2 Change in temperature from one location to another results in a disturbance in the global-scale north–south temperature gradient between low and high latitudes, as shown by the isotherms. The counterclockwise flow around the low-pressure center aloft brings warm, tropical air from the south into the vicinity of the warm-air column, which tends to heat this region even further. Conversely, the high-pressure center aloft brings cold air from the north into the vicinity of the cold-air column, cooling it even further.

Step 3 As the warm-air column continues to warm, it expands, producing even higher pressures aloft. In addition, the cold-air column continues to cool, thereby decreasing pressures further. The pressure gradient 

between the cold-air region and the warm-air region intensifies, and so the wind speeds also intensify.

As the undulation grows, warm air pushes poleward, while a tongue of cold air is brought to the south. Eventually, the cold tongue is pinched off, leaving a pool of cold air at a latitude far south of its original location. This cold pool may persist for some days or weeks, slowly warming with time. Because of its cold 

center, it will contain low pressures aloft. In addition, the cold air in the core will descend and diverge at 

the surface, creating surface high pressure. Similarly, a warm air pool will be pinched off far to the north of its original location. Within the core of the warm pool will be rising air, with convergence and low pressure at the surface, as well as high pressure aloft.

Some MCQs
Q1. Which of the following applies to waves that are no longer in the area in which they were generated in and have lost energy and height?
a. Rossby waves
b. Swell Waves
c. Kelvin waves
d. Sea waves

Q2. Which type of atmospheric waves are jet streams positioned within?
a. Sound waves
b. Kinetic Waves
c. Rossby waves
d. Kelvin waves

Q3. Which one of the following statements about the polar jet stream is NOT true?
a. The general flow is from west to east.
b. It moves northward during summer.
c. Its velocity is greater during the summer.
d. Its location roughly coincides with that of the polar front.
e. It supplies energy to the circulation of surface storms

Q4. The polar jet stream occurs along a major frontal zone called the __________ front.
a. Secondary
b. Polar
c. Mid-Latitude
d. Asian
e. Sub tropical

Source:
Alan H. Strahler's book