Monsoons and Long Waves

Objectives

 

 

 

Describe the climatic reasons for the development of monsoons in the northern Indian Ocean.

 

Describe the seasonal circulation in the Indian Ocean and the reasons why this differs from a typical ocean gyre.

 

Discuss the role of long Kelvin and Rossby waves in oceanic circulation.

 

Calculate the Rossby radius of deformation and explain its importance.

 

Key terms: amphidromic systems, wave guides.

The Northeast Monsoon

 

 

 

 

 

In the northern winter, the air over the Eurasian land mass is cooler and denser than the air over the ocean.

 

This results in a winter high pressure area.

 

The resulting pressure gradient results in a northerly or northeasterly flow of air from Asia to south of the Equator.

 

This is the Northeast Monsoon.

 

The Northeast Monsoon brings dry cool air to India from Eurasia; this corresponds to a dry season.

The Southwest Monsoon

 

 

 

 

 

In the northern summer, the air over the Eurasian land mass is warmer and lighter than the air over the ocean.

 

This results in a summer low pressure area.

 

The resulting pressure gradient results in southwesterly flow of air.

 

This is the Southwest Monsoon.  This is the stronger of the two monsoons.

 

The Southwest Monsoon brings humid air from the ocean to India.

Somali Current

 

 

 

During the Northeast Monsoon, the Somali Current flows to the southwest in response to it.

 

During the Southwest Monsoon, the Somali Current flows to the northeast in response to it.

Normalcy

 

 

 

 

With respect to the other gyres (Atlantic and Pacific), there is ~  normal clockwise atmospheric flow in the Northern I.O. during the summer.

 

During the winter, there is ~ normal current flow along the equator.

 

Other contributors to the I.O.'s uniqueness:

 

Eurasian land mass is massive.

 

Northern Indian Ocean is relatively small

Kelvin Waves

 

 

 

 

 

Important "transmitters" of change; contribute to the oceans' ability to change rapidly in response to changes in the overlying wind field.

 

Kelvin Wave: perturbations of the sea surface or of the thermocline which propagate along “wave guides.”

 

Smaller waves do not disrupt isopycnals.

 

These are termed “barotropic waves”

 

Surface Kelvin waves are nearly always barotropic

 

Thermocline-riding and large waves that cause disruptions in the isopycnic surfaces are “baroclinic waves”

Kelvin Waves (con’t)

 

 

 

 

 

Travel along “wave guides” that trap wave motion through a combination of Coriolis Force and horizontal pressure gradient forces.

 

Equator

 

Coriolis Force deflects wave back on to equator

 

Coastal boundaries

 

Boundary to the right in N. Hemi (e.g. North America)

 

Boundary to the left in S. Hemi (e.g. South America)

 

Amplitude of vertical displacement is greatest at the coast and decreases exponentially seaward.

Rossby Radius of Deformation

 

 

 

 

 

The distance at which the amplitude of the Kelvin Wave has decayed to the extent that it is hardly discernible.

 

In mathematical form:

 

L = c/f

 

c = wave speed

 

f = Coriolis parameter

 

Mathematically, it can be seen that equatorial Kelvin waves are not as closely trapped to the coast as they are in the mid-latitudes.

... Deformation (con’t)

 

 

 

 

 

Generally “L” is the distance that a wave with speed c can travel in time 1/f.

 

It acts as a guide to the distance that a wave can travel before being significantly affected by the Coriolis Force.

 

The implication of this is that the tendency for a current to take on a curved or gyral pattern increases with increasing distance from the equator.

 

Note that L = 4 at the equator (since f = 0) and is a minimum at the poles.

Rossby Waves

 

 

 

 

Sometimes called “planetary waves,” these arise from the need to conserve potential vorticity.

 

A water parcel may oscillate about its original latitude alternately gaining and losing planetary vorticity while respectively losing or gaining relative vorticity.

 

Overall effect of the clockwise and counterclockwise rotations associated with Rossby waves is to cause the waveforms to travel westwards.

 

The “jet stream” is a perfect example of an atmospheric Rossby wave

For Next Time...

 

 

 

Read El Nino, High Latitude; OC 145-155

 

Describe what happens in the atmosphere and the oceans during an El Nino event.

 

Discuss the difference in circulation in the Arctic Sea and around Antarctica and the reasons for this difference.

 

Describe the circulation pattern around Antarctica.

Questions?