CHAPTER FOUR

ATMOSPHERIC AND OCEANIC CIRCULATIONS

Introduction

Wind is the movement of air relative to the Earth’s surface. As with all moving things, it is

caused by a force acting on it. Force is a pull or push that changes the resting state, motion, or

direction of an object. Force can also cause objects to accelerate. Human skin can sense wind

when an uncountable number of molecules collide with us as they flow along in the air, and

sense the pressure changes in the air flow.

Forces Causing Atmospheric Circulation of Winds

1. Pressure Gradient Force

This refers to the rate of change of pressure of winds with horizontal distance. In simple

terms, it is the decrease in pressure per unit of horizontal distance. If distance between high-

and low-pressure zones are short, pressure gradients are steep and wind velocities are great.

More gentle air movement occur when zones of different pressure are far apart and the degree

of difference is not great. Isobars are lines on maps showing places with equal atmospheric

pressure.

In a simple sense, one place has more air than another. The atmosphere is always attempting

to balance out imbalances. Winds blow when there is a pressure imbalance in the atmosphere,

as the atmosphere strives to balance the pressure differential. When a strong area of low

pressure sweeps over an area, this is the most usual occurrence. Strong winds are caused by

the pressure differential between the low and the nearby high pressure.

2. Coriolis Force

The Coriolis effect, also known as the Coriolis force, is the outcome of the earth's rotation. It

has a significant influence on wind direction. Winds are deflected from their initial direction

due to the earth's rotation, rather than crossing the isobars at right angles as the pressure

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gradient force directs. Winds in the northern hemisphere are deflected to the right of their

course, whereas those in the southern hemisphere are deflected to the left, according to

Farrell's Law (the law that wind is deflected to the right in the Northern Hemisphere and to

the left in the Southern Hemisphere, derived from the application of the Coriolis effect to air

masses). This deflection force does not appear until the air is placed in motion, and it grows

as wind velocity, air mass, and latitude increase. The Coriolis force is perpendicular to the

pressure gradient force (the pressure gradient force is perpendicular to an isobar).

3. Frictional force
Friction is created by the imperfections of the earth's surface, which provide resistance to

wind motion. The angle at which air flows across the isobars, as well as the speed at which it

moves, is determined by this force. It may also change the direction of the wind. Friction

slows the speed of wind near the surface, thereby lowering the Coriolis force.

Frictional force is a force which exist between two surfaces which are in contact. This force

prevents the sliding of one over the other. Frictional force is caused by the ruggedness of the

earth’s surface. The lower the distance between two bodies, the stronger the Frictional Force.

It is more felt below 500m called Frictional Layer beyond which is called Free Atmosphere

or Frictionless Layer where air blows parallel to the isobars. The main function or effect of

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frictional force on air or wind is reduction in speed. It thus prevents sliding of objects such as

a vehicle on a road.

Diagram depicting Frictional force

4. Centrifugal force
This is the force which causes a body to move outward along a curved path. It is also

referred to as the radial force. When a body is moving along a curved surface or path, a

force accelerates its radially or outwardly from the centre. The centrifugal effect due to

rotation of the earth has resulted in slight bulging of the earth’s mass in low latitudes

especially at the equator. The centrifugal force is 0 at the poles and maximum around the

equator.

Processes of Wind Circulation in The Atmosphere

The pattern of planetary winds largely depends on: (i) latitudinal variation of atmospheric

heating; (ii) emergence of pressure belts; (iii) the migration of belts following apparent

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path of the sun; (iv) the distribution of continents and oceans; (v) the rotation of earth.

The pattern of the movement of the planetary winds is called the general circulation of the

atmosphere. The general circulation of the atmosphere also sets in motion the ocean water

circulation which influences the earth’s climate.

The air at the Inter Tropical Convergence Zone (ITCZ) rises because of convection caused by

high insolation and a low pressure is created. The winds from the tropics converge at this

low-pressure zone. The converged air rises along with the convective cell. It reaches the top

of the troposphere up to an altitude of 14 km. and moves towards the poles. This causes

accumulation of air at about 30° N and S. Part of the accumulated air sinks to the ground and

forms a subtropical high. Another reason for sinking is the cooling of air when it reaches 30 °

N and S latitudes.

Down below near the land surface the air flows towards the equator as the easterlies. The

easterlies from either side of the equator converge in the Inter Tropical Convergence Zone

(ITCZ). Such circulations from the surface upwards and vice-versa are called cells. Such a

cell in the tropics is called Hadley Cell.

In the middle latitudes the circulation is that of sinking cold air that comes from the poles and

the rising warm air that blows from the subtropical high. At the surface these winds are called

westerlies and the cell is known as the Ferrell cell. At polar latitudes the cold dense air

subsides near the poles and blows towards middle latitudes as the polar easterlies. This cell is

called the polar cell. These three cells set the pattern for the general circulation of the

atmosphere. The transfer of heat energy from lower latitudes to higher latitudes maintains the

general circulation.

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 General atmospheric Circulation

Oceanic Circulation

The ocean covers 71% of Earth’s surface and is constantly in motion. Large masses of water

that move together, called ocean currents, transport heat, marine organisms, nutrients,

dissolved gasses such as carbon dioxide and oxygen, and pollutants all over the world.

Climate and ecosystems everywhere on Earth, even those far from the ocean, are affected by

the ocean circulation.

Ocean circulation patterns, the movement of large masses of water both at and below the

surface, are determined by atmospheric circulation patterns, variation in the amount of

sunlight absorbed with latitude, and the water cycle. Surface currents, also called

horizontal currents, are primarily the result of wind pushing on the surface of the water, and

the direction and extent of their movement is determined by the distribution of continents.

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Currents, like winds in the atmosphere, do not move in straight lines because of the spin of

the Earth, which causes the Coriolis effect.

Currents that move up and down in the water column, also called vertical currents, are

created by differences in the density of water masses, where heavier waters sink and lighter

waters rise. This type of ocean circulation is called thermohaline circulation (thermo=heat,

halos=salt) because the vertical movement is caused by differences in temperature and

salinity (the amount of salt in water). Adding heat decreases the density of water, while

adding salt increases the density of water. Thermohaline circulation occurs because winds

move warm surface waters from the equator towards the poles, where the water cools and

increases in density. Some of this water gets so cold that it freezes, leaving its salt behind in

the remaining water, further increasing the density of this water. This cold, salty water near

the poles (primarily in the North Atlantic and near Antarctica) sinks and spreads along the

bottom and eventually rises back towards the surface of the ocean. It takes about 1000 years

for water to circulate around what is called the global conveyor belt that moves water three

dimensionally throughout the world’s ocean basins.

The general circulation of the atmosphere also affects the oceans. The large-scale winds of

the atmosphere initiate large and slow-moving currents of the ocean. Oceans in turn provide

input of energy and water vapour into the air. These interactions take place rather slowly over

a large part of the ocean.