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Chapter 12

Objectives

The ocean and the atmosphere are one interconnected system. The surface processes on

planet Earth are the result of H2O moving through this water cycle which is being driven byenergy from the Sun. The ocean's energy and water exchange with the atmosphere produce

atmospheric circulation cells, pressure belts, and wind belts. The idealized circulation

patterns are modified by the continents' own particular effects on heating, cooling, and

precipitation. The result is an intriguing mix of weather and climate, producing somewhat

predictable but widely variable atmospheric and oceanographic phenomena that are the topics

covered in this chapter.

After studying this chapter, you should be able to:

Describe the causes of uneven solar heating on Earth. Understand why Earth has seasons and how seasonal changes in solar energy affectatmospheric temperature, pressure, and density.

Explain the nature, origin, and consequences of the Coriolis effect in both theNorthern and Southern Hemisphere.

Discuss the locations and characteristics of Earth's major atmospheric circulationcells, pressure belts, wind belts, and boundaries.

Know the difference between weather (meteorology) and climate (climatology). Indicate the conditions required for the formation of tropical cyclones (hurricanes)and explain what types of destruction are caused by them.

Describe the cause of Earth's greenhouse effect and why it has increased in the recentpast.

Insolation

From Wikipedia, the free encyclopedia

Not to be confused withInsulation (disambiguation).

Insolation is a measure ofsolar radiationenergy received on a given surface area in a given

time. It is commonly expressed as averageirradiancein watts per square meter (W/m2) or

kilowatt-hours per square meter per day (kWh/(m2day)) (or hours/day). In the case

ofphotovoltaicsit is commonly measured as kWh/(kWpy) (kilowatt hours per year per

kilowatt peak rating).

The object or surface that solar radiation strikes may be a planet, a terrestrial object inside the

atmosphere of a planet, or any object exposed to solar rays outside of an atmosphere,

includingspacecraft. Some of the solar radiation will be absorbed, while the remainder will

be reflected. Usually the absorbed solar radiation is converted to thermal energy, causing an

increasing in the object's temperature. Some systems, however, may store or convert a

portion of the solar energy into another form of energy, as in the case of photovoltaics
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orplants. The proportion of radiation reflected or absorbed depends on the

object'sreflectivityoralbedo.

[edit]Projection effect

The insolation into a surface is largest when the surface directly faces the Sun. As the angle

increases between the direction at a right angle to the surface and the direction of the rays of

sunlight, the insolation is reduced in proportion to thecosineof the angle; seeeffect of sun

angle on climate.

Figure 2

One sunbeam one mile wide shines on the ground at a 90 angle, and another at a 30 angle.

The one at a shallower angle distributes the same amount of light energy over twice as much

area.

In this illustration, the angle shown is between the ground and the sunbeam rather than

between the vertical direction and the sunbeam; hence the sine rather than the cosine is

appropriate. A sunbeam one mile (1.6 km) wide falls on the ground from directly overhead,

and another hits the ground at a 30 angle to the horizontal.Trigonometrytells us that

thesineof a 30 angle is 1/2, whereas the sine of a 90 angle is 1. Therefore, the sunbeam

hitting the ground at a 30 angle spreads the same amount of light over twice as much area (if

we imagine the sun shining from the south atnoon, the north-south width doubles; the east-

west width does not). Consequently, the amount of light falling on each square mile is only

half as much.

This 'projection effect' is the main reason why thepolar regionsare much colder

thanequatorial regionson Earth. On an annual average the poles receive less insolation than

does the equator, because at the poles the Earth's surface are angled away from the Sun.
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Solar Radiation Map of Africa and Middle East

Apyranometer, a component of a temporary remote meteorological station, measures

insolation onSkagit Bay,Washington.

[edit]Earth's insolation

Direct insolationis the solarirradiancemeasured at a given location on Earth with a surface

element perpendicular to the Sun's rays, excluding diffuse insolation (the solar radiation that

is scattered or reflected by atmospheric components in the sky). Direct insolation is equal to

thesolar constantminus the atmospheric losses due toabsorptionandscattering. While the

solar constant varies with theEarth-Sun distanceandsolar cycles, the losses depend on the

time of day (length of light's path through the atmosphere depending on theSolar elevation

angle),cloud cover,moisturecontent, and otherimpurities. Insolation is a fundamental abiotic

factor[1]

affecting the metabolism of plants and the behavior of animals.

Over the course of a year the average solar radiation arriving at the top of the Earth's

atmosphere at any point in time is roughly 1,366wattsper square meter[2][3]

(seesolar

constant). The radiant power is distributed across the entireelectromagnetic spectrum,

although most of the power is in thevisible lightportion of the spectrum. The Sun's rays

areattenuatedas they pass through theatmosphere, thus reducing the insolation at the Earth's
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surface to approximately 1,000 watts per square meter for a surface perpendicular to the Sun's

rays at sea level on a clear day.

The actual figure varies with the Sun angle at different times of year, according to the

distance thesunlighttravels through theair, and depending on the extent of atmospheric hazeand cloud cover. Ignoring clouds, the average insolation for the Earth is approximately 250

watts per square meter (6 (kWh/m2)/day), taking into account the lower radiation intensity in

early morning and evening, and its near-absence at night.

The insolation of the sun can also be expressed in Suns, where one Sun equals 1,000 W/m2

at

the point of arrival, with kWh/(m2day) displayed as hours/day.

[4]When calculating the

output of, for example, a photovoltaic panel, the angle of the sun relative to the panel needs to

be taken into account as well as the insolation. (The insolation, taking into account the

attenuation of the atmosphere, should be multiplied by the cosine of the angle between thenormal to the panel and the direction of the sun from it). One Sun is a unit ofpower flux, not

a standard value for actual insolation. Sometimes this unit is referred to as a Sol, not to be

confused with a sol, meaning one solar day on, for example, a different planet, such as

Mars.[citation needed]

[edit]Distribution of insolation at the top of the atmosphere

Spherical triangle for application of the spherical law of cosines for the calculation the solar

zenith angle for observer at latitude and longitude from knowledge of the ho ur angle h

and solar declination . ( is latitude of subsolar point, and h is relative longitude of subsolarpoint).
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, the theoretical daily-average insolation at the top of the atmosphere. The calculation

assumed conditions appropriate for 2000 A.D.: a solar constant ofS0 = 1367 W m2

, obliquity

of = 23.4398, longitude of perihelion of = 282.895, eccentricity e = 0.016704. Contour

labels (green) are in units of W m2.

The theory for the distribution of solar radiation at the top of the atmosphere concerns

how the solarirradiance(the power of solar radiation per unit area) at the top of the

atmosphere is determined by the sphericity and orbital parameters of Earth. The theory could

be applied to any monodirectional beam of radiation incident onto a rotating sphere, but is

most usually applied to sunlight, and in particular for application innumerical weather

prediction, and theory for theseasonsand theice ages. The last application is known

asMilankovitch cycles.

The derivation of distribution is based on a fundamental identity fromspherical trigonometry,

thespherical law of cosines:

where a, b and c are arc lengths, in radians, of the sides of a spherical triangle. Cis the angle

in the vertex opposite the side which has arc length c. Applied to the calculation of

solarzenith angle, we equate the following for use in the spherical law of cosines:
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The distance of Earth from the sun can be denoted RE, and the mean distance can be denoted

R0, which is very close to 1AU. The insolation onto a plane normal to the solar radiation, at a

distance 1 AU from the sun, is thesolar constant, denoted S0. The solar flux density

(insolation) onto a plane tangent to the sphere of the Earth, but above the bulk of the

atmosphere (elevation 100 km or greater) is:

and

The average ofQ over a day is the average ofQ over one rotation, or the hour angle

progressing from h = toh = :

Let h0 be the hour angle when Q becomes positive. This could occur at sunrise

when , or for h0 as a solution of

or

cos(ho) = tan()tan()

If tan()tan() > 1, then the sun does not set and the sun is already risen at h = , so ho = . If

tan()tan() < 1, the sun does not rise and .

is nearly constant over the course of a day, and can be taken outside the integral
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Let be the conventional polar angle describing a planetaryorbit. For convenience, let = 0

at the vernalequinox. Thedeclination as a function of orbital position is

where is the obliquity. The conventionallongitude of perihelion is defined relative to thevernal equinox, so for the elliptical orbit:

or

1.

With knowledge of , ande from astrodynamical calculations

[5]

and So from aconsensus of observations or theory, can be calculated for any latitude and . Note

that because of the elliptical orbit, and as a simple consequence ofKepler's second

law,does not progress exactly uniformly with time. Nevertheless, = 0 is exactly the time

of the vernal equinox, = 90 is exactly the time of the summer solstice, = 180 is exactly

the time of the autumnal equinox and = 270 is exactly the time of the winter solstice

.

[edit]Application to Milankovitch cycles

Obtaining a time series for a for a particular time of year, and particular latitude, is a

useful application in the theory ofMilankovitch cycles. For example, at the summer solstice,

the declination is simply equal to the obliquity . The distance from the sun is

Past and future of daily average insolation at top of the atmosphere on the day of the summer

solstice, at 65 N latitude. The green curve is with eccentricity e hypothetically set to 0. The

red curve uses the actual (predicted) value ofe. Blue dot is current conditions, at 2 ky A.D.

For this summer solstice calculation, the role of the elliptical orbit is entirely contained within

the important product , which is known as the precession index, the variation of
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which dominates the variations in insolation at 65 N when eccentricity is large. For the next

100,000 years, with variations in eccentricity being relatively small, variations in obliquity

will be dominant.

[edit]Applications

Inspacecraftdesign andplanetology, it is the primary variable affectingequilibrium

temperature.

In construction, insolation is an important consideration when designing a building for a

particular climate. It is one of the most important climate variables for human comfort and

building energy efficiency.[6]

The projection effect can be used inarchitectureto design buildings that are cool in summer

and warm in winter, by providing large vertical windows on the equator-facing side of thebuilding (the south face in thenorthern hemisphere, or the north face in thesouthern

hemisphere): this maximizes insolation in the winter months when the Sun is low in the sky,

and minimizes it in the summer when the noonday Sun is high in the sky. (The Sun's

north/south paththrough the sky spans 47 degrees through the year).

Insolation figures are used as an input to worksheets to sizesolar power systemsfor the

location where they will be installed.[7]

This can be misleading since insolation figures

assume the panels are parallel with the ground, when in fact they are almost always mounted

at an angle

[8]

to face towards the sun. This gives inaccurately low estimates for winter.

[9]

Thefigures can be obtained from an insolation map or by city or region from insolation tables that

were generated with historical data over the last 3050 years. Photovoltaic panels are rated

under standard conditions to determine the Wp rating (watts peak),[10]

which can then be used

with the insolation of a region to determine the expected output, along with other factors such

as tilt, tracking and shading (which can be included to create the installed Wp

rating).[11]

Insolation values range from 800 to 950 kWh/(kWpy) inNorwayto up to 2,900

inAustralia.

In the fields ofcivil engineeringandhydrology, numerical models of snowmelt runoff useobservations of insolation. This permits estimation of the rate at which water is released from

a melting snowpack. Field measurement is accomplished using apyranometer.

Conversion factor (multiply top row by factor to obtain side column)

W/m2

kWh/(m2day)

sun

hours/daykWh/(m

2y) kWh/(kWpy)
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W/m2 1 41.66666 41.66666 0.1140796 0.1521061

kWh/(m2day) 0.024 1 1 0.0027379 0.0036505

sun hours/day 0.024 1 1 0.0027379 0.0036505

kWh/(m2y) 8.765813 365.2422 365.2422 1 1.333333

kWh/(kWpy) 6.574360 273.9316 273.9316 0.75 1

1. Insolation is the energy received on the earths surface from the sun. It is the most

important single source of atmospheric heat.

2. The earths surface does not absorb all the energy that it receives. The proportion of the

solar radiation reflected from the surface is called Albedo.

3. On an average, insolation is highest near the tropics, marginally lower at the equator and

lowest at the poles.

4. Although the earth receives energy continuously from the sun, its temperature remains

fairly constant, the only variations being the long-term climatic changes.

This is so because the atmosphere loses an amount of heat equal to the gain through

insolation. This mechanism of maintaining the same temperature by the atmosphere is called

the Heat Budget or Heat Balance.

5. Assuming that 100 units of energy reach the top of the atmosphere of the earth, 14 units are

absorbed directly by the atmosphere and 35 units are lost to space through reflection.

The remaining 51 units reach the earths surface and are absorbed by the earth due to which

the surface gets heated. The heated surface of the earth starts radiating energy in the form of

long waves and this process is called Terrestrial Radiation.

Out of the total 51 units given up by the surface in the form of terrestrial radiation, the

atmosphere (mainly carbon dioxide and water vapour) absorbs about 34 units and the

remaining 17 units escape to space.

In this manner, the atmosphere receives a total of 14 + 34 = 48 units and this amount is

radiated back to space by the atmosphere. The total loss of energy to space thus amounts to

100 units: 35 units reflected by the atmosphere, 17 units lost as terrestrial radiation and 48


10/47

units from the atmosphere. In this manner, no net gain or loss of energy occurs in the earths

surface.

6. Although the earth and its atmosphere as a whole have a radiation balance, there are

latitudinal variations. The heat/energy is transferred from the lower latitudes to the higher

latitudes through winds and ocean currents.

Earth's radiation balance is the equation of the incoming and outgoing thermal radiation.

An instrument for measuring the net radiation balance and albedo. Model shown CNR 1.

Courtesy of Kipp & Zonen

The incoming solar radiation is short wave, therefore the equation below is called the shortwave radiation balance Qs:

Qs = G - R = D + H - R or depending on thealbedo(back-reflection to space): = (D+H)(1 - a)

G = global radiation D = direct shortwave radiation H = diffuse shortwave radiation R = reflected portion of global radiation (ca. 4%) a = albedoThe Earth's surface and atmosphere emits heat radiation in the infrared spectrum, called long

wave radiation. There is little overlap between the long wave radiation spectrum and the solar

radiation spectrum. The equation below expresses the long wave radiation balance Ql:

Ql = AE = AO - AG

AE = effective radiation AO = radiation of the Earth's surface AG = trapped radiation (radiation forcing, also known as the so called greenhouseeffect)
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The two equations on incoming and outgoing radiation can be combined to show the net total

amount of radiation energy, total radiation balance Qt:

Qt = Qs - Ql = G - R - AE

The difficulty is to precisely quantify the various internal and external factors influencing the

radiation balance. Internal factors include all mechanisms affecting atmospheric composition(volcanism, biological activity, land use change, human activities etc.). The main external

factor is solar radiation. Thesun's average luminositychanges little over time.

External and internal factors are also closely interconnected. Increased solar radiation for

example results in higher average temperatures and higher water vapour content of the

atmosphere. Water vapour, a heat trapping gas absorbing infrared radiation emitted by the

Earth's surface, can lead to either higher temperatures through radiation forces or lower

temperatures as a result of increased cloud formation and hence increased albedo.

Atmospheric circulation is the large-scale movement of air, and the means (together with

the smallerocean circulation) by whichthermal energyis distributed on the surface of

theEarth.

The large-scale structure of the atmospheric circulation varies from year to year, but the basic

climatological structure remains fairly constant. However, individual weather systems - mid-

latitude depressions, or tropical convective cells - occur "randomly"[citation needed], and it is

accepted that weather cannot be predicted beyond a fairly short limit: perhaps a month in

theory, or (currently) about ten days in practice (seeChaos theoryandButterfly effect).

Nonetheless, as the climate is the average of these systems and patterns - where and when

they tend to occur again and again -, it is stable over longer periods of time.

As a rule, the "cells" of Earth's atmosphere shift polewards in warmer climates

(e.g.interglacialscompared toglacials), but remain largely constant even due tocontinental

drift.Tectonic upliftcan significantly alter major elements of it, however - for example

thejet stream-, andplate tectonicsshiftocean currents. In the extremely hot climates of

theMesozoic, indications of a thirddesertbelt at theEquatorhas been found; it was perhaps

caused byconvection. But even then, the overalllatitudinalpattern of Earth's climate was not

much different from the one today.

The wind belts girdling the planet are organised into three cells: the Hadley cell, theFerrelcell, and thePolar cell. Contrary to the impression given in the simplified diagram, the vast

bulk of the vertical motion occurs in the Hadley cell; the explanations of the other two cells

are complex. Note that there is one discrete Hadley cell that may split, shift and merge in a

complicated process over time[citation needed]

. Low and high pressures on earth's surface are

balanced by opposite relative pressures in the upper troposphere.

[edit]Hadley cell

Main article:Hadley cell
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12/47

TheITCZ's band of clouds over theEastern Pacificand theAmericasas seen from space

The Hadley cell mechanism is well understood. The atmospheric circulation pattern

thatGeorge Hadleydescribed to provide an explanation for thetrade windsmatches

observations very well. It is a closed circulation loop, which begins at the equator with warm,

moist air lifted aloft in equatoriallow pressure areas(theIntertropical Convergence Zone,

ITCZ) to thetropopauseand carried poleward. At about30N/S latitude, it descends in ahigh

pressure area. Some of the descending air travels equatorially along the surface, closing the

loop of the Hadley cell and creating theTrade Winds.

Though the Hadley cell is described as lying on the equator, it is more accurate to describe it

as following the sunszenithpoint, or what is termed the "thermal equator," which undergoes

a semiannual north-south migration.

[edit]Polar cell

Main article:Polar vortex

The Polar cell is likewise a simple system. Though cool and dry relative to equatorial air, air

masses at the60th parallelare still sufficiently warm and moist to undergoconvectionand

drive athermal loop. Air circulates within thetroposphere, limited vertically by thetropopause at about 8 km. Warm air rises at lower latitudes and moves poleward through the

upper troposphere at both the north and south poles. When the air reaches the polar areas, it

has cooled considerably, and descends as a cold, dry high pressure area, moving away from

the pole along the surface but twisting westward as a result of theCoriolis effectto produce

thePolar easterlies.

The outflow from the cell createsharmonicwaves in the atmosphere known asRossby

waves. These ultra-long waves play an important role in determining the path of the jet

stream, which travels within the transitional zone between thetropopauseand theFerrel cell.

By acting as aheat sink, the Polar cell also balances the Hadley cell in the Earths energy

equation.It can be argued that the Polar cell is the primary weathermaker for regions above the middle

northern latitudes. While Canadians and Europeans may have to deal with occasional heavy

summer storms, there is nothing like a winter visit from aSiberianhigh to give one a true

appreciation of real cold. In fact, it is the polar high which is responsible for generating the

coldest temperature recorded on Earth: -89.2C atVostok Stationin 1983 inAntarctica.

The Hadley cell and the Polar cell are similar in that they are thermally direct; in other words,

they exist as a direct consequence of surface temperatures; their thermal characteristics

override the effects of weather in their domain. The sheer volume of energy the Hadley cell

transports, and the depth of theheat sinkthat is the Polar cell, ensures that the effects of

transient weather phenomena are not only not felt by the system as a whole, but exceptunder unusual circumstancesare not even permitted to form. The endless chain of passing
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highs and lows which is part of everyday life for mid-latitude dwellers is unknown above the

60th and below the 30th parallels. There are some notable exceptions to this rule. In Europe,

unstable weather extends to at least70 north.

These atmospheric features are also stable, so even though they may strengthen or weaken

regionally or over time, they do not vanish entirely.[edit]Ferrel cell

The Ferrel cell, theorized byWilliam Ferrel(1817-1891), is a secondary circulation feature,

dependent for its existence upon the Hadley cell and the Polar cell. It behaves much as an

atmospheric ball bearing between the Hadley cell and the Polar cell, and comes about as a

result of theeddycirculations (the high and low pressure areas) of the mid-latitudes. For this

reason it is sometimes known as the "zone of mixing." At its southern extent (in the

Northern hemisphere), it overrides the Hadley cell, and at its northern extent, it overrides the

Polar cell. Just as the Trade Winds can be found below the Hadley cell, theWesterliescan be

found beneath the Ferrel cell. Thus, strong high pressure areas which divert the prevailing

westerlies, such as aSiberian high(which could be considered an extension of the Arctichigh), could be said to override the Ferrel cell, making it discontinuous.

While the Hadley and Polar cells are truly closed loops, the Ferrel cell is not, and the telling

point is in theWesterlies, which are more formally known as "the Prevailing Westerlies."

While the Trade Winds and the Polar Easterlies have nothing over which to prevail, their

parent circulation cells having taken care of any competition they might have to face, the

Westerlies are at the mercy of passing weather systems. While upper-level winds are

essentially westerly, surface winds can vary sharply and abruptly in direction. A low moving

polewards or a high moving equator wards maintains or even accelerates a westerly flow; the

local passage of a cold front may change that in a matter of minutes, and frequently does. A

strong high moving polewards may bring easterly winds for days.

The base of the Ferrel cell is characterized by the movement of air masses, and the location of

these air masses is influenced in part by the location of the jet stream, which acts as a

collector for the air carried aloft by surface lows (a look at a weather map will show that

surface lows follow thejet stream). The overall movement of surface air is from the 30th

latitude to the 60th. However, the upper flow of the Ferrel cell is not well defined. This is in

part because it is intermediary between the Hadley and Polar cells, with neither a strong heat

source nor a strong cold sink to drive convection and, in part, because of the effects on the

upper atmosphere of surface eddies, which act as destabilizing influences.

[edit]Longitudinal circulation features

While the Hadley, Ferrel, and Polar cells are major factors in global heat transport, they do

not act alone. Disparities in temperature also drive a set of longitudinal circulation cells, and

the overall atmospheric motion is known as the zonal overturning circulation.

Latitudinal circulation is the consequence of the fact that incident solar radiation per unit area

is highest at the heat equator, and decreases as the latitude increases, reaching its minimum at

the poles. Longitudinal circulation, on the other hand, comes about because water has a

higher specific heat capacity than land and thereby absorbs and releases more heat, but the

temperature changes less than land. Even atmesoscales(a horizontal range of 5 to several

hundred kilometres), this effect is noticeable; it is what brings the sea breeze, air cooled by

the water, ashore in the day, and carries the land breeze, air cooled by contact with theground, out to sea during the night.
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Diurnal wind change in coastal area.

On a larger scale, this effect ceases to be diurnal (daily), and instead is seasonal or

evendecadalin its effects. Warm air rises over theequatorial,continental, and western PacificOcean regions, flows eastward or westward, depending on its location, when it reaches the

tropopause, andsubsidesin theAtlanticandIndian Oceans, and in the easternPacific.

The Pacific Ocean cell plays a particularly important role in Earth's weather. This entirely

ocean-based cell comes about as the result of a marked difference in the surface temperatures

of the western and eastern Pacific. Under ordinary circumstances, the western Pacific waters

are warm and the eastern waters are cool. The process begins when strong convective activity

over equatorial East Asia and subsiding cool air off South America's west coast creates a

wind pattern which pushes Pacific water westward and piles it up in the western Pacific.

(Water levels in the western Pacific are about 60 cm higher than in the eastern Pacific, a

difference due entirely to the force of moving air.)

[1][2][3]

[edit]Walker circulation

Main article:Walker circulation

The Pacific cell is of such importance that it has been named the Walker circulation afterSir

Gilbert Walker, an early-20th-century director of British observatories inIndia, who sought a

means of predicting when themonsoonwinds would fail. While he was never successful in

doing so, his work led him to the discovery of an indisputable link between periodic pressure

variations in the Indian Ocean and the Pacific, which he termed the "Southern Oscillation".

The movement of air in the Walker circulation affects the loops on either side. Under

"normal" circumstances, the weather behaves as expected. But every few years, the wintersbecome unusually warm or unusually cold, or the frequency of hurricanes increases or

decreases, and the pattern sets in for an indeterminate period.

The behavior of the Walker cell is the key to the riddle, and leads to an understanding of

theEl Nio(more accurately, ENSO or El Nio - Southern Oscillation) phenomenon.

If convective activity slows in the Western Pacific for some reason (this reason is not

currently known), theclimatedominoes next to it begin to topple. First, the upper-level

westerly winds fail. This cuts off the source of cool subsiding air, and therefore the surface

Easterlies cease.

The consequence of this is twofold. In the eastern Pacific, warm water surges in from thewest since there is no longer a surface wind to constrain it. This and the corresponding effects
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of the Southern Oscillation result in long-term unseasonable temperatures and precipitation

patterns in North and South America, Australia, and Southeast Africa, and disruption of

ocean currents.

Meanwhile in the Atlantic, high-level, fast-blowing Westerlies which would ordinarily be

blocked by the Walker circulation and unable to reach such intensities, form. These windstear apart the tops of nascenthurricanesand greatly diminish the number which are able to

reach full strength.

[edit]El Nio - Southern Oscillation

Main article:El Nio-Southern Oscillation

El Nio andLa Nia are two opposite surface temperature anomalies in the Southern Pacific,

which heavily influence the weather on a large scale. In the case of El Nio warm water

approaches the coasts of South America which results in blocking the upwelling of nutrient-

rich deep water. This has serious impacts on the fish populations.

In the La Nia case, the convective cell over the western Pacific strengthens inordinately,resulting in colder than normal winters in North America, and a more robust cyclone season

in South-East Asia and Eastern Australia. There is increased upwelling of deep cold ocean

waters and more intense uprise of surface air near South America, resulting in increasing

numbers of drought occurrence, although it is often argued that fishermen reap benefits from

the more nutrient-filled eastern Pacific waters.

The neutral part of the cycle - the "normal" component - has been referred to humorously by

some as "La Nada", which means "the nothing" in Spanish.

[edit]See also

Chapter 6 - Atmospheric and Oceanic Circulation

Air Pressure and Wind Basics

Air pressure depends on the density and temperature of the air in addition to the altitude.

Air pressure was first measured by Torricelli (1600s), Galileo's pupil.

Barometer (Gk: baros = weight)

- mercury barometer: the weight of the air pushes mercury up an evacuated glass tube

- anneroid barometer (anneroid = without fluid): weight of air pushes in on partiallyevacuated chamber

Atmospheric pressure is commonly measured in inches or milimeters of mercury, or as

milibars or kilopascals of pressure.

Winds are produced bypressure differences between two locations. Wind speedis measured

by ananemometer. Wind speed is recorded in miles per hour (mph), kilometers per hour

(kmph), or knots (1 knot = 1.15 mph). Wind direction is measured by a wind vane. Wind

direction is the direction that the wind is coming from (not the direction it is moving toward).

TheBeaufort Wind Scale was developed in 1806 to estimate wind speed from visual cues atsea. It was modified for using visual cues on land by Simpson in 1926.
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Driving Forces of the Wind

Gravitational Force - compresses the atmosphere

Pressure Gradient Force - air moves from high pressure (more compressed) toward low

pressure (less compressed)

Coriolis Force - air in motion appears to be deflected as Earth spins west to east

Friction Force - drag against the Earth's surface slows the wind; slows surface winds the

most, high level winds the least

- Pressure Gradient Air pressure differences are the result of uneven heating of the

atmosphere. In some areas, stronger heating leads to expansion of air, making it less dense

(fewer molecules and less weight per cubic foot or cubic meter of air). Hot, rising air is

associated with low pressure. Cold, dense, sinking air is associated with high pressure.

Pressure gradient refers to the pressure difference (inches or mm of mercury or milibars)divided by the distance over which the pressure drop occurs. Since wind (moving air) is

caused by pressure differences, the greater the pressure gradient the stronger the wind.

Air pressure maps plot lines of equal pressure called isobars. Isobars are drawn at equal

increments of air pressure. The more closely spaced the isobars are, the greater the pressure

gradient and the stronger the winds.

The pressure gradient force, if it acted by itself (which it doesn't), would produce winds thatmoved at very high speeds at right angles to the isobars, the shortest path from high to low

pressure. But...(see the next two forces)

- Coriolis Force As winds move north or south they are deflected due to the rotation of the

Earth. As the Earth spins on its axis, a person standing on the equator moves from west to

east at around 1000 mile per hour. At the poles, on the other hand, that person would not

move at all, just spin around in place. So, the equator and anything on it moves west to east

faster than any other place on Earth. The west to east motion decreases from the equator to

the pole.

As winds move away from the equator, their west to east momentum carries them to the east

of a true poleward trajectory. In the northern hemisphere they are deflected to the right. In the

southern hemisphere they are deflected to the left. For the opposite case, as air masses movetoward the equator, their west to east momentum lags behind the west to east motion of the

Earth at lower latitude and they curve to the west.In the northern hemisphere moving air

(wind) is deflected to the right. In the southern hemisphere winds are deflected to the left.

The strength of the coriolis force is zero at the equator, half its maximum strength at 30

latitude, and maximum at the poles. Fast winds and winds covering the greatest distances are

deflected the most. In the absence of friction (approximated in the upper atmosphere), the

coriolis force would cause the winds to blow parallel to isobars, in circles, clockwise around

high pressure and counterclockwise around lows in the northern hemisphere. In the southern

hemisphere the effect is the opposite, counterclockwise around highs and clockwise around

lows. These isobar parallel circular winds,geostrophic winds, only occur in the upperatmosphere, away from friction with the Earth's surface.


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- Friction Force Friction with the Earth's surface only affects wind speed up to altitudes

around 500 m (1640 ft). Friction prevents geostrophic winds at low altitude. Rather, low

level winds move at an angle across isobars.

The net effect of these forces is that near surface winds spiral outward away from high

pressure centers and inward toward low pressure centers.

- In the northern hemisphere, where winds are deflected to the right,

winds spiralclockwise around high pressure andcounterclockwise around low pressure

- In the southern hemisphere, where winds are deflected to the left, winds

spiralcounterclockwise around high pressure andclockwise around low pressure

Low pressure centers are zones of convergence, with winds spiralling inward. These are

calledcyclones.

Tornadoes and hurricanes are strong cyclonic storms.

High pressure centers are zones of diveregence, with winds spiralling outward. These are

calledanticyclones.

Primary Gobal Pressure Systems, Atmospheric Circultion, and Climate Belts

Equatorial Low-Pressure - Intertropical Convergence Zone (ITCZ) The equatorial region

is the most strongly heated area on the Earth. It is there that we find the most vigorous

upward convection. Low pressure is found all along the equator. Winds converge on the

intertropical convergence zone from the northeast (northeast tradewinds) and the

southeast (southeast tradewinds). The trades are fairly strong and consistent. Right at the

ITCZ winds are weak and variable. Sailors in the days of sailing ships called thisthedoldrums.


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Subtropical Highs At latitudes around 25 to 30 north and south of the equator there are

several more-or-less continuous and stationary centers of high pressure. For example

theBermuda High (or Azores High) in the north Atlantic Ocean, thePacific High (or

Hawaiian High) in the north Pacific, and highs over the south Atlantic, south Pacific, and

south Indian oceans. Winds diverging from these highs toward the equator form the

tradewinds. Winds diverging from the highs towards the poles are deflected to the east innorthern and southern hemispheres forming the prevailing westerlies in midlatitudes.

Subpolar Lows A series of low pressure centers encircle Antarctica summer and winter. In

the northern hemisphere, theIcelandic Low in the north Atlantic andAleutian Low in the

north Pacific spawn cyclonic storms in winter but weaken or die out in summer as the

subtropical highs strengthen in the north Atlantic and north Pacific.

Polar Highs High pressure dominates in the polar regions because the air is very cold and

dense. Antarctica is the coldest place on Earth because it lies over the south pole, because it

is continental, and because the ice sheet is very thick and so the surface elevation is also

high. High pressure dominates Antarctica year-round. The north pole, however, lies in theArctic Ocean. The ocean has a moderating effect on the arctic. High pressure is less well

developed in the north polar summer, but devlops over land as the Canadian and Siberian

Highs in winter.

Climate Belts Simplified

It is commonly known that when materials (such as atmospheric gases) are heated they

expand thereby becoming less dense or "lighter." In a room with a radiator or other such

heater, as the air around the radiator becomes heated it expands and rises to the ceiling and is

replaced by cooler denser air. This is an example ofconvection. A similar process occurs

near the coast in summer. Land heats up faster during the day than the water does. Air over

the land is heated, expands, and rises. It is replaced by cooler, denser air from over the water

forming a coolsea breeze. Convection also occurs on a global scale driven by the uneven

heating of the Earth.

equatorial: hot & wet

The equatorial regions are the most strongly heated areas on the Earth's surface. It is there

that we find the most vigorous upward convection. Hot air is capable of holding much water

vapor. Hot, humid air rises over the equator. As it rises to high altitude it expands because the

air pressure decreases (there is less mass of air above it). As air expands due to thisdecreasing pressure, it also cools. Since cool air is able to hold less water vapor than warm

air, condensation occurs. This is why the equatorial regions normally have very high rainfall.

It is here that we find tropical rainforests such as those in the Amazon, Congo, and Indonesia.

Areas of upward convection are dominated by low atmospheric pressure.

Atmospheric Convection and Rainfall

desert belts: hot & dry

The rising air at the equator is replaced by low-level air from higher latitudes north and south

of the equator. To balance the air moving toward the equator at low altitude, the convecting
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air moves away from the equator, toward the north and south, at high altitude. It is now cool

because of expansion and dry because it has dropped off excess moisture. To complete the

convection loop, in the regions around 25 degrees north and 25 degrees south of the equator,

this cool and dry air descends back to the surface (subtropical highs). As it descends, the

pressure increases (because there is now more air overhead). The increased pressure increases

the temperature of the air and therefore increases the capacity of the air to hold water vapor.Now the air is very dry and has the capacity to soak up much evaporation. Consequently,

these latitudes are very dry with high evaporation and low rainfall. These are the desert belts,

including the Sahara, Mojave and Sonoran deserts of the U.S. southwest and Mexico, the

Kalahari and Namib in southern Africa, the Australian desert, and the Atacama Desert on the

west coast of South America. Areas of descending air are dominated by high atmospheric

pressure.

midlatitude: temperate - cool and moist

The midlatitudes are a battleground between very cold, dense polar air and warm air moving

poleward from the subtropical highs. The boundary between them is called thepolarfront. The polar front is an undulating boundary. The undulations are calledRossby Waves.

In the midlatitudes, these undulations are sites where cold air pushes equatorward and warm

air pushes poleward. Cyclonic circulation (convergence, counterclockwise in northern

hemisphere) develops around the southward bulges of the polar front. As these undulations

of the polar front sweep southward and eastward (northern hemisphere) cold dense air pushes

under warmer, less dense air. As the warmer air rises, it expands, cools, and condenses some

of its water vapor. That is why cold fronts bring clouds and rain. During the summer the

polar front lies farther north and we seldom see summer cold fronts on Long Island.

polar: cold & dry

The polar regions are the coldest on Earth. The air is very cold, dense, and dry. High

pressure dominates. Much of Antarctica is essentially a desert because so little precipitation

falls (though what does fall remains frozen).

Ocean Circulation

surface currents Wind drives both waves and surface currents in the oceans. Warm surface

waters at the equator are driven westward by the easterly trade winds. When the equatorial

currents reach the western edge of the ocean basin (east coast of some continent) they are

diverted to the north and south along the continents. In the central Atlantic this northwardflowing branch is called the Gulf Stream. It carries warm water from the equator, Caribbean

and Gulf of Mexico, northward along the east coast of North America, across the North

Atlantic, and to arctic Ocean off northwestern Europe. The westerlies help to drive the Gulf

Stream eastward across the Atlantic toward Europe. The warmth of the Gulf Stream

moderates the climate of northwestern Europe. The waters in the North Atlantic cool. The

cooled waters then flow southward along the coast of Europe and Africa. This southern

current is called the Canary Current. It brings cool waters down to northwest Africa and

keep the coast here relatively cool. Eventually this current approaches the equator where the

waters warm again and the easterly winds drive them westward across the Atlantic again to

start another clockwise loop. Such large surface current loops, calledsubtropical gyres, are

found in all the open ocean basins. They generally flow clockwise in the northern hemisphereand counterclockwise in the southern hemisphere.


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deep ocean currentsAs water chills in the North Atlantic it becomes more dense. Also, when

the cold water begins to freeze to form sea ice the ice that forms is from pure water; the salt is

left behind in the remaining sea water. The sea water gets saltier. The saltier the water the

denser it becomes. These cold, salty, dense surface waters sink down to the bottom of the

Atlantic. The sinking waters are replaced by less dense surface waters from the south. The

sinking waters flow southward along the bottom of the ocean as surface waters flownorthward to replace them. The North Atlantic Deep Water continues south until they meet a

northward flowing bottom current of even denser waters that formed off the coast of

Antarctica. The North Atlantic Deep Water then rides up above the Antarctic Bottom Water

and continues southward at intermediate depths until they eventually rise to the surface near

Antarctica. From there they follow other currents that carry them throughout the oceans.

ocean conveyor belt There is an ocean conveyor belt that mixes waters through all the ocean

basins from the sea bottom to the sea surface, connecting the surface, intermediate, and

bottom water currents. This mixing moves heat, dissolved gases, and nutrients through the

oceans in one grand cycle. Breakdown of this conveyor belt may have been responsible for

sudden changes in the Earth's climate in the past.

global warming and sudden cooling in Europe? The Earth is warming, largely due to the

release of greenhouse gases from industrial and agricultural activity. As a result of the

warming, the rate of melting and release of icebergs from Greenland into the far north

Atlantic and Arctic Ocean is increasing. The increased influx of fresh water into the sea willmake these surface waters less dense which could slow or stop the formation of deep water


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(sinking of surface water). This should slow the northward movement of the Gulf Stream.

Because heat carried into the far north Atlantic helps to moderate the climate of densely

populated northwestern Europe, a weakening or cessation of the Gulf Stream would cause

major social, agricultural, and economic problems. Geologic evidence has shown that this has

happened very rapidly in the past yielding a sudden cooling of northwestern Europe within

about two decades time.

Climate change refers to a statistically significant variation

in either the mean state of the climate or in its variability,

persisting for an extended period (typically decades or

longer). Climate change may be due to natural internal

processes or external forcings, or to persistent anthropogenic

changes in the composition of the atmosphere or in land use.

The Earth is the only planet in our solar system that supports life. The complex process ofevolution occurred on Earth only because of some unique environmental conditions that were

present: water, an oxygen-rich atmosphere, and a suitable surface temperature.

Mercury and Venus, the two planets that lie between Earth and the sun, do not support life.

This is because Mercury has no atmosphere and therefore becomes very hot during the day,

while temperatures at night may reach -140C. Venus, has a thick atmosphere which traps

more heat than it allows to escape, making it too hot (between 150 and 450 C) to sustain

life.

Only the Earth has an atmosphere of the proper depth and chemical composition. About 30%

of incoming energy from the sun is reflected back to space while the rest reaches the earth,

warming the air, oceans, and land, and maintaining an average surface temperature of about

15C.

The chemical composition of the atmosphere is also responsible for nurturing life on our

planet. Most of it is nitrogen (78%); about 21% is oxygen, which all animals need to survive;

and only a small percentage (0.036%) is made up of carbon dioxide which plants require for

photosynthesis.

The atmosphere carries out the critical function of maintaining life-sustaining conditions on

Earth, in the following way: each day, energy from the sun (largely in the visible part of the

spectrum, but also some in the ultraviolet, and infra red portions) is absorbed by the land,

seas, mountains, etc. If all this energy were to be absorbed completely, the earth would

gradually become hotter and hotter. But actually, the earth both absorbs and, simultaneously

releases it in the form of infra red waves (which cannot be seen by our eyes but can be felt as

heat, for example the heat that you can feel with your hands over a heated car engine). All

this rising heat is not lost to space, but is partly absorbed by some gases present in very small

(or trace) quantities in the atmosphere, called GHGs (greenhouse gases).

Greenhouse gases (for example, carbon dioxide, methane, nitrous oxide, water vapour,ozone), re-emit some of this heat to the earth's surface. If they did not perform this useful


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function, most of the heat energy would escape, leaving the earth cold (about -18C) and

unfit to support life.

However, ever since the Industrial Revolution began about 150 years ago, man-made

activities have added significant quantities of GHGs to the atmosphere. The atmospheric

concentrations of carbon dioxide, methane, and nitrous oxide have grown by about 31%,

151% and 17%, respectively, between 1750 and 2000 (IPCC 2001).

Variations of the Earth's surface temperature for the past 140

years

The Earths surface temperature is shown year by year (red bars)

and approximately decade by decade (black line, a filtered annualcurve suppressing fluctuations below near decadal time-scales).

There are uncertainties in the annual data (thin black whisker bars

represent the 95% confidence range) due to data gaps, random

instrumental errors and uncer

12 air sea interactions

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