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INTER-TROPICAL CONVERGENCE ZONE (ITCZ)

SUMMARY NOTE:

ITCZ– A zone of convergence is where winds meet– Converging winds include the trade winds which meet at the

equator – called the ITCZ– Associated low pressure: calm, warm conditions

EFFECT OF THE EARTH’S ORBIT ON THE ITCZ– Latitude receiving the most intense heating from the sun– This areas moves between the tropics due to the Earth’s orbit– The area of maximum heating is known as the thermal equator– Sun overhead at the Tropics at the solstices (once in N.

Hemisphere and once in the S.)– Sun directly over the equator twice a year – equinoxes– This migration of the thermal equator shifts the belts of winds and

pressure systems north and south over the year

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INTER-TROPICAL CONVERGENCE ZONE (ITCZ)

SUMMARY NOTE (Cont.):

EFFECT OF LAND AND SEA MASSES ON ITCZ– Rates of heating vary greatly over land and sea– Temperature extremes are greater over land – it heats and cools

at a faster rate– This means that the ITCZ (zone of low pressure) moves at a

different rate around the world– As a result the ITCZ moves further N and S over land than sea

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INTER-TROPICAL CONVERGENCE ZONE (ITCZ)

CASE-STUDY – EQUITORIAL & SAVANNA REGIONS OF AFRICA

Main climatic regions:

1. Equatorial Climate

2. Tropical Climate

3. Hot Deserts

These zones merge into one another

Key difference is the rainfall rather than temperature

Therefore seasons tend to be wet season and dry season rather than

winter and summer

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23º

The most intense heating of the sun, occurring at the so-called thermal equator, annually moves

between the tropics.

On or around June 20th each year the sun is overhead at 23½ºN, the Tropic of Cancer.

On or around December 20th the the sun is at overhead at 23½ºS, the Tropic of Capricorn.

These two dates are the solstices.

Twice a year, at the equinoxes, on or around March 20th and September 20th the overhead

sun crosses the equator.

This annual north to south and back again "shift" of the thermal equator shifts the belts of

planetary winds and pressure systems to the north and to the south as the year turns.

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0ºEQUATOR

23½ºNTROPIC of CANCER

23½ºSTROPIC of CAPRICORN

December Winter Solstice

September Autumn Equinox

June Summer Solstice

March Spring Equinox

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ITCZ

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ITCZ JULYITCZ

JANUARY

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The location of the ITCZ varies throughout the year The ITCZ over land moves farther north or south than the ITCZ over the oceans

due to the variation in land temperatures.

ITCZ JULYITCZ JANUARY

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http://www.cla.sc.edu/geog/faculty/carbone/modules/newmods/africa-itcz/

The blue shading on the map shows the areas of highest cloud reflectivity,

which correspond to the average monthly position of the ITCZ.

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The migration of the inter-tropical convergence zone (ITCZ) in Africa affects seasonal precipitation patterns across that continent. http://www.bbc.co.uk/scotland/learning/learningzone/clips/538/

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DESERT

SAVANNA

RAINFOREST

dry all year

ITCZ moves north in summer

dry ‘winter’wet ‘summer’

wet all year

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Tropical rainforest

savanna

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The further North of the Equator in tropical Africa:-

the lower the annual rainfall

the more the rainfall is concentrated in the summer months

the more variable the rainfall.

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0º 20ºN10ºN

RAINFORESTGUINEA

SAVANNASAHEL

SAVANNADESERT

rainfall decreases

seasonality increasesvariability increases

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LAGOS SOKOTO TIMBUKTU

A B C A B C A B C

JAN 27 28 65 20 0 13 22 0 22

FEB 29 41 69 22 0 13 24 0 19

MAR 29 96 72 31 0 11 28 2 18

APR 28 143 72 34 10 17 32 0 15

MAY 28 274 76 33 51 31 35 5 18

JUN 26 460 80 30 89 41 36 23 31

JUL 25 282 80 28 147 55 32 79 45

AUG 25 69 76 26 236 64 30 81 57

SEP 25 140 77 27 145 59 32 38 45

OCT 26 208 76 28 13 37 31 3 23

NOV 27 69 72 27 0 18 28 0 17

DEC 28 25 68 25 0 15 22 0 19

annual 27 1835 74 28 691 31 29 231 27

A = average monthly temperature C

B = mean monthly rainfall mmC = % relative humidity at midday

LAGOS

SOKOTO

TIMBUKTU

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savanna savanna climateclimate

tropical summer rain

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savanna savanna vegetationvegetation

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savanna ‘parkland’

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savanna ‘parkland’

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savanna ‘parkland’

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savanna ‘parkland’

23baobab tree

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acacia tree

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acacia thorns

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desertification

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INTER-TROPICAL CONVERGENCE ZONE (ITCZ)

CASE-STUDY – EQUITORIAL & SAVANNA REGIONS OF AFRICA

Using pages 19 26

Answer questions 4 6 on page 39 under the heading ‘Tropical Africa:

Climate’

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3 Marks

Name the air masses

Describe where they come from

Give their characteristics

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Air Mass A: Tropical Continental or cT Origin: Over the Sahara Desert (ie large land mass in tropical latitudes) Nature: Warm, dry, stable air

Air Mass B: Tropical Maritime OR mT Origin: Over Atlantic Ocean (Gulf of Guinea) in tropical latitudes Nature: Warm, moist, unstable air

Allow one mark for each correctly named air mass,origin and nature. Candidates who incorrectly name (or omit the name) of the air mass can still gain credit for the correct

origin and/or nature of the air mass.

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12 Marks

Describe the climate graphs: i. Highest rainfall ii. Least rainfall iii. Rain all year iv. One peak or two

Give figures

Explain why this happens.

Match movement of ITCZ to rainfall i. Dry = cT ii. Wet = mT iii. Peak Rainfall = ITCZ overhead

Why does the ITCZ move?

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Description should be able to focus on the marked contrasts in rainfall amounts between a very dry north (with only 250 mm per annum) and a much wetter south (where some coastal areas receive over 2000 mm per annum) as shown on the map provided. Candidates could also refer to the graphs and note the variations between the three stations with Tombouctou in the north and Jos “in the middle” experiencing obvious wet and dry seasons whilst coastal Lagos in the south has a vastly greater annual rainfall total, no dry months and a “twin peak” regime.

Explanation ought to concentrate on the role of the I.T.C.Z and its associated Tropical Maritime air mass (warm, moist and unstable) and Tropical Continental air mass (warm, dry and stable). For example, Lagos – On the coast of the Gulf of Guinea – is influenced by warm moist Tm air for Most of the year. This accounts for its much higher annual rainfall total. The twin rainfall peaks can be attributed to the I.T.C.Z migrating northwards and then southwards again later in the year, following the overhead sun or thermal equator. Tombouctou, in contrast, lies well to the north of the I.T.C.Z in January and is under the influence of the hot, dry Tc air from the Sahara Desert. In May/June the I.T.C.Z. moves north bringing moist Tm air and rainfall to Jos and, to a much lesser extent, Tombouctou which lies closer to its point of maximum extent.