Assess the relative roles of sub-tropical anticyclones and the inter-tropical convergence zone in creating the characteristic features of the climate of one tropical region that you have studied. (40 marks)
The area that is affected by subtropical anticyclones and the inter-tropical convergence zone (ITCZ) that we studied was the transitional climate of Kano in Nigeria. The wet dry/climate of Kano is split into 6-month periods, November to April is the dry season and March to October is the wet season. The dry season sees no more than 20mm of rainfall in one month whereas the wet season can bring more than 260mm of rainfall in the same time period. The wet season also receives 95% of the annual rainfall. Kano is located at 12 degrees north of the equator and is around 630m above sea level. Temperatures range from 26 degrees to 34 degrees all year round and the annual precipitation is 920 mm of rainfall. This wet dry climate is due to many effects, most of them caused by subtropical and anticyclone and the movement of the ITCZ. In this essay I will assess factors that create Kano’s wet dry climate.
In Kano temperatures are high throughout the year due to the sun being overhead for many months. The angle of the sun affects the amount of atmosphere the sun's radiation has to pass through. This then determines the amount of radiation that reaches the earth's surface. When the sun is directly overhead and radiation travels through least amount of atmosphere enroute to the earth's surface temperatures are higher. However, there is a short cooler season (in comparison with the equatorial climate) in tropical continental areas Like Kano during the summer months due to two factors. Firstly, this is the time of maximum rainfall and the increased cloud cover reduces incoming solar radiation. Secondly, during the summer months the sun is not directly overhead in the northern hemisphere (it is overhead further north at the Tropic of Cancer in the Northern Hemisphere, June 21st). This increases the amount of atmosphere that the sun's radiation has to travel through reducing the amount of heating and also increases the land area being heated as the insolation is spread over a larger area.
Rainfall is the most important element in the climate ofLKano. The amount and seasonal distribution of rainfall in Kano is largely determined by fluctuations in the position of two important air masses and their associated wind systems. The tropical continental air mass (Tc) originates over the Sahara Desert, and consequently is warm and dry. Associated with the Tc air mass are easterly or northeasterly winds, which is known as the Harmattan, which have a drying influence on the areas over which they pass. The tropical maritime air mass (Tm) originates over the Atlantic Ocean to the south of the equator, and consequently is warm and moist. Associated with the Tm air mass are moisture-laden winds called the Southwest Monsoon.
The migration pole ward of the ITCZ during the summer months brings rainfall to this area of West Africa (Kano in Nigeria) giving it a wet season. Rainfall results from the moist unstable tropical maritime (Tm) air brought in by the Southwest Monsoon winds from the Atlantic Ocean. Areas at the pole ward limit of ITCZ movement are only briefly affected (i.e. latitude 20°N), thus they have only a brief wet season and low annual rainfall totals. Nearer the equator the wet season lasts whilst the ITCZ is pole ward, and the area is under the influence of the Tm air mass brought in by the Southwest Monsoon winds. Maximum rainfall occurs with the passage pole wards of the ITCZ and on its return. The migration equator wards of the ITCZ during the winter months brings the hot, dry desert Harmattan winds across this area of West Africa giving it a dry season. Rainfall is restricted to the far south of the area and results from the moist unstable Tm air brought in by the Southwest Monsoon winds from the Atlantic Ocean (e.g. Calabar in Nigeria). Areas way to the north have the longest dry season due to the drying influence of the Harmattan most of the year and due to the fact that the moist Southwest Monsoon winds are pushed back way to the south (e.g. Atar in Mauritania).

So, the alternating wet and dry seasons in West Africa are directly related to the positions of the Tc and Tm air masses. When the Tm air mass moves over the area in the summer months it is hot and wet, with winds blowing from the south, and when the Tc air mass moves over the region in the winter months it is hot and dry, with the winds blowing from the north. But what causes these two air masses to move over the region at different times?

Surface winds always blow from areas of high pressure to areas of low pressure. A band of high pressure can be seen along the line of the Tropic of Capricorn (23.5 S) and another just north of the Tropic of Cancer (23.5 N), these pressure systems are the subtropical highs. In between these two areas of high pressure is an area of low pressure occurring roughly along the line of the equator, hence generally termed equatorial low pressure. The relative movement of the surface winds over Kano is toward the equator. The winds blowing from the high pressure in the north are part of the Tropical continental air mass (Tc) and the winds blowing from the south are part of the Tropical maritime air mass (Tm). The low-pressure area where these two air masses (Tc & Tm) meet is known as the Inter-Tropical Convergence Zone (ITCZ). The location of the ITCZ is linked to intense heating by the overhead sun. Over the equatorial regions the sun is directly overhead for most of the year and heats this area up more than any other. This intense heating of the land surface causes the air directly above to be warmed up, causing it to eventually rise. Rising air forms a low-pressure area beneath it. This is the low pressure that the surface winds flow into from north and south. However, the position of the ITCZ is not stationary, but fluctuates slowly throughout the year, following with a lag of a month or two. For example In July the ITCZ is much further south than in the January.

In conclusion the seasonal wind patterns, variations in temperature and the alternate wet and dry seasons of the tropical continental climate of Kano are all caused by the influence of the Tc and Tm air masses. These two air masses move over Kano due to the way air moves from areas of high pressure to areas of low pressure. In the winter the low pressure is situated over the equator or south of the equator, causing the dry Tc winds from the Sahara to influence Kano. In the summer months the low-pressure area is situated further north (about 20°N) towards to Tropic of Cancer, allowing the wet Tm winds from the Atlantic to influence Kano. The area of low pressure where the winds converge form the surrounding high-pressure system is known as the Inter- tropical convergence zone (ITCZ). The ITCZ is associated with the heating from the overhead sun. Intense heating from the overhead sun heats the ground up causing convectional uplift to occur. This uplift causes a low-pressure area to develop at the ITCZ and follows the movement of the sun through the year. So finally why does the sun seem to move through the year?

It is the tilt of the Earth (23.5°) on its axis, which causes the position of the overhead sun to move during the year. As the Earth circles the sun for one half of the year the northern hemisphere is tilted towards the sun giving it its summer months. For the other half of the year the northern hemisphere is tilted away from the sun giving it its winter. The net effect is that the overhead sun seems to move from the Tropic of Cancer to the Tropic of Capricorn over a year. See the diagram below. This causes the pressure belts to move which in turn creates transitional climates with seasonal rainfall patterns such as the tropical continental climate of West Africa.