Area Climatology — Page 412, Lesson 379

I want to walk you through the area climatology of Northwest Africa. This is a region that stretches from the Mediterranean Sea in the north, down to the coast of Nigeria, Ghana, and Senegal in the south — so we're talking roughly between 35° north and 5° north, and west of 10° east. The region includes parts of Ivory Coast, Guinea, Liberia, Mauritania, Morocco, Mali, and Algeria. Let's start with the geographical setting. The area is bounded to the east by the Sahara Desert, centred near 23° north. That desert is a source of Tropical Continental air — very hot, very dry air — and it brings a lot of dust into the region. Along the Atlantic coast, we have the cold Canaries sea current, which runs southward close to the coast. That cold current helps advection fog to form — advection fog being fog that forms when warm, moist air moves over a colder surface, in this case the cold sea current. Now let's look at the pressure systems that drive the weather here. The ITCZ — the Intertropical Convergence Zone, also called the equatorial trough — traverses the southern half of the region. As it passes, it brings rain and a change in surface wind velocity, which we abbreviate as W/V. In January, the ITCZ is south of the coastal regions of Ghana and Nigeria. Then it pushes north to between 18° and 20° north in July, and after that it moves south again, clearing the south coast by the next January. So it's a seasonal north-south migration. North of the ITCZ lies the Subtropical High. In winter, this high pressure extends from the west across the Sahara Desert, and the surface outflow from it brings dry, dusty conditions to all parts of the region, especially the south and west. As summer approaches, the subtropical high and its associated dry dusty conditions become increasingly restricted northwards, because the ITCZ is advancing from the south. For weather and surface winds, it's convenient to split the region into two areas at the mid-latitude of 20° north. The southern region includes Dakar on the west coast at 15° north. Let's focus on the area south of 20° north during the winter season. In winter, the ITCZ is south of the area. High pressure is dominant over the Sahara, and there is no cloud or precipitation. The northeast trade wind outflow from the Sahara, blowing toward the ITCZ, is extremely dusty, and this wind is known as the Harmattan. The duration of the Harmattan period decreases as you go southwards — that's because the ITCZ recedes southwards in autumn, then advances north in spring. Visibility in the dust is frequently down to 4000 metres, and occasionally it drops all the way down to fog limits — that's below 1000 metres. Additionally, the outflow over the cold Canaries sea current favours advection sea fog, which can then drift inland when there is a sea breeze. So to summarise: the key players here are the ITCZ bringing rain as it moves north in summer, the subtropical high bringing dry dusty conditions in winter, the Harmattan wind carrying dust from the Sahara, and the cold Canaries current producing advection fog along the coast.