Area Climatology — Page 422, Lesson 381

I want to walk you through the area climatology of Northwest Africa, starting with the winter season north of 20°N. This is a region where the weather is shaped by a battle between high pressure and invading low-pressure systems. North of 20°N during winter, we have high pressure sitting over the Sahara and extending to the west. But that high pressure can be modified—meaning it gets disrupted—by encroaching polar front lows and their associated cold fronts. These are low-pressure systems that form along the polar front, the boundary between cold polar air and warmer mid-latitude air. When those cold fronts push in, they bring onshore westerlies or northwesterlies to the coasts of Mauritania and Morocco. "Onshore" means the wind blows from the sea onto the land. Now, here's an important detail: those winds pass over the cold Canaries Current. The Canaries Current is a cool ocean current flowing southward along the northwest coast of Africa. As the westerlies or northwesterlies travel over that cold water, they bring cold fronts with low cloud and precipitation—and this is what defines the wet season for those coastal areas. Cold fronts from the Mediterranean can also affect northern Algeria, but they are prevented from moving further south by the Atlas Mountains. So the mountains act as a barrier. Elsewhere in the region, away from those frontal influences, the prevailing wind is the NE Harmattan. The Harmattan is a dry, dusty wind that blows from the northeast. It traverses the area, bringing dry, dusty conditions. At times, outflow to the north—air flowing out from the high pressure toward the north—will produce the dusty but dry Scirocco wind to the Mediterranean. So Scirocco is another wind name you'll encounter: it's dusty and dry, and it flows toward the Mediterranean from this region. Now let's move to the summer season south of 20°N. This is driven by the movement of the ITCZ—the Intertropical Convergence Zone. The ITCZ is a belt of low pressure where the trade winds from the northern and southern hemispheres meet. During spring, the ITCZ advances northwards across the region. As it passes, the NE Harmattan wind veers through east to become the SW Monsoon wind. Let me explain that term "veer." In meteorology, veer means the wind changes direction clockwise. So the Harmattan, which is from the northeast, turns clockwise through east—meaning it becomes easterly first—and then continues veering until it becomes southwesterly. That southwesterly wind is the SW Monsoon. Why does it become southwesterly? The excerpt tells us the SW direction results from the SE trades—the southeast trade winds—which have crossed the equator and have therefore veered. When air from the southern hemisphere crosses the equator, it veers—turns clockwise—and becomes a southwesterly flow in the northern hemisphere. The SW Monsoon brings the wet season. It brings much cumulus cloud—CU—and cumulonimbus cloud—CB—along with heavy rain showers and thunderstorms. So this is a very active, convective weather pattern. Then in autumn, the ITCZ recedes southwards. As it passes back, the SW Monsoon backs through east to become the dry, dusty NE Harmattan once more. "Back" is the opposite of veer: it means the wind changes direction anticlockwise. So the wind turns from southwest back through east to northeast. Note this important point: the surface wind velocity—abbreviated W/V—is easterly at each ITCZ passage. That means whether the ITCZ is advancing north in spring or receding south in autumn, at the moment the ITCZ passes over a location, the surface wind is from the east. Finally, let's look at summer north of 20°N. In summer, the winter Sahara High has moved north to the Mediterranean region. The outflow from this high gives NE dusty winds flowing south toward the ITCZ, and also flowing beyond the west coast. Over the cold Canaries Current, that outflow can cause advection sea fog to form. Advection fog forms when warm, moist air moves—is advected—over a colder surface. That fog can then be drawn inland by sea breezes—the local daytime wind that blows from the sea onto the land. So to summarise the big picture: the seasonal shift of the ITCZ and the movement of the Sahara High drive a complete reversal of wind and weather between winter and summer, with the Harmattan dominating in winter north of 20°N and the SW Monsoon bringing thunderstorms south of 20°N in summer.