Area Climatology — Page 422, Lesson 383

I want to walk you through the area climatology of two major regions: West Africa and the North Atlantic. Let's start with West African tornadoes. These tornadoes occur in the southeast of the West African area, specifically over the Southern Nigerian valleys. The conditions that produce them are moist air combined with strong surface heating. Now, these are not the classic rotating supercell tornadoes you might think of from the US Midwest — in this context, they are thunderstorms that form in a north/south line above those valleys. The trigger for their formation is the movement of the Intertropical Convergence Zone, or ITCZ. The ITCZ passes northbound in March and April, and then southbound in September and October. At these times, the wind is temporarily from the east. Because the wind is from the east, the storms are carried westwards, affecting other coastal countries before they pass out over the Atlantic. Let's move on to upper winds over Northwest Africa, starting with winter conditions. In winter, the ITCZ, with light easterly winds aloft, lies well to the south. Over land, you get light westerly winds in the south. As you go further north, those westerlies increase into the westerly subtropical jet stream, which reaches 100 knots or more over Morocco. Now for summer conditions. In summer, the ITCZ, again with light easterlies aloft, affects the south of the region. In the north, the wind becomes light westerly only. Notice the key difference: the subtropical jet has moved out of the area entirely — it shifts to Bordeaux. Let's cover the tropopause and freezing level for this region. The tropopause averages 54,000 feet throughout the year. The freezing level averages 15,000 feet throughout the year. Both are constant year-round. Now we shift to the North Atlantic Region. Let's define the geographical area first. The area considered reaches from 10° North latitude up to 70° North latitude. It stretches from the Caribbean and New York in the west, to London and the Norwegian Sea in the northeast. This area lies across two major climatic belts: the Disturbed Temperate belt and the Subtropical High belt. Let's look at winter pressure systems. I'll list the key pressure centres you need to know. First, the North American High, with a central pressure of 1020 hectopascals. Second, the Icelandic "Statistical" Low, at 1000 hectopascals. Third, the Azores High at 30° North, also at 1020 hectopascals. And fourth, Polar Air Depressions, which occur between 65° North and 55° North. Now, how do these systems interact? Polar front activity is dominant across the disturbed temperate region. In the west, air diverging from the North American High moves southeastward over the sea. There, it meets warm Tropical maritime air, which we call Tm air, overlying the warm Gulf Stream waters flowing northbound off the North American East Coast. This convergence of air masses causes much instability and the formation of depressions where the two air masses meet. This well-defined but erratic frontal line forms the western end of the polar front. In winter, that polar front lies near SW Florida and stretches across the Atlantic. These depressions are then driven east/northeastwards by the thermal mid-latitude winds. They track along the polar front towards the UK and the Norwegian Sea. Some of these lows become slow-moving and/or occluded between southern Greenland and Norway. As the depressions pass by, they give rise to what we call the "Statistical" Low near Iceland — it's called statistical because it's a mean low-pressure area created by the frequent passage of these depressions. In the eastern Atlantic, the northeastward outflow from the subtropical Azores High ensures that the travelling frontal depressions track to the northeast. A typical winter landfall for these systems is SW England. That covers the winter pressure patterns and the life cycle of the depressions that affect the North Atlantic region.