A low (or depression) is a pattern of reduced pressure, leading to rings of isobars about the point of lowest pressure. The circular pattern was recognised by Heinrich Dove in 1828 and led to the name 'cyclone', which comes from the Greek for a coiled snake.
We will discuss various kinds of low in this section, at the risk of seeming to imply that they are distinct bodies of air. They are not. Air masses and winds are real, but a low is simply part of a pattern. The movement of a low in the atmosphere is like a ripple across a wheatfield: the wheat does not shift position, except temporarily. As a low moves within the westerly winds, its rotation drives air ahead towards the pole, air behind in the direction of the equator, it accelerates air on the equatorward side and slows it down on the poleward side.
Lows occur at any level. For instance, they occur automatically within a clockwise meander in winds at 300 hPa level in the southern hemisphere (Figure 12.15). Any low, aloft or near the surface, becomes a cut-off low once the meander has become so extreme that the rotation forms a complete whirl.
Here we are primarily concerned with surface lows, i.e. minima in the MSLP pattern. They are caused by a reduced weight of air above (Note 1.G), usually because of warmth. There are several possible arrangements:
1 There is warm air just above the surface in the case of heat lows. These form within cT air masses over the southern continents in summer, and remain almost stationary over the hot land. They are more shallow and are weaker than midlatitude lows, i.e. the lowering of pressure below normal is relatively slight. Heat lows are not associated with any front or with a jet stream aloft. Typically the tropopause above them is as high as 16 km, and flat.
There are often two distinct heat lows over northern Australia in summer, helping form part of the ITCZ and driving moist air and monsoonal rains inland from the Timor Sea, lying to the north (Section 12.1). The heat lows are, respectively, the Cloncurry low over Queensland and the Pilbara low at about 22°S in West Australia (Figure 13.11). The Pilbara low directs hot dry winds from inland towards south-west Australia, which is why Perth experiences hot arid summers. The lows may either weaken or amplify, and they move around, depending on the tracks of fronts to the south and tropical cyclones to the north, but their preferred positions can be seen in the December diagram of Figure 13.11. They are generally connected together into a single elongated trough across northern Australia. Sometimes it extends along the west coast of Australia, bringing extremely hot easterlies towards Perth, exceeding 40°C on occasion. Troughs of this kind are not found along the west coasts of Southern Africa and South America.
2 A frontal disturbance produces a deep low in its mature stage, normally between 45-60°S. Such frontal lows (or extra-tropical lows) are unusually deep in the southern hemisphere (Figure 1.8) and result from a lowering of the
Figure 13.11 Pressure patterns (and hence winds) for Australia in December (summer) and July (winter), showing heat lows in the north in summer and a high over the continent in winter. A complex frontal disturbance affects south-east Australia in the July chart. The low in the Great Bight in December is unusual: a high there is more common.s
Figure 13.11 Pressure patterns (and hence winds) for Australia in December (summer) and July (winter), showing heat lows in the north in summer and a high over the continent in winter. A complex frontal disturbance affects south-east Australia in the July chart. The low in the Great Bight in December is unusual: a high there is more common.s tropopause and consequent raising of the air column's average temperature (Note 13.B).
3 Polar lows or 'comma' lows, are like frontal lows except for being much more shallow, with little cyclonic circulation above 500 hPa so that there is no interaction with any jet stream. They result from the temperature difference across the Antarctic front and the strong surface winds at high latitudes. They often occur in the Southern Ocean, especially within an outbreak north of cold air, in the wake of a major frontal low.
4 Subtropical lows between 25-40°S are due to a combination of warm air thoughout the troposphere and a weak depression at the tropopause (Note 13.C). Such lows are far less common than frontal lows but cause most floods between 25-40°S. They often form beneath the poleward part of an unusually wide swing of the subtropical jet (Section 12.5), and may occur along the east coasts of the southern continents, in which case they are called east coast lows. A subtropical low lingers a day or more, under a cut-off low at 300 hPa level, while frontal lows at the surface always move along with the shortwaves in the jet stream.
5 An orographic low occurs in the shelter of a mountain range, partly as a result of the warming due to subsidence of the wind on the lee, and partly because of the conservation of vorticity (Note 13.D). An example is a depression by as much as 4 hPa on the southeast coast of South Africa, offshore from the 1,200 m escarpment, whenever north-westerly winds blow towards the sea. Such an orographic low (or coastal low) and its resulting cyclonic circulation hardly extend above the escarpment, but the onshore winds on the equatorward side of the low may produce light rain, as happens in Durban a few times a month in winter. The low also brings great temperature changes, firstly due to the subsiding northwesterly winds, bringing warm, dry air to Durban (Day 2 in Figure 13.7), then cool cloudy weather as the orographic low moves eastward up the coast (Day 3). This is often followed by even cooler south-westerly winds, due to the passage of the cold front (Day 4), which was the cause of the north-westerly winds in the first place.
Similarly, a north-westerly wind often creates troughs along the south-east coast of New Zealand. 6 Warm air extends right through the troposphere in a tropical cyclone, creating an intense low (Section 13.5).
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