Highs

Centres or ridges of high pressure at the surface imply a relatively cold atmosphere (Note 1.G) and regions with subsiding air. So they tend to be found over continents in winter, when the land is colder than the surrounding seas. Thus, a large anticyclone sits over Siberia every winter (Figure 3.4 and Figure 12.1) and a pressure has been measured there equivalent to a record MSLP of 1,084 hPa (Section 1.5). This is a cold high. There is one permanently over Antarctica. Also, there tends to be one over southeast Australia in winter, whereas highs are located south of the continent in summer, when it is the sea that is colder (Figure 13.11 and Figure 13.16)

Cold highs are shallow and often stationary, although a mobile cold high follows any midlatitude frontal disturbance (Note 13.B), forming below the equatorward part of the jet stream, where upper-level convergence takes place (Note 12.L).

Another type is the subtropical high (or warm-anticyclone) due to cold air at the tropopause, not near the surface. In this case, the tropopause is higher than usual, i.e. colder, so there is cold air even above the 300 hPa level. In other words, subtropical highs are associated with a high or ridge at that level, as well as at sea-level, unlike the situation with cold highs, which occur below an upper-level trough.

Figure 1316 The positions of highs and lows in summer and winter around Australia. The continuous lines show the number of hours in a month when a high's centre is within the indicated 5-degree square, i.e. the anticyclonicity of the region. The dashed lines show the corresponding cyclonicity (Section 13.4). There is a high centred for more than 24 hours each month in the areas shown shaded.

Figure 1316 The positions of highs and lows in summer and winter around Australia. The continuous lines show the number of hours in a month when a high's centre is within the indicated 5-degree square, i.e. the anticyclonicity of the region. The dashed lines show the corresponding cyclonicity (Section 13.4). There is a high centred for more than 24 hours each month in the areas shown shaded.

A ring of highs lies around the southern hemisphere at 30-35°S (Figure 12.1), resulting in an average pressure at that latitude higher than elsewhere (Figure 1.8). The precise latitude of these subtropical highs corresponds to the subsiding part of the Hadley cell, so it alters in response to the seasonal shift of the ITCZ (Figure 12.11), but only over 5-10 degrees. On average, highs cross the east coast of Australia south of Sydney in summer (Figure 13.16), so that the anticlockwise rotation of winds brings easterly maritime breezes in that season, whereas dry westerly winds prevail in winter when the centres of highs pass further north.

Subtropical highs tend to lie over the oceans between the southern continents, especially in summer, when there are heat lows over the continents. One subtropical high is anchored over the eastern Pacific by an anticyclonic swirl induced by the Andes when they deflect westerly winds north (Figure 12.1). The centre of this South Pacific high shifts from 32°S in January to 23°S in July, in response to the Sun's declination (Section 2.2). The high is particularly strong because of the cold ocean surface (Figure 11.2), except during an El Niño, when it is weaker and displaced to the south. Another semi-permanent high lies over the Indian ocean, moving nearer Australia in summer and towards Africa in winter.

Highs are usually elliptic in shape, with a zonal diameter of 2,000-4,000 km and a meridional diameter of 1,000-2,000 km. In other words, they tend to be larger than lows. Therefore they involve smaller pressure gradients, so that winds are lighter, with calm conditions at the centre of any high (Note 12.D).

The belt of subtropical highs is intersected by cold fronts, located in troughs which connect to frontal lows to the south. As a result, it appears that the highs themselves are moving eastward at the same speed as the frontal disturbances. One study showed that sixteen large highs with central pressures over 1,024 hPa travelled at about 30 km/h (the range being 6-72 km/h) across Australia, whilst fifteen smaller, less intense highs moved at about 50 km/h. They tend to move more slowly in winter than summer. A typical speed of 40 km/h, and an east-west dimension of 4,000 km, implies that they pass across in four days if they keep moving.

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