Middle-latitude climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 119

Seasonally, the strength of the subtropical highs varies (Leighton 1994 ). In the
Indian Ocean, the high is strongest in winter and weakest in summer. In the
Atlantic, the high is strongest in winter, but weakest in both summer and autumn.
In the Pacific, the high is strongest in spring, and weakest in autumn.
4.6.5 Characteristics and role of the synoptic-scale
pressure systems
At a smaller temporal and spatial scale than the subtropical highs and their
anomalies discussed above are the characteristics and role of the synoptic highs
and lows that process within the mid-latitude westerly zone (Leighton 1994 ;
Sinclair 1995 , 1996 ; Chen and Yen 1997 ). These systems transfer heat and
moisture between latitude zones and, supported by the jet stream and its varia-
tions, help maintain the general circulation.
Anticyclones
In January (SH summer), anticyclonicity dominates the 30-408 S latitudinal
band, and especially occurs at the longitudes 108W, 908W, south of Australia
at 1308 E, and around 908 E. These are also areas of higher immobility, suggest-
ing blocking. In July (SH winter), the anticyclonicity zone has moved north, to
between 25 and 358 S, with the prevalences in the longitudes listed above
reduced by 10-30 percent (Figure 4.9 ). It is in winter, however, that the frequency
of pressure systems with central pressure greater than 1035 hPa increases, espe-
cially southeast of Southern Africa, Australia, and New Zealand. The frequency of
blocking is the most intense SE of New Zealand, and west of South America
(1108W, 558 S), and doubles in winter compared to summer.
Anticyclones tend to form near the SE coasts of continents, especially
those with mountain barriers, such as South America. Ridging is enhanced
by orography, and stability by low-level cooling (especially the Benguela
and Humboldt cold ocean currents). While high mountain chains, such as
the Andes, will prevent normal west-to-east movement of anticyclones,
Leighton ( 1994 ) describes offshoots (or ''budding'') that form on the lee
side of the mountains. Anticyclones will tend to move eastward with the
direction of movement affected by the location of the subtropical high ridge
axis. Speed of movement is on the order of 10m s 1 ,andissomewhatstronger
at higher latitudes. Faster movements are usually linked to the subtropical
jet stream.
South of about 458 latitude, there are few anticyclones, since this is where the
majority of storm tracks occur, although in the Pacific anticyclones may extend
to 558 S. On occasion, if there is a breakdown in westerly circulation, anti-
cyclones may extend below 508 S over any ocean, increasing the mean sea
level pressure by up to 30 hPa compared to normal.