Cool ocean currents

Between the western coasts of the continents and the eastern rims of the subtropical high-pressure cells the ocean surface is relatively cold (see Figure 7.33). This is the result of: the importation of water from higher latitudes by the dominant currents; the slow upwelling (sometimes at the rate of about 1 m in twenty-four hours) of water from intermediate depths due to the Ekman effect (see Chapter 7D.1); and the coastal divergence (see Figure 7.31). This concentration of cold water gently cools the local air to dew-point. As a result, dry, warm air degenerates into a relatively cool, clammy, foggy atmosphere with a comparatively low temperature and little range along the west coast of North America off California, off South America between latitudes 4 and 3°S, and off southwest Africa (8 and 32°S). Thus Callao, on the Peruvian coast, has a mean annual temperature of 19.4°C, whereas Bahia (at the same latitude on the Brazilian coast) has a corresponding figure of 25°C.

The cooling effect of offshore cold currents is not limited to coastal stations, as it is carried inland during the day at all times of the year by a pronounced sea breeze effect (see Chapter 6C.2). Along the west coasts of South America and southwest Africa the sheltering effect from the dynamically stable easterly trades aloft provided by the nearby Andes and Namib Escarpment, respectively, allows incursions of shallow tongues of cold air to roll in from the southwest. These tongues of air are capped by strong inversions at between 600 and 1500 m, reinforcing the regionally low trade wind inversions (see Figure 11.6) and thereby precluding the development of strong convective cells, except where there is orographically forced ascent. Thus, although the cool maritime air perpetually bathes the lower western slopes of the Andes in mist and low stratus cloud, and Swakopmund (southwest Africa) has an average of 150 foggy days a year, little rain falls on the coastal lowlands. Lima (Peru) has a total mean annual precipitation of only 46 mm, although it receives frequent drizzle during the winter months (June to September), and Swakopmund in Namibia has a mean annual rainfall of 16 mm. Heavier rain occurs on the rare instances when large-scale pressure changes cause a cessation of the diurnal sea breeze or when modified air from the South Atlantic or South Indian Ocean is able to cross the continents at a time when the normal dynamic stability of the trade winds is disturbed. In southwest Africa, the inversion is most likely to break down during either October or April, allowing convectional storms to form, and Swakopmund recorded 51 mm of rain on a single day in 1934. Under normal conditions, however, the occurrence of precipitation is limited mainly to the higher seaward mountain slopes. Further north, tropical west coast locations in Angola and Gabon show that cold upwelling is a more variable phenomenon in both space and time; coastal rainfall varies strikingly with changing sea-surface temperatures (Figure 11.55). In South America, from Colombia to northern Peru, the diurnal tide of cold air rolls inland for some 60 km, rising up the seaward slopes of the western Cordillera and overflowing into the longitudinal Andean valleys like water over a weir (Figure 11.56). On the west-facing slopes of the Andes of Colombia, air ascending or banked up against the mountains may under suitable conditions trigger off convectional instability in the overlying trades and produce thunderstorms. In southwest Africa, however, the 'tide' flows inland for some 130 km and rises up the 1800-m Namib Escarpment without producing much rain because convectional instability is not generated and the adiabatic cooling of the air is more than offset by radiational heating from the warm ground.

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