Tropical weather and climate

Learning objectives

When you have read this chapter you will:

Understand the characteristics and significance of the intertropical convergence zone, Be familiar with the principal weather systems that occur in low latitudes and their distribution, Know some of the diurnal and local effects that influence tropical weather, Know where and how tropical cyclones tend to occur,

Understand the basic mechanisms and characteristics of El Niño and La Niña events.

Tropical climates are of especial geographical interest because 50 per cent of the surface of the globe lies between latitudes 30°N and 30°S, and over 75 per cent of the world's population inhabit climatically tropical lands. This chapter first describes the trade wind systems, the intertropical convergence zone and tropical weather systems. The major monsoon regimes are then examined and the climate of Amazonia. The effects of the alternating phases of the El Niño - Southern Oscillation in the equatorial Pacific Ocean are discussed as well as other causes of climatic variation in the tropics. Finally, the problems of forecasting tropical weather are briefly considered.

The latitudinal limits of tropical climates vary greatly with longitude and season, and tropical weather conditions may reach well beyond the Tropics of Cancer and Capricorn. For example, the summer monsoon extends to 30°N in South Asia, but to only 20°N in West Africa, while in late summer and autumn tropical hurricanes may affect 'extra-tropical' areas of East Asia and eastern North America. Not only do the tropical margins extend seasonally poleward, but also in the zone between the major subtropical high-pressure cells there is frequent interaction between temperate and tropical disturbances. Elsewhere and on other occasions, as illustrated in Plate 23 over the western North Pacific, distinct tropical and mid-latitude storms are observed. In general, however, the tropical atmosphere is far from being a discrete entity and any meteorological or climatological boundaries must be arbitrary. There are nevertheless a number of distinctive features of tropical weather, as discussed below.

Several basic factors help to shape tropical weather processes and also affect their analysis and interpretation. First, the Coriolis parameter approaches zero at the equator, so that winds may depart considerably from geostrophic balance. Pressure gradients are also generally weak, except for tropical storm systems. For these reasons, tropical weather maps usually depict streamlines, not isobars or geopotential heights. Second, temperature gradients are characteristically weak. Spatial and temporal variations in moisture content are much more significant diagnostic characteristics of climate. Third, diurnal land/sea breeze regimes play a major role in coastal climates, in part as a result of the almost constant day length and strong solar heating. There are also semi-diurnal pressure oscillations of 2 to 3 mb, with minima around 04:00 and 16:00 hours and maxima around 10:00 and 22:00 hours. Fourth, the annual regime of incoming solar radiation, with the sun overhead at the equator in March and September and over the Tropics at the respective summer solstices, is reflected in the seasonal variations of rainfall at some stations. However, dynamic factors greatly modify this conventional explanation.

Continue reading here: The Intertropical Convergence

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