Introduction

Based on the third assessment reports (TAR) of IPCC (2001), the globally averaged surface temperatures have increased 0.6 ± 0.2 °C over the 20th century with sharper increases in the mid- and high latitudes of the northern hemisphere continents. It is very likely that precipitation has increased by 0.5-1% per decade over most of the mid- and high latitudes of the northern hemisphere continents and that rainfall has increased by 0.2-0.3% per decade over the tropical land areas. Although the changes of mean climate for the tropics have not been obvious, climate variability and extreme events, (dominated by inter-decadal to multi-decadal variability) have likely increased in intensity and frequency in some places. The projected climates in the future include not only changes of mean climate such as global warming that varies with region, and increases and decreases in precipitation, but also changes in the variability of climate and changes in the frequency and intensity of some extreme climate phenomena.

Climate variability will affect agriculture and forestry through effects on crops and forests; soils; weeds, pests and diseases; and also livestock. Variation of climatic conditions across the world leads to different local and regional impacts although the humid and sub-humid tropics have much more special climates and ecosystem

Climatic Change (2005) 70: 73-116

© Springer 2005

Figure 1. World map showing Koppen's climate classification (Af: tropical rainy; Aw: tropical wet-and-dry; Am: tropical monsoon).

environments. Climate variability and extreme events will definitely bring impacts on agriculture and forestry in these latitudes.

2. Characteristics of Climate of Humid and Sub-humid Tropics

Humid and sub-humid tropics usually refer to the tropical rainy climate zones and most parts of the tropical monsoon or tropical wet-dry climate zones. The tropical rainy climate prevails mainly in lowlands within 5° N and 5° S of the equator (also known as the equatorial climate). The climate is characterized by a combination of constantly high temperatures and abundant rainfall well distributed throughout the year. Humid tropics are commonly associated with the luxurious evergreen forests. The tropical monsoon climate differs from the tropical rainy climate in that it has a distinct dry season; its annual rainfall totals and temperature conditions are similar to those in the tropical rainy climate but rainfall regime is similar to that of tropical wet-dry climate. The tropical wet-dry climate has alternating wet and dry seasons. Annual rainfall totals are less than those in the tropical rainy and monsoon climates. The dry season here is severe and has a profound effect on vegetation and crops unlike in the monsoon tropical climate.

Humid and sub-humid tropical conditions are found over nearly 50% of the tropical land mass and 20% of the earth's total land surface (Figure 1). This total is distributed among three principal regions (Oliver and Hidore, 1984). Tropical Central and South America contain about 45% of the world's humid and subhumid tropics; Africa, about 30%; and Asia, about 25%. As many as 62 countries are located partly or entirely within the region.

Tropical monsoon Asia is dominated by the two monsoons; the summer southwest monsoon influences the climate of the region from May to September, and the winter northeast monsoon controls the climate from November to February. The monsoons bring most of the region's precipitation. The equatorial monsoon climate comprises of the Indonesian archipelago, Malaysia, New Guinea and some other islands. The wet-dry monsoon climate prevails in the Indian subcontinent, including the northern parts of Sri Lanka and coastal parts of Bangladesh, Burma, Thailand and Indo-China; Philippine archipelago; and northern Australia, and southeastern Indonesia. Tropical cyclone is another important feature of the weather and climate in this region. Two core areas of cyclogenesis exist; one in the northwestern Pacific Ocean, which particularly affects Philippines and Vietnam, and the other in the northern Indian Ocean, which particularly affects Bangladesh. In addition, geographically much more extensive is the El Nino-Southern Oscillation (ENSO) phenomenon, which has an especially important influence on the climate and in-terannual variability of climate in a number of countries in these latitudes and also over the globe.

The region of humid and sub-humid central and western Africa consists of three main areas; the Congo Basin, between about 5o N and 5o S, the southern coast of western Africa, at latitudes between 5o N and 9o N; and the remaining parts of the region at latitudes between 15o N and 15o S. Climatic conditions differ from those in the tropical monsoon Asia in many ways. The climate of the first two areas is characterized by heavy rainfall throughout the year. Convection is the main process causing precipitation; seasonal variations are mainly the result of large-scale airmass movements and the movements of the intertropical convergence zone (ITCZ). The common feature of this climate is the gradual decrease in the length of the rainy season and in the total amount of rainfall with latitude, basically characterized by wet-dry climate.

The most important topographic feature of Central America and the northern part of South America is the high and continuous mountain backbone of the Andes range, which has important climatological consequences. The Caribbean islands and Atlantic coastal areas of Central America all have a similar climate. The trade winds are the dominating element of its general circulation, which bring much rainfall. The Pacific side of Central America is generally drier than the Atlantic side because of the influence of the subtropical high-pressure cell over the North Pacific. The largest humid part of tropical South America lies to the east of the Andes Mountains. It is a predominantly flat area, with a low center, occupied by the Amazon Basin. The general circulation over this region, the seasonal movements of which are much smaller than over Southeast Asia or tropical Africa, is controlled by the position of the ITCZ. Although Central America and the northern part of South America are characterized largely by humid and sub-humid conditions, important areas (e.g., northeastern Brazil) are subject to droughts and floods due to the ENSO phenomenon.

Atmospheric extremes which are seen as hazards in the humid and sub-humid tropics mainly include droughts, floods, and tropical cyclones. Droughts and floods often occur in the tropical monsoon and wet-dry climate zones. For a number of countries in tropical Asia, the major hazards are tropical cyclones and floods. Droughts, on the other hand, are severe in the northern part of South America and some regions of sub-humid Africa.

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