Tropical climates

The climate is hot throughout the year, with mean monthly temperatures of about 27°C in every month and a very small annual temperature range of only around 2°C.

Rainfall is high and falls in every month, with each month receiving more than 150 mm, so there is no dry season and the annual total is high (well over 2000 mm).

These features — constantly high temperatures with a tiny annual range and heavy rain in every month — are characteristic of a humid tropical (tropical rainforest) climate.

Seasonally humid tropical (savanna) environments are found in a belt roughly between 10° and 20° north and south of the Equator.

They lie on the poleward side of the tropical rainforest and between the rainforest and the hot deserts, so they form two belts, one in each hemisphere, that are broadly symmetrical about the Equator.

Major savanna regions include the Sahel and East Africa, the Llanos and Cerrado of South America, parts of northern Australia and parts of India.

The further a savanna lies from the Equator and the closer it lies to the deserts, the longer and more intense its dry season becomes.

The ITCZ is the zone where the trade winds of the two hemispheres converge, forcing warm, moist air to rise. As this air rises it cools, water vapour condenses, and heavy convectional rainfall is produced, so the ITCZ is a belt of low pressure, cloud and rain.

The ITCZ is not fixed: it migrates north and south through the year, following the position of the overhead Sun. It moves towards the Tropic of Cancer (about 23.5°N) during the northern summer (June and July) and towards the Tropic of Capricorn (about 23.5°S) around December and January.

When the Sun, and therefore the ITCZ, is overhead in the savanna's hemisphere — the high-Sun season — the rain belt lies over the savanna and brings its wet season, with heavy convectional storms.

When the ITCZ migrates into the other hemisphere, it is replaced by dry, sinking air from the subtropical anticyclone. This subsiding air warms and dries, suppressing cloud and rain, and gives the savanna its dry season.

So the alternation is simple: the savanna is wet when the ITCZ is overhead and dry when it has moved away. Places nearer the Equator may even experience a double wet season as the ITCZ passes overhead twice, while those nearer the deserts catch it only briefly and have a longer dry season.

The rainforest lies close to the Equator, where the midday Sun is high in the sky throughout the year and day length stays close to twelve hours. Insolation is therefore intense and almost unchanging from month to month, so mean temperatures stay around 26–28°C and the annual range is only a few degrees. In fact the day-to-night (diurnal) range here is larger than the annual range.

The savanna lies further from the Equator (about 10°–20°), where the angle of the midday Sun and the length of day change more between the high-Sun and low-Sun seasons. This greater seasonal change in heating produces a larger annual temperature range, and temperatures often peak just before the rains break, when skies are clear.

The rainfall difference is explained by the ITCZ. Over the rainforest the ITCZ is overhead or close to it for most of the year, so convectional rain falls in every month and there is no dry season. Over the savanna the ITCZ is overhead only during the high-Sun season, giving a single wet season, while for the rest of the year dry, sinking air from the subtropical anticyclone gives a pronounced dry season. This is why the savanna has a marked wet/dry rainfall pattern and a lower annual total than the rainforest.

Tropical climates fall into two broad types — the humid tropical (rainforest) climate near the Equator and the seasonally humid tropical (savanna) climate in a belt on either side, between the rainforest and the hot deserts. Their distribution is undeniably shaped by the Inter-Tropical Convergence Zone (ITCZ), but the ITCZ does not act alone: the subtropical anticyclones, seasonal monsoons and the El Niño Southern Oscillation (ENSO) all help to determine where these climates lie and how they behave. This essay assesses the ITCZ's role and argues that, while it is the single most important control of the wet season and the broad belt pattern, the distribution as a whole is the product of the ITCZ and the subtropical anticyclones acting together as one circulation, modified regionally by monsoons and from year to year by ENSO. The view is therefore true only in part.

The case for the ITCZ is strong. The ITCZ is the belt where the trade winds converge, air rises, and heavy convectional rain falls, and it migrates north and south through the year following the overhead Sun. Because it sits over the Equator for most of the year, the equatorial zone receives rain in every month, producing the rainforest belt of the Amazon, Congo and Indonesia. Because it sweeps over the savanna latitudes only during the high-Sun season, it gives those regions — such as the Sahel and East Africa — their single wet season. The very symmetry of the rainforest and savanna belts about the Equator mirrors the ITCZ's migration, and the number of rainfall peaks a place receives depends on how many times the ITCZ passes overhead. In explaining the position of the wet belts, the ITCZ is clearly the dominant control.

However, the ITCZ cannot by itself explain the full distribution, because it does not account for the dry season or the deserts that bound the savanna. These are produced by the subtropical anticyclones — the zones of sinking, warming, drying air at about 20°–30° where the poleward limb of the Hadley cell descends. It is this subsiding air, replacing the ITCZ as it migrates away, that gives the savanna its dry season and creates the hot deserts beyond. The savanna's seasonally humid character is therefore a see-saw between the wet ITCZ and the dry anticyclone, and the boundary between savanna and desert is set by the anticyclone, not the ITCZ. Crucially, the ITCZ and the anticyclones are two limbs of the same Hadley circulation, so it is artificial to credit one without the other.

Further controls modify the distribution regionally and inter-annually. In monsoon regions such as South Asia, the differential heating of land and sea reverses the wind direction between seasons and drags the ITCZ far over the continent, producing an exceptionally wet summer and a dry winter; here the monsoon reshapes the tropical climate well beyond what a simple zonal ITCZ model would predict. ENSO adds variability from year to year: in an El Niño the weakening trade winds shift warm water and rain eastward across the Pacific, bringing drought to the normally wet western Pacific (Australia and Indonesia) and floods to the usually dry South American coast, while La Niña intensifies the normal pattern. These show that the distribution of tropical rainfall is neither fixed nor controlled by the ITCZ alone.

Weighing these factors, the ITCZ is the most important SINGLE control of the wet belts and the broad symmetry of tropical climates, and no other factor explains the rainforest's year-round rain so directly. But the distribution as a whole emerges from the ITCZ and the subtropical anticyclones working together as one circulation — the ITCZ setting the wet season, the anticyclone the dry — with monsoons reshaping the pattern in particular regions and ENSO driving year-to-year change. The controls are best ranked as primary zonal controls (ITCZ and anticyclones), then regional (monsoon) and inter-annual (ENSO) modifiers.

In conclusion, the statement is correct that the ITCZ is the principal control of the wet season and the belt pattern, but it overstates the case by saying 'mainly'. The distribution of tropical climates is most accurately explained as the product of the ITCZ and the subtropical anticyclones acting as a single Hadley circulation, modified by monsoons regionally and by ENSO from year to year. To attribute the distribution mainly to the ITCZ is therefore a partial truth that overlooks the controls that produce the dry season and the year-to-year variability.

The climatic characteristics of tropical environments — the constant heat and year-round rain of the rainforest, and the marked wet and dry seasons of the savanna — are produced by several interacting factors: the migration of the ITCZ, the subtropical anticyclones, seasonal monsoons and the El Niño Southern Oscillation (ENSO). This essay assesses their relative importance and argues that the ITCZ and the subtropical anticyclones are the primary controls that generate the basic temperature and rainfall regimes, while monsoons and ENSO are powerful but secondary modifiers that operate regionally and from year to year. No single factor acts in isolation.

The ITCZ is arguably the most important control of the rainfall characteristics. As the belt of converging trade winds and rising air, it brings heavy convectional rainfall wherever it lies, and its migration with the overhead Sun explains the defining rainfall patterns of both tropical climates. Over the equatorial rainforest it is overhead for most of the year, giving rain in every month and no dry season; over the savanna it is overhead only in the high-Sun season, producing the single wet season. The ITCZ therefore directly generates the most distinctive characteristic of each climate — the rainforest's year-round rain and the savanna's wet season.

The subtropical anticyclones are of equal importance for explaining the dry characteristics. The sinking, warming and drying air of these high-pressure cells, where the Hadley cell descends at about 20°–30°, suppresses cloud and rain. It is this subsiding air, taking over as the ITCZ migrates away, that produces the savanna's dry season and the hot deserts beyond. Because the savanna's defining feature is its seasonality, and the dry half of that seasonality comes from the anticyclone, the ITCZ and the anticyclone must be assessed together: they are the two limbs of one Hadley circulation, alternately delivering the wet and dry seasons. Temperature characteristics also follow from this large-scale geometry — the small annual range of the rainforest reflects the steady high-Sun insolation near the Equator, while the larger range of the savanna reflects the greater seasonal change in Sun angle further from the Equator.

Seasonal monsoons are a major regional modifier rather than a universal control. Where a large landmass adjoins a warm ocean, as in South Asia, the differential heating of land and sea reverses the winds between seasons: moist onshore winds and a continental low draw the ITCZ far inland in summer, giving an exceptionally heavy wet season, while dry offshore winds in winter give a marked dry season. The monsoon therefore intensifies the wet/dry contrast far beyond the textbook savanna pattern, but only in particular regions, which is why it is best judged a powerful regional amplifier of the basic regime set by the ITCZ and the anticyclones.

ENSO is the most variable factor, important for explaining why tropical climates are not the same every year. In an El Niño the weakening trade winds shift warm water and rising air eastward across the Pacific, bringing drought to the western Pacific (Australia, Indonesia) and heavy rain to the eastern Pacific coast; La Niña intensifies the normal pattern. ENSO does not create the basic climate types, but it explains the recurrent droughts and floods superimposed on them, so its importance lies in the inter-annual variability rather than in the underlying characteristics.

Weighing these factors, the ITCZ and the subtropical anticyclones are the primary controls, because together they generate the fundamental temperature and rainfall characteristics of both tropical climates every year. Monsoons are a strong regional modifier and ENSO an inter-annual one, each crucial in its own domain but neither responsible for the basic regime. The most accurate judgement is that the characteristics of tropical environments are produced by the ITCZ and anticyclones acting as one zonal circulation, with monsoons and ENSO modifying that circulation in space and time.

In conclusion, the factors are not of equal weight. The ITCZ and the subtropical anticyclones are the most important because they create the year-round rain, the wet/dry seasonality and the temperature regimes that define tropical climates; the seasonal monsoon and ENSO are significant but secondary, reshaping the pattern regionally and varying it from year to year. Tropical climatic characteristics are therefore best understood as a single zonal system of rising and sinking air, modified by land–sea heating and by ocean–atmosphere oscillations.