Tropical meteorology

(meteorology) The study of the tropical atmosphere; the dividing lines, in each hemisphere, between the tropical easterlies and the mid-latitude westerlies in the middle troposphere roughly define the poleward boundaries of this region.

The study of atmospheric structure and behavior in the areas astride the Equator, roughly between 30° north and south latitude. The weather and climate of the tropics involve phenomena such as trade winds, hurricanes, intertropical convergence zones, jet streams, monsoons, and the El Niño Southern Oscillation. More energy is received from the Sun over the tropical latitudes than is lost to outer space (infrared radiation). The reverse is true at higher latitudes, poleward of 30°. The excess energy from the tropics is transported by winds to the higher latitudes, largely by vertical circulations that span roughly 30° in latitudinal extent. These circulations are known as Hadley cells.

For the most part, the oceanic tropics (the islands) experience very little change of day-to-day weather except when severe events occur. Tropical weather can be more adverse during the summer seasons of the respective hemispheres. The near equatorial belt between 5°S and 5°N is nearly always free from hurricanes and typhoons: the active belt lies outside this region over the tropics. The land areas experience considerable heating of the Earth's surface, and the summer-to-winter contrasts are somewhat larger there. For instance, the land areas of northern India experience air temperatures as high as 108°F (42°C) in the summer (near the Earth's surface), while in the winter season the temperatures remain 72°F (22°C) for many days. The diurnal range of temperature is also quite large over land areas on clear days during the summer (32°F or 18°C) as compared to winter (18°F or 10°C).

The steady northeast surface winds over the oceans of the Northern Hemisphere between 5° and 20°N and southeast winds over the corresponding latitudes of the southern oceans constitute the trade winds. Trade winds have intensities of around 5–10 knots (2.5–5 m/s). They are the equatorial branches of the anticyclonic circulation (known as the subtropical high pressure). The steadiness of wind direction is quite high in the trades. See also Wind.

Hurricanes are also known as typhoons in the west Pacific and tropical cyclones in Indian Ocean and south Pacific. If the wind speed exceeds 65 knots (33 m/s) in a tropical storm, the storm is labeled a hurricane. A hurricane usually forms over the tropical oceans, north or south of 5° latitude from the Equator. See also Hurricane.

Intertropical convergence zones are located usually between 5 and 10°N latitude. They are usually oriented west to east and contain cloud clusters with rainfall of the order of 1.2–2 in. (30–50 mm) per day. The trade winds of the two hemispheres supply moisture to this precipitating system. See also Cloud physics; Precipitation (meteorology).

A number of fast-moving air currents, known as jets, are important elements of the tropical general circulation. With speeds in excess of 30 knots (15 m/s), they are found over several regions of the troposphere. See also Atmospheric general circulation; Jet stream; Troposphere.

Basically the entire landmass from the west coast of Africa to Asia and extending to the date line experiences a phenomenon known as the monsoon. Monsoon circulations are driven by differential heating between relatively cold oceans and relatively warm landmasses. See also Monsoon meteorology.

Every 2–6 years the eastern equatorial Pacific Ocean experiences a rise in sea surface temperature of about 5–9°F (3–5°). This phenomenon is known as El Niño, which is part of a larger cycle referred to as the El Niño Southern Oscillation (ENSO). The other extreme in the cycle is referred to as La Niña. El Niño has been known to affect global-scale weather. See also El Niño; Maritime meteorology.

In this field, the boundaries of the tropics fluctuate, being marked by the descending limbs of the Hadley cells, and the centres of the subtropical anticyclones. The tropical atmosphere is characterized by high temperatures, the transfer of energy in the form of latent heat via the Hadley cell, small temperature and pressure gradients (so that there are no fronts), high humidity, easterly winds, and low values of the Coriolis parameter, such that small pressure gradients will produce a stronger geostrophic wind than in mid-latitudes.

The following synoptic scale phenomena are characteristic of tropical meteorology:

1. tropical wave disturbances. These have a wavelength of 2000-4000 km, travel across 6-7° of longitude a day, and last for about two weeks. Ahead of the trough is a ridge of high pressure bringing fine weather. With the approach of the trough, cumulus cloud develops, wind veers, and heavy showers fall.
2. tropical cyclones. See hurricane.
3. monsoon depressions.
4. subtropical cyclones. These occur when the cold upper air from high latitudes is cut off to form a wave some 300 km in width. They bring cloud and some rain.
5. tropical cloud clusters. See squall line.
6. easterly waves.

Small-scale variables such as relief, local winds, and ocean currents are vital. Cold ocean currents cause offshore fog, taking moisture from the winds. Sea breezes then carry cool, dry air far inland. See also inter-tropical convergence zone.