Tropical climatology

050-08-02Pablo Asensio Martínez2026-04-013 min

Tropical climatology is dominated by the Intertropical Convergence Zone (ITCZ), the trade winds, and the monsoons.

ITCZ January-July Seasonal average displacement of the Intertropical Convergence Zone (ITCZ) between January and July.

Intertropical Convergence Zone (ITCZ)

Also known as the Doldrums, Thermal Equator, or Equatorial Trough.

  • Definition: It is a zone of low pressure where the trade winds from the Northern Hemisphere (NE) and the Southern Hemisphere (SE) converge.
  • Cause: Intense solar heating near the equator forces warm, moist air to rise (convection).
  • Associated Weather:
    • Clouds of great vertical development (Cu, Cb) and frequent thunderstorms.
    • Heavy precipitation, severe turbulence, and icing (above 16,000 ft).
    • Cloud tops can reach 50,000 - 60,000 ft, making it difficult to fly over.
  • Seasonal Movement: Follows the sun (thermal equator).
    • NH Summer (July): Shifts North.
    • NH Winter (January): Shifts South.
    • The displacement is much greater over continents than over oceans (due to the thermal inertia of water).

Trade Winds

Persistent winds blowing from the subtropical high-pressure belts (Horse Latitudes, ~30° N/S) towards the ITCZ.

  • Northern Hemisphere: Northeast Trades (NE Trades).
  • Southern Hemisphere: Southeast Trades (SE Trades).
  • Characteristics:
    • They are low-level winds (up to ~10,000 ft).
    • They pick up moisture as they pass over oceans (moist in lower layers, dry aloft due to subtropical subsidence).
    • Upon crossing the equator, they change direction due to the Coriolis force (e.g., SE becomes SW).

Monsoons

Seasonal changes in the prevailing wind direction, caused by the displacement of the ITCZ and land-sea thermal differences.

Southwest Monsoon (SW Monsoon) - Summer

  • Season: June, July, August.
  • Mechanism: The ITCZ moves North. The SE trades cross the equator, turn right (Coriolis), and become SW winds.
  • Weather: Warm, moist, and unstable air (long maritime track).
    • Cb clouds, strong thunderstorms, heavy rains, low ceilings.
  • Affected Regions: India, Southeast Asia, West African Coast (Guinea, Ghana).

Northeast Monsoon (NE Monsoon) - Winter

  • Season: December, January, February.
  • Mechanism: The ITCZ moves South. The wind blows from the Siberian High towards the equator.
  • Weather: Cool and dry air (continental origin).
    • Clear and stable weather over India and Thailand.
    • Exception: If it travels over the sea (e.g., Bay of Bengal towards Sri Lanka), it picks up moisture and can bring rains.

Northwest Monsoon (NW Monsoon)

  • It is the extension of the NE Monsoon crossing the equator into the Southern Hemisphere, turning NW. Affects northern Australia in January (austral summer).

Other Tropical Phenomena

  • Easterly Waves: Low-pressure troughs moving from East to West in the trade winds. They originate in West Africa and can evolve into tropical cyclones in the Atlantic/Caribbean.
  • Harmattan: Dry and dusty NE trade wind blowing from the Sahara over West Africa in winter.
  • Pampero: Outbreak of cold polar air in South America (Argentina, Uruguay, Brazil) with severe storms.
  • 0°C Isotherm: In the tropics, it is found very high, typically at 15,000 - 17,000 ft.
  • Sandstorms: Require winds > 20 kt and instability. Drastically reduce visibility.