At the surface the air moves from high pressure at the Tropics back to the Equator as a result of pressure gradient force.
This moving air however is affected by Coriollis’ force, and is deflected in the respective direction depending on the Hemisphere it’s in.
This moving air results in the formation of prevailing winds, the low pressure region where these winds meet is called the Inter- Tropical Convergence Zone (ITCZ).
Another cell to affect the tropical circulation is Walker’s cell. Unlike Hadley’s cell this refers to longitudinal movement of air, rather than latitudinal movement. In the South Pacific hot air rises over Indonesia giving heavy rainfall. On the other side of the Pacific Ocean on the West Coast of South America a cold ocean current maintains stable air and a large high pressure is kept even when the ITCZ is overhead. The difference in pressures across the ocean causes the rising air to move eastwards across the Pacific and then fall on the West coast of South America – intensifying the high pressure found there. At the surface the air moves back towards Indonesia as part of the South East trade winds. Walker’s cell results in a west coast desert climate, with very hot summers and dry, warm winters giving droughts. This is caused by deep lows and the intense highs that develop over the continents, giving very stable air.
This is can be dramatically altered however in an El Nino year. Around Christmas time in some years for a reason that is so far unknown, the water off Peru suddenly warms. The whole circulation system that was before set in the South Pacific is broken down. Air pressure rises over the West Pacific near to Indonesia. The pressure falls over the East pacific coast off the coast of Peru. The trades weaken and the east winds at the equator are replaced by west winds. The circulation in the ocean alters so that the warm current flows down the coast of Peru. This event can have drastic effects on the weather of these regions.
The tropics climatic changes occur as the ITCZ follows the Zenithal sun. The further you go from the Equator to the Tropics the greater the difference between summer and winter. A region on the tropics that is affected greatly by the movement of the Zenithal sun and the ITCZ is India. India experiences an extreme change between summer and winter. In the winter season, as expected there is a high pressure cell over India, as a result of air sinking from the upper atmosphere. The high is intense due to the air sinking from the sub-tropical jet over the North of India. Air also subsides down the Himalayas as it travels from the High over Siberia. The air moves from the high pressure to the low pressure at the ITCZ giving a North Easterly air flow, which is dry as it comes over the land.
In summer however the air comes from the sea so it is wet. The sub tropical jet moves north of the Himalayas and the ITCZ follows the Zenithal sun towards the tropic of Cancer. Establishing air rising and low pressure. The air is pulled in towards an intense low pressure in the North West of the sub-continent. Air moves up from the high pressure cells in the Southern Hemisphere, becoming the South West Monsoon. They carry huge quantities of moisture, giving heavy thunderstorms and cause floods.
The Tropics are areas that generally are hot all year as described by Hadley’s cell. Although we must remember that within the Tropics seasonal changes occur in the circulation as the ITCZ moves to follow the Zenithal sun. Also tropical circulation is affected by Walker’s cell and El Nino.
Explain how atmospheric circulation gives rise to the formation of low and high pressure areas in your chosen climatic region. Illustrate and explain the seasonal changes in the distribution of these pressure systems.
The Tropics
Sarah Bennett
