Understanding Climate-3

Earth’s energy distribution (circulation patterns)

Atmospheric circulation

In the tropical regions, the planet is girdled by a belt of intense convective activity and rising air, known as the intertropical convergence zone (ITCZ). Here, hot air rises, releasing its heat energy to the atmosphere. As this rising air cools, moisture condenses to form clouds and rainfall. Where the air rises buoyantly within short-lived convective clouds (called ‘hot towers’), rainfall can be very intense. When the rising air reaches an altitude of around 12 to 15 km and virtually all the moisture has been extracted, it spreads out. Descending air on each side of the ITCZ creates zones of dry, hot air that maintain the deserts in the subtropical regions of the world. At the surface the Trade Winds flow back towards the ITCZ. First interpreted by the amateur meteorologist George Hadley in 1735, this basic circulation pattern is named after him and is known as the Hadley circulation or Hadley cell.

Further poleward, the middle latitude depressions swirl endlessly around the globe, often steered by concentrated cores of strong high westerly winds known as jet streams. These ‘rivers’ of air are usually found between altitudes of 9 and 12 km. Wind speeds are at a maximum during winter and often average near 180 km/h, although peak speeds can exceed twice this value. Jet streams can be very turbulent and hazardous for aircraft.

Ocean circulation

Ocean Currents.Image: IPCC

Ocean circulations transport roughly the same amount of energy towards the poles as does the atmosphere. The basic form in both hemispheres is a basin-wide vortex or circulation known as a gyre, with wind-driven westward flow in low latitudes close to the equator and poleward-directed currents along the western margins. Beyond about 35°N and 35°S the major currents sweep eastward carrying warm water to higher latitudes. This pattern is seen most clearly in the North Atlantic and North Pacific in the form of the Gulf Stream/North Atlantic Current and the Kuroshio/North Pacific Current. To balance the poleward flow there are returning currents of cold water moving toward the equator on the eastern sides of the ocean basins. In the Southern Hemisphere, because of the virtual absence of land between 35°S and 60°S, the ocean gyres are linked with a strong circumpolar current around Antarctica. There are also regions of significant vertical motion associated with these global ocean circulations. These circulations are show in the image above.

Hydrological cycle

All the water in the atmosphere gets replaced every 8 days, while it can take up to 100 years for the water in a lake to be renewed.

The continual recycling of water between the oceans, land surface, underground aquifers, rivers and the atmosphere (the hydrological cycle) is an essential part of the climate system. Ice requires much energy to melt (latent heat of fusion) and water needs even more energy to evaporate (latent heat of vaporization), so the cycling of water through the atmosphere by evaporation and its subsequent precipitation is a significant mechanism through which energy is transported throughout the climate system.