Лекция: The Southern Oscillation

The locations of the furnaces, the convective zones of rising air and low surface pressures, are determined by temperature patterns at Earth's surface. The air ascends where surface temperatures have maxima. The seasonal north-south migrations of the convective zones therefore tend to keep those zones in the summer hemisphere. Over Africa and South America the zones of heavy rainfall are difficult to dislodge from the continents because surface temperatures can attain higher values on land than over the oceans. The maritime continent of southeastern Asia is an entirely different matter because its eastern boundary coincides with that of the pool of warm water that covers the western tropical Pacific. Should this pool expand eastward, so would the region of rising air and heavy rainfall, which is exactly what happens interannually during El Nino. On such occasions, the eastern tropical Pacific experiences an increase in sea surface tem­peratures and in rainfall, a decrease in surface pressure, and a relax­ation of the trade winds. Because of this eastward shift, the tropical regions west of the date line, including India and southeastern Africa, experience decreases in rainfall.

In the same way that the seasonal cycle is an oscillation between winter and summer, so the Southern Oscillation is a fluctuation between El Nino and a complementary state, which has been given the opposite name La Nina. Whereas the seasonal cycle is forced by regular variations in the intensity of sunlight, the Southern Oscillation corresponds to a natural mode of oscillation of the coupled ocean and atmosphere and is literally the music of our spheres (the atmosphere and hydro­sphere).

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