Winds, Westerlies

THE WESTERLIES ARE the prevailing winds in the middle latitudes blowing from the subtropical high pressure toward the poles. The westerlies originate as a result of pressure differences between the subtropical high-pressure zone and the subpolar low-pressure zone. The westerlies curve to the east because of the Coriolis effect, caused by the Earth's rotation. In the Northern Hemisphere, the westerlies blow predominantly from the southwest, whereas in the Southern Hemisphere, they blow predominantly from the northwest. The equator-ward boundary is fairly well defined by the subtropical high-pressure belts, whereas the poleward boundary is more variable. The westerlies can be quite strong, particularly in the Southern Hemisphere, where less land causes friction to slow them down. The strongest westerly winds typically occur between 40 degrees and 50 degrees latitude.

Winds transport heat from warmer areas to cooler areas and help the Earth to maintain equilibrium of its thermal environment. In the midlatitude, the westerlies play a big role in the weather and atmospheric circulation in the middle latitudes. They transport warm, moist air to polar fronts and are also responsible for the formation of extratropical cyclones. In winter, they [Page 1094]collect warm, moist air from over the oceans, move it to the cooler continents, and bring heavy rainfall to the areas like the northwest coast of the United States. In summer, they collect hot, drier air from over the continents and move it to the oceans.

Does global warming influence westerlies? A recent study of westerlies in the Southern Hemisphere shows that the westerlies are shifting southward, toward Antarctica. No conclusion has been made yet, however. Some scientists believe that recent observations are related to global warming, but others think of them as a part of natural variations. North Atlantic Oscillation (NAO) is one indicator that shows the relationship between global warming and westerlies. NAO is calculated by the difference in pressure between the permanent low-pressure system located over Iceland (Icelandic Low) and the permanent high-pressure system located over the Azores (Azores High). Global warming can reduce the difference in pressure between the two places. At a high NAO index, a large pressure difference between the two places induces stronger westerlies flow. Storm tracks advance northward, and Europe experiences milder winters but more frequent rainfall in central Europe and nearby. At a low NAO index, with suppressed westerlies, storm tracks move more toward the Mediterranean, which results in colder winters in Europe and southern Europe and in North Africa receiving more storms and higher rainfall.

El Nino-Southern Oscillation (ENSO) is another indicator. In winters of EINino years, thepolar Jetstream in the Northern Hemisphere moves farther poleward and brings warmer winter weather to the northeastern part of the United States. In the winter of 2006 to 2007, the warming induced was about 9 degrees F (5 degrees C), which was as much as five times the typical air temperature increase compared with a warming in a typical El Nino year. Changes in both surface and upper-level westerlies resulting from El Nino patterns can also influence the development, intensity, and track of hurricane over the tropical Atlantic Ocean. In fall 2006, El Nino strengthened the upper-level westerlies, increased wind shear, and discouraged tropical cyclogenesis over the tropical Atlantic. Whether or not global warming is behind these stronger El Nino patterns is still being researched. A recent climate model (Joellen L. Russell et al, 2007) indicates that westerlies influence the temperature of the Southern Ocean. According to the model, the southward movement of the Southern Hemisphere westerlies in recent years transfers more heat and carbon dioxide into the deeper waters of the Southern Ocean. This poleward shift of the westerlies has intensified the strength of the westerlies near Antarctica. The pattern could slow down global warming somewhat but also induce ocean levels to rise in Antarctica.

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