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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 Niño-Southern Oscillation (ENSO) is another indicator. In winters of El Niño years, the polar jet stream 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 Niño year. Changes in both surface and upper-level westerlies resulting from El Niño patterns can also influence the development, intensity, and track of hurricane over the tropical Atlantic Ocean. In fall 2006, El Niño 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 Niño 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.

How global warming influences the westerlies still remains in question. The recent observation, however, suggests that global warming brings noticeable change in the westerlies.

sEE ALso: Climate Change, Effects; Winds, Easterlies.

BIBLioGRAPHY: C. Donald Ahrens, Meteorology Today, 8th ed. (Thompson Brooks/Cole, 2007); Peter J. Robinson and Ann Henderson-Sellers, Contemporary Climatology, 2nd ed. (Prentice Hall, 1999).

Jongnam Choi Western Illinois University

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