Westerlies Winds

The westerlies are the prevailing winds in the middle latitudes blowing from the subtropical high pressure toward the poles. The westerlies originate due to pressure differences between the subtropical high-pressure zone and the subpolar low-pressure zone. The westerlies curve to the east due to the Coriolis effect caused by the Earth’s rotation. In the Northern Hemisphere, the westerlies blow predominantly from the southwest, while in the Southern Hemisphere, they blow predominantly

from the northwest. The equatorward boundary is fairly well defined by the subtropical highpressure belts, while 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 and 50 degrees latitude.

Winds transport heat from warmer areas to cooler areas and help the Earth maintain equilibrium of its thermal environment. In the midlatitude, the westerlies play a large 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 collect warm, moist air from over the oceans, move it to the cooler continents, and bring heavy rainfall to 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.

• Global Warming and Westerlies

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, while others believe they are a part of natural variations. The 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 two places. At a high NAO index, a large pressure difference between the two places induces stronger westerly flows. The storm track advances northward and Europe experiences milder winters but more frequent rainfall in central Europe and nearby. At a low NAO index with suppressed westerlies, the storm track moves more toward the Mediterranean and results in colder winters in Europe and southern Europe, and North Africa receives 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 further poleward and brings warmer winter weather to the northeastern part of the United States. In the winter of 2006–07, the warming induced was about 9 degrees F (5 degrees C), which was as much as five times the air temperature increase compared to warming in a typical El Niño year. Changes in both surface and upperlevel westerlies due to El Niño patterns can also influence the development, intensity, and track of hurricanes over the tropical Atlantic Ocean. In the fall of 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 2007 climate model by Joellen L. Russell and colleagues 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 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.