B North Atlantic Oscillation

The relative strength of the Icelandic low and Azores high was first observed to fluctuate on annual to decadal scales by Sir Gilbert Walker in the 1920s. Fifty years later, van Loon and Rogers discussed the related westeast 'seesaw' in winter temperatures between western Europe and western Greenland associated with the north-south change in pressure gradient over the North Atlantic. The North Atlantic Oscillation (NAO) is a north-south oscillation in the pressure field between the Icelandic low (65°N) and the Azores high (40°N). The relationship between the positive and negative modes of the NAO noted by Walker, and the associated temperature and other anomaly patterns, are shown in Plate E. When the two pressure cells are well developed as in January 1984, the zonal westerlies are strong. Western Europe has a mild winter, while the intense Icelandic low gives strong northerly flow in Baffin Bay, low temperatures in western Greenland and extensive sea ice in the Labrador Sea. In the negative phase the cells are weak, as in January 1970, and opposite anomalies are formed. In extreme cases, pressure can be higher

Figure 7.24 (A) Mean 700-mb contours (in tens of feet) for December 1957, showing a fast, westerly, small-amplitude flow typical of a high zonal index. (B) Mean 700-mb zonal wind speed profiles (m s-1) in the western hemisphere for December 1957, compared with those of a normal December. The westerly winds were stronger than normal and displaced to the north.

Source: After Dunn (1957).

Figure 7.24 (A) Mean 700-mb contours (in tens of feet) for December 1957, showing a fast, westerly, small-amplitude flow typical of a high zonal index. (B) Mean 700-mb zonal wind speed profiles (m s-1) in the western hemisphere for December 1957, compared with those of a normal December. The westerly winds were stronger than normal and displaced to the north.

Source: After Dunn (1957).

near Iceland than to the south giving easterlies across western Europe and the eastern North Atlantic.

The NAO appears to be the major component of a wider pressure oscillation between the north polar region and mid-latitudes - the Arctic Oscillation (AO). However, the mid-latitude zone responds with varying intensity both geographically and temporally. There is a much weaker mid-latitude signature of the Arctic Oscillation over the North Pacific Ocean than over the North Atlantic. Nevertheless, in the southern hemisphere there is a corresponding Antarctic Oscillation between the polar region and southern mid-latitudes. For this reason, some researchers consider the two zonally symmetric modes to be more fundamental features of the global circulation. They also extend upward throughout the troposphere. In the twentieth century, the NAO index (of south-north pressure difference) was generally low from 1925 to 1970. Air temperatures in the northern hemisphere were above normal and cyclones along the east coast of North America tended to be located over the ocean, thus causing longer, drier east coast summers. Prior to 1925, a regime of colder climatic conditions was associated with a higher NAO index. Since 1989, the NAO has been mostly positive, except for the winters of 1995 to 1996 and 1996 to 1997. This recent phase has given rise to winters that, compared to normal, are warmer over much of Europe, wetter (drier) over northern Europe-Scandinavia (southern Europe-Mediterranean), in association with a northward shift of storm tracks.

Figure 7.25 (A) Mean 700-mb contours (in tens of feet) for February 1958. (B) Mean 700-mb zonal wind speed profiles (m s-1) in the western hemisphere for February 1958, compared with those of a normal February. The westerly winds were stronger than normal at low latitudes, with a peak at about 33°N.

Source: After Klein (1958), by permission of the American Meteorological Society.

Figure 7.25 (A) Mean 700-mb contours (in tens of feet) for February 1958. (B) Mean 700-mb zonal wind speed profiles (m s-1) in the western hemisphere for February 1958, compared with those of a normal February. The westerly winds were stronger than normal at low latitudes, with a peak at about 33°N.

Source: After Klein (1958), by permission of the American Meteorological Society.

Was this article helpful?

0 0
Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment