Monthly to seasonal variability

As we discussed in chapter 1, the seasonal cycle itself arises because of the tilt of Earth's axis of rotation relative to the Earth-sun axis. The axis of rotation of Earth is fixed relative to the distant stars, so that as Earth moves around the sun, the North Pole points generally toward the sun (giving the Northern Hemisphere summer), away from the sun (giving the Northern Hemisphere winter) or somewhere in between, as illustrated in figure 1.2 in chapter 1. If we think of a season as lasting roughly three months, are there any climate phenomena that have timescales between the weather timescale and the seasonal timescale? The answer is, to a degree yes, there is at least one such phenomenon, known as the North Atlantic Oscillation (NAo). The NAo is a phenomenon at the interface between weather and climate that dictates variability on a monthly timescale over the North Atlantic and surrounding regions, thus from the eastern seaboard of the United States, over Greenland, to Europe, and from the Arctic region to the canaries. Indeed, to some extent the NAo affects the weather and climate over the entire Northern Hemisphere. There is an analogue of the NAo in the Southern Hemisphere (called the Southern Annular Mode), although it has a more hemispheric extent, and a somewhat similar pattern in the Pacific Basin (called the Pacific-North American pattern), but none are quite as well studied as the NAo, so let us focus on that.

So what is the NAO? It may be thought of as a north-south oscillation of the main patterns of weather variability over the Atlantic region, especially in the winter, as illustrated in figure 6.1. During the positive phase of the NAO, the main path of weather systems tracks a little further north than usual. Because the air is coming

Figure 6.1. A schematic of the two phases of the NAO. The top panel shows the positive phase of the NAO, with storms tracking northward bringing mild but wet weather to northern Europe. The bottom panel shows the negative phase, with storms tracking southward bringing wet weather to southern Europe and colder weather to northern Europe.

Figure 6.1. A schematic of the two phases of the NAO. The top panel shows the positive phase of the NAO, with storms tracking northward bringing mild but wet weather to northern Europe. The bottom panel shows the negative phase, with storms tracking southward bringing wet weather to southern Europe and colder weather to northern Europe.

straight over the ocean, it tends to be relatively warm and so brings warm, wet weather to the United Kingdom and other parts of northern Europe, with precipitation often falling as rain rather than snow in the United Kingdom, with similar effects downstream into eastern Europe and even Asia. Southern Europe and North Africa tend to have somewhat cooler weather than usual during these periods. Meanwhile, stronger northerly and northeasterly winds over Greenland and northeastern Canada bring cold, dry air to these parts, decreasing the temperature, with the eastern parts of the United States getting higher temperatures and more precipitation than normal, rather like northern Europe.

During the negative phase of the NAO, the storm track swings southward, bringing mild, wet weather to southern Europe. During these periods, northern Europe tends to receive air that has come from the east, which, since it has come from a continental land mass in winter, tends to be very cold, with precipitation often in the form of snow. Greenland, on the other hand, is now somewhat warmer than usual. The signal of the NAo is evidently quite large and coherent and in fact accounts for about one-third of the Northern Hemisphere's inter-annual surface variance during winter.

The distinctive pattern of the NAO, whether positive or negative, tends to last for several days to a few weeks, perhaps a little longer than regular weather patterns, but undoubtedly the main mechanism for the NAO lies in the atmosphere itself. However, there may be a longer timescale to the NAO, as illustrated in figure 6.2. Large climate va ri ability climate va ri ability

Figure 6.2. The NAO index from 1860 to 2005. The index is based on the normalized difference in average winter surface pressure between Lisbon, Portugal, and Stykkisholmur, Iceland. The heavy solid line shows the index after it has been smoothed to remove fluctuations of less than four years.

changes in the NAO index occur from year to year, and on the whole atmospheric behavior in one winter is largely independent of its behavior the previous winter. (The correlation in the NAO index from one year to another is only about 0.1.) Nevertheless, there is some indication that there are certain periods of time when the NAO persists from year to year. For example, from about 1905 to 1915 the NAO index was largely positive. It was negative from about 1950 to 1970, and then unusually positive in the 1980s and 1990s. Among other things, the positive index in the 1980s and 1990s brought higher than usual precipitation to Scandinavia and may have ameliorated the effects of global warming in the retreat of the glaciers. In contrast, over the Alps, a positive NAO index and less than normal precipitation may have worked in conjunction with global warming to cause a significant retreat of the Alpine glaciers. The Iberian peninsula and other areas around the Mediterranean also experienced drought in the late twentieth century.

It is an open question whether such long periods of persistent behavior are much more than the random variations of a chaotic system (rather like throwing six sixes in a row with a die) or whether they have a specific cause, most likely in the ocean. Why is the cause most likely in the ocean? It is because the timescales of the interannual variability correspond most closely to the timescales occurring in large-scale ocean dynamics, for example, in the dynamics of the ocean gyres. The dominant internal dynamics of the atmosphere are most likely too short to produce coherent dynamics that last over decadal timescales, whereas the dynamics of land ice sheets, or of changes in insolation at the top of the atmosphere because of orbital variations, are too long. One possibility is that volcanoes can have a climatic effect on the decadal timescale, but the correlation of the NAO with volcanoes is small. So let us look at the role of the ocean—that is, after all, the topic of this book.

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