strong-wind/weak-wind) of such oscillations (Palmer, 1993). However, the scientific community is divided on the issue of whether analogous “regime-like” behavior exists in instrumental records related to the real climate system. Hansen and Sutera (1995), Corti et al. (1999), and Monahan et al. (2000) found evidence of such mode-shift behavior. However observational evidence has been questioned (e.g., Nitsche et al., 1994; Berner and Branstator, 2001). Also, questions remain about whether such behavior should be characteristic of an entity with as many degrees of freedom as the climate system (Dymnikov and Gritsoun, 2001).

The possibility that mode shifts participated in or provide clues to the large, abrupt climate changes of preinstrumental times suggests common mechanisms or even common causes. Thus, the study of abrupt climate change should involve consideration of the preferred modes of the climate system.

Annular Modes

The annular modes—the Arctic Oscillation (AO) and the Antarctic Oscillation (AAO)—primarily affect polar to middle-latitude regions in both the North and South and are the dominant modes of climate variability in these areas, especially in the winter. The AO and AAO represent a transfer of atmospheric mass between subtropical high-pressure regions and polar lows. A strongly positive state of an annular mode is associated with intensified highs and lows driving strong atmospheric circulation. The negative state has much less difference between high- and low-pressure regions and thus is related to weaker atmospheric circulation.

The southern annular mode is moderately symmetric about the pole, but owing to the complex geometry of northern continents, the AO is especially strong over the North Atlantic and less evident in other regions. Thus, the mode was originally described as the North Atlantic Oscillation (NAO), and an NAO index was based on the difference in atmospheric pressure between Portugal and Iceland (Hurrell, 1995). When the winter pressure difference is large, frequent strong storms take a northeasterly track across the North Atlantic, producing warm and wet weather in northern Europe, cold and dry conditions in northern Canada, and mild and wet conditions along the US East Coast. In contrast, a small pressure difference produces fewer, weaker storms, taking an easterly track to produce a moist Mediterranean, cold northern Europe, and a snowy US East Coast in response to frequent cold-air outbursts.

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