Figure 1: Updated winter NAO index based on instrumental data. Courtesy of P.D. Jones to T. Osborn (http://www.cru.uea.ac.uk/~timo/profpages/nao_update.htm)

The normal atmospheric situation over the North Atlantic Ocean has surface westerlies blowing across the ocean at about 40'N between the surface expression of the Icelandic low and the Azores high, with the most intense westerlies existing during the winter season. On times scales ranging from monthly to interdecadally, there is an oscillation of the strength of these pressure features which can be conveniently measured by the difference in surface pressure between the Azores (or some nearby station) and Iceland. The state of this North Atlantic Oscillation (NAO) is positive when the Azores high is strong and the Icelandic low is deep and negative when reversed. A time series of this normalized winter index is given in Fig. 1.

             
Figure 2: Schematic representations of the (a) positive and (b) negative phases of the North Atlantic Oscillation. (Courtesy of B. Dickson, CEFAS).

The extraordinary climatic interest in the NAO arises from two observations: the unusual locking of the NAO in its positive phase almost continuously since 1976 and the concomitant collection of climatic phenomenon which can be associated with its positive and negative phases. These two phases are given by the two parts of Figure 2. The Positive phase of the NAO is mostly easily characterized during the winter and has the following effects:

There are also direct effects of NAO variability on the ocean, both in term of direct driving of fluxes by NAO (Cayan, 1992) and in convective responses to NAO changes in heat and freshwater inputs (Dickson et al., 1996).

             
Figure 3: Normalized time series of the reconstructed mean winter (DIF) and autumn (SON) indices from 1675 to 1990. Red lines are 7-point low pass filtered time series. (Source: Luterbacher et al., 1999)

We may note that the Pacific manifestation of the NAO are consistent with the idea that the NAO itself is part of a more annular (circumpolar) mode of variability that has expression in both the North Atlantic and Pacific (Thompson and Wallace, 1998). For the purposes here, we will not distinguish between the NAO and the so?called Arctic Oscillation (AO). We might also note that the above-mentioned positive phase of NAO since 1976 is coincident with (and may be related to), the rapid global surface warming, especially at high latitudes, evident in the record.

Paleoclimatic Opportunities

The instrumental record of the NAO index extends back to about 1850 since long surface pressure records have been available at the antipodes of the NAO. As pointed out by Wunsch (1999) there are numerous difficulties involved in determining if features seen in this extant, relatively short, instrumental record of the NAO are statistically significant, let alone understanding any underlying dynamical mechanisms which may exist. For example, the instrumental record is not nearly long enough to decide if the locking in the positive phase since 1976 is truly unusual, or, if in a short record dominated by decadal signals, it has appeared many times before. Therefore, in order to better interpret the instrumental record of NAO variability it is imperative that a longer record be obtained.

Several paleoclimatic proxies have the potential to record aspects of North Atlantic climatic variability, and thereby the NAO index, with annual or higher resolution to well before the year 1700. Recent paleo?proxy NAO reconstructions with annual or better resolution include, for example, those from tree rings (Cook et al., 1998), ice cores (Appenzeller et al., 1998), stalagmites (Proctor et al., in press) as well as combined tree ring and ice core data (Stockton and Glueck, 1999). Regional synthesis of paleo?proxy indicators with subdecadal resolution can provide information regarding historical impacts of the NAO on regional moisture balance. One example is the multiproxy regional synthesis of historical records, tree?rings, laminated lake sediments, speleothems, geomorphological and other sources in the Mediterranean region currently being undertaken as part of the PAGES PEP III synthesis (detailed information on this program will be published in the upcoming PAGES Newsletter Vol.8, N'2). Multiple proxies in the Scandinavian region, including annually laminated lake sediments, tree rings; glaciers and speleothems provide another fruitful area for future paleoclimatic synthesis of NAO variability and regional expression in the past.

Luterbacher et al. 1999) have published a multiproxy derived NAO index with monthly resolution from 1675 to the present. Their reconstruction is shown in figure 3. In addition to the reconstruction, Luterbacher et al. show that the correlations between the many individual paleo reconstructions that are now available are not high enough to regard any one of them as definitive. An approach which includes multiple, independent, paleoclimatic archives and proxies is clearly required in order to provide an extended record of NAO variability. Such studies are underway (e.g. Cullen et al., submitted) and will lead, in the next few years, to both an improved record of NAO variability as well as better understand the underlying dynamics associated with this important mode of climatic variability.

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