Partenaires et organisations internationales
(Anglais)
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Max-Planck-Institut, Hamburg (D), Météo-France, Toulouse (F), Hadley Center, Bracknell (GB), Laboratoire de Météorologie Dynamique, Paris (F), Université de Louvain-la-Neuve (B),
Danish Meteorological Institute, Copenhagen (DK)
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Résumé des résultats (Abstract)
(Anglais)
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The North Atlantic Oscillation (NAO) is one of the large-scale modes of variability in the climate system. The state of the oscillation is usually measured by the NAO index, which is the standardized difference in sea level pressure (SLP) between Lisbon (Portugal) and Stykkisholmur (Iceland). However, in order to understand the NAO, the definition of this index may be too simple since several mechanisms are proposed that could influence the NAO, which could have centers of action different from the classical NAO pattern.
In the final phase of the SIDDACLICH project, the NAO index between 1970 and 1993 has been investigated. This period shows a significant 8-year oscillation in wavelet analysis (Appenzeller et at., 1998). An oscillating component of the NAO index (a sine function with a period of 8 years) has been extracted, to determine whether it correlates with another center of action than the remaining part of the NAO index.
These two different components (based on the index of Hurrell, 1995) are then correlated with the mean winter SLP fields. A comparison shows that the 8-year oscillating signal correlates better with the western part of the NAO pattern, and the remaining part of the NAO index with the eastern part of NAO pattern. A relationship between the 8-year oscillation in sea level pressure and the underlying sea-surface temperatures (SST) may be expected, since the ocean could have a multiple-year memory, which is probably not the case for the atmosphere. Indeed the underlying SST also correlates very closely (>0.8) with the sine function, but lags by 2 years or with a phase shift of 90 degrees. Thus, a direct relationship between the oscillation in SLP and SST can almost be excluded.
This study shows that a very simple decomposition of the NAO index is able to separate the NAO pattern into two significant sub-patterns for the period between 1970 and 1993. A similar result has been obtained by Bresch (1998), who showed in limited-area simulations, that only the western part of the NAO pattern is influenced by SST-anomalies. It is not able to explain the physical mechanisms behind the eight-year oscillation, however. In order to find such a mechanism, one should look for eight-year oscillations in climate variables such as sub-surface temperature and salinity, as well as sea surface salinity.
References:
· Appenzeller, C., Stocker, T. F. and Anklin, M. 1998: North Atlantic Oscillation Dynamics Recorded in Greenland Ice Cores, Science, conditional accepted.
· Hurrell, J.W., 1995: Decadal trends in the North Atlantic Oscillation: regional temperatures and precipitation. Science, 269, 676-679.
· Bresch, D., 1998: Coupled flow and SST patterns of the North Atlantic, Diss. Naturwiss. ETH Zuerich, Nr. 12878
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