E. Xoplaki

NORTH ATLANTIC OSCILLATION - CONCEPTS AND STUDIES

This paper aims to provide a comprehensive review of previous studies and concepts concerning the North Atlantic Oscillation. The North Atlantic Oscillation (NAO) and its recent homologue, the Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM), are the most prominent modes of variability in the Northern Hemisphere winter climate. The NAO teleconnection is characterised by a meridional displacement of atmospheric mass over the North Atlantic area. Its state is usually expressed by the standardised air pressure difference between the Azores High and the Iceland Low. ThisNAO index is a measure of the strength of the westerly flow (positive with strong westerlies, and vice versa). Together with the El Niño/Southern Oscillation (ENSO) phenomenon, the NAO is a major source of seasonal to interdecadal variability in the global atmosphere. On interannual and shorter time scales, the NAO dynamics can be explained as a purely internal mode of variability of the atmospheric circulation. Interdecadal variability maybe influenced, however, by ocean and sea-ice processes.

Reconstruction of monthly NAO and EU indices back to AD 1675

Instrumental station pressure, temperature and precipitation measurements and proxy data were used to statistically reconstruct monthly time series of the North Atlantic Oscillation (NAO) and the Eurasian (EU) circulation indices back to 1675. Systematic testing of the reconstruction procedure indicated generally reliable reconstructions throughout the entire period, except for summertime before about 1750. Predictive skill varied for different sub-periods depending on data availability. It was highest for autumn and winter and was generally better for the EU than for the NAO index. Wavelet analysis suggested significant low-frequency variability, especially for the spring, summer and annual averaged indices. The co-variability between the NAO and EU indices was found to exhibit large decadal to century timescale variations, indicating that climate variability over the continent is temporarily decoupled from the NAO.

Can we trust proxy‐based NAO index reconstructions?

Three existing cold season North Atlantic Oscillation (NAO) indices which were reconstructed from proxy data were assessed for their consistency and robustness at the inter annual and decadal timescales. All three indices were found to be inconsistent with an observed NAO index, whereas two other, mainly instrument ally based indices, showed good agreement with observations. The combination of the proxy-based indices into one new index by means of multivariate regression yielded some improvement, such that the skill of the new index was comparable or higher than the individual indices. However, the new index still did not correlate significantly with the instrumentally based reference time series prior to 1850. Our results suggest non-stationarities in the statistical relationships between the proxy-based indices and the NAO. This has important implications with regard to the choice of verification periods and predictor data-base for upcoming reconstruction attempts.

WARM AND COLD MONTHLY ANOMALIES ACROSS THE MEDITERRANEAN BASIN AND THEIR RELATIONSHIP WITH CIRCULATION; 1860-1990

Anomalously wet and dry months in the Mediterranean basin were identified during the period 1860–1990 from observations at five stations located along the west-east axis of the Mediterranean basin (Barcelona, Florence, Malta, Athens and Jerusalem), supplemented by data from Madrid and Lisbon. Wet and dry months were characterized by hydric indices (HI) based on values of the standardized precipitation anomalies. Different patterns of anomalously wet and dry months were qualitatively identified on the basis of the spatial distributions of the hydric indices. The standardized sea level pressure values at 56 grid points in the domain 35° N–65° N, 30° W–40° E, for each of the anomalously wet and dry months, were subjected to T-mode Principal Component Analysis.

Spatial Winter Precipitation Distribution over the Iberian Peninsula and Greece and its Relation to the Large Scale 500 hPa Circulation

Using Canonical Correlation Analysis (CCA) in EOF space, the spatial distribution of winter precipitation over the Iberian Peninsula and Greece has been found to related to the 500 hPa geopotential height fields over the eastern North Atlantic and European area. This study is based on 4 stations in Portugal, 38 in Spain, and 21 in Greece and covers the winter months (DJF) during the period 1958–1994.