The Iberian Peninsula precipitation and river flow regimes are characterized by large values of inter-annual variability, with large disparities between wet and dry years, especially in southern Iberia. This situation is a major problem for water resources management in general, and for the production of hydroelectricity in particular. Hydroelectric production represents, in an average year of precipitation, 20% of the total Spanish electricity production (and 35% for Portuguese production). Its absolute value, however, can vary by a factor of three between wet and dry years. We have assessed the impact of the North Atlantic oscillation (NAO) on winter precipitation and river flow regimes for the three main international Iberian river basins, namely the Douro (north), the Tejo (centre) and the Guadiana (south). Results show that the large inter-annual variability in the flows of these three rivers is largely modulated by the NAO phenomenon. Throughout most of the 20th century, the January-to-March river flow is better correlated with the December to February (DJF) NAO index than is the simultaneous (DJF) river flow. Correlation values for the period 1973-98 are highly significant (−0.76 for Douro, −0.77 for Tejo and −0.79 for Guadiana), being consistently of higher magnitude than those obtained over previous decades. This impact of the NAO on winter river flow was quantified in terms of total Spanish potential hydroelectricity production. The important control exerted by the NAO and the recent positive trend in the NAO index contribute to a significant decrease in the available flow. This reduction represents an important hazard for the two Iberian economies because of its negative impact on water-dependent resources, such as intensive agriculture and hydroelectric power production.

This paper is part of a research programme devoted to the analysis of historical climate in Spain. A reconstruction of rainfall characteristics in Southern Spain from 1500 to the present is described. Weather information was taken from original documentary sources in the region. A numerical index was established to characterize the rainfall and its evolution. Results were calibrated with modern precipitation data and with the results of other studies of historical climate. The main objective is to obtain a long precipitation record. A preliminary analysis of the obtained series shows a fluctuating behavior with alternating dry and wet periods. The wettest periods occurred at the end of 16th century, the beginning of 17th century, and at the end of 19th century. The driest periods in the pre-instrumental era occurred during the first half of the 16th century, and around 1750. A general decreasing trend in precipitation can be observed from 1960 onwards.

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The winter precipitation anomalies in the European area have been analysed over the period 1900-98 based on the El Niño-Southern oscillation (ENSO) state. A set of winter and autumn ENSO events is first selected using the Sea-Surface temperature (SST) data of the Niño 3 region, with the constraint that the ENSO event is well developed during the winter and autumn of study, and that it is an extreme event. Cold and warm ENSO events and periods that can be regarded as normal are selected. For the selected winter ENSO events and for the winter following the selected autumn ENSO events, composites of European winter precipitation anomalies have been obtained and compared with each other. A study of the consistency among events of the relationship between ENSO and precipitation anomalies was also carried out. The analysis of the winter precipitation anomalies based on the selected winter ENSO events shows the existence, for the European area and during La Niña events, of a statistically significant precipitation anomaly pattern with positive precipitation anomalies north of the British Isles and in the Scandinavian area and negative anomalies in southern Europe, resembling the precipitation pattern associated with the positive phase of the North Atlantic oscillation (NAO). Particularly, for the southwestern area of the Iberian Peninsula, the negative anomaly reaches 20% of the winter average precipitation. The consistency analysis shows that this precipitation pattern is not the result of a few major events, but rather that it is stable and qualitatively similar to that found during the positive phase of the NAO. A non-linear response to ENSO is found in the eastern Mediterranean area: negative precipitation anomalies are found, having similar amplitude anomalies, both during El Niño and La Niña events. The analysis of the precipitation anomalies for the winter following the selected autumn ENSO events shows very similar results to those found for the previous analysis, thus suggesting the existence of a potential source of seasonal forecasting of European precipitation. An analysis of the sensitivity of the precipitation anomalies to the strength of the ENSO events shows that, when the strength of the ENSO increases, the magnitude of the rainfall anomalies does not change, but the area influenced and the coherence between events do increase slightly.

In this work a long rainfall series in Andalusia (southern Spain) is analysed. Methods of historical climatology were used to reconstruct a 500-year series from historical sources. Different statistical tools were used to detect and characterize significant changes in this series. Results indicate rainfall fluctuations, without abrupt changes, in the following alternating dry and wet phases: 1501-1589 dry, 1590 -1649 wet, 1650 -1775 dry, 1776-1937 wet and 1938-1997 dry. Possible causal mechanisms are discussed, emphasizing the important contribution of the North Atlantic Oscillation (NAO) to rainfall variability in the region. Solar activity is discussed in relation to the Maunder Minimum period, and finally the past and present are compared. Results indicate that the magnitude of fluctuations is similar in the past and present.

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[1] For the period 1826-2000, we analyze the spectral characteristics of the winter North Atlantic Oscillation (NAO) index and its predictability based on Singular Spectral Analysis (SSA) and Autoregressive Moving Average (ARMA) models. In the first part, SSA is applied to the winter NAO index to isolate its main spectral characteristics. Based on the SSA, a reconstruction (filtering) of the winter NAO index series was carried out. Results of the SSA indicate that the winter NAO index can be broken down into some modulated amplitude oscillations with periods around 7.7 and 4.8 years, some oscillations associated with a broadband peak of period around 2.4 years along with nonlinear trends. The sum of these components, the SSA-filtered series, explains 56% of the variance of the raw winter NAO index. The SSA-filtered series is particularly reliable, reproducing the NAO phase during extreme events (winter NAO index ! 1 or À1); for this subset of events, the phase of the actual and SSA-filtered series shows to be the same in 91% of the cases. The high positive values observed in the winter NAO index in the last eighties and nineties appear to be associated with the simultaneous presence of a positive trend, starting in the early eighties and of unprecedented steepness, and an oscillation with period around 7.7 years, having very high amplitude. In the second part, an ARMA model has been fitted to the filtered winter NAO index and a forecasting experiment was conducted; results are tested against the raw winter NAO index. Results show that the ARMA modeling has useful 1-year-ahead forecasting abilities. Particularly, over the period 1986-2000, not used to fit the model, the model skill is 27.8% better than climatology and 43.3% better than persistence (38.5% and 47.6%, respectively, when taking into account only extreme NAO events). Additionally, percentage of cases in which the NAO phase was accurately predicted proved to be 80% (88% for extreme NAO events). For

[1] We explore the relationship between the NAO and the solar radiation spatio-temporal variability in the European North Atlantic area during winter. Measured monthly sums of sunshine duration and short-wave downward solar flux reanalysis data have been used. Correlation analysis between the NAO index and the measured sunshine duration shows a dipolar pattern, with maximum positive values (+0.75) over the Iberian Peninsula, and maximum negative values (À0.71) over Norway. Reanalysis results confirm these findings. Composite analysis shows, for northern Europe, negative anomalies (À10% to À20%) associated with NAO > 1 and positive anomalies (10% to 20%) associated with NAO < À1; while for southern Europe anomalies are, respectively, 10% to 20% and À10% to À20%. A stronger influence is found during the NAO negative phase; particularly, the northern British Isles, Norway and the Iberian Peninsula present a significant non-linear response, with higher anomalies (10% to 20%) during this negative phase.