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Since 1974, there has been a significant increasing trend in land and sea surface temperatures of 0.5 and 0.24°C decade -1 , respectively, in the NW Iberian Peninsula. Over the same period, annual precipitation does not show any trend, although some tendencies have been detected at seasonal scales. A significant positive trend, on average of 2 cm decade -1 , was also observed in sea level rise from 1943 onwards. Ekman transport perpendicular to the coast (upwelling index) showed a decrease from 1975 to 2008 at both annual and seasonal scales. In addition, the flow of the Miño River (the main river in the area) has also decreased at a mean rate of 18 m 3 s -1 decade -1 since 1970. At a synoptic scale, winter cyclone frequency and winter and spring blocking activity have decreased since the 1950s, which may partially explain the winter precipitation decline and the winter and spring temperature increases. These changes in synoptic systems are also in agreement with reported trends in the dominant variability modes of atmospheric circulation affecting NW Iberia, particularly a pronounced positive trend in the North Atlantic Oscillation from the 1970s to the 1990s.

The influence of the most representative winter patterns of atmospheric variation in the Northern Hemisphere on precipitation variability and on river flow regimes was evaluated for the Miño River basin (NW Iberian Peninsula). The Arctic Oscillation (AO) was also considered in order to analyze the effect of a larger-scale atmospheric mode on precipitation and river flow. Precipitation data were obtained from 22 rain-gauge stations located within the Miño River basin from 1976 to 1997, and the Miño River flow data were recorded at Frieira gauge station from 1970 to 2005. Both hydrological-year (October to September) time series were averaged for the wet season (December to February). The precipitation variability and river flow in the Miño catchment were shown to have high positive correlation. Relationships with atmospheric circulation modes show that the main precipitation variability, and consequently the river-flow regimes, during winter cannot be explained by a single atmospheric pattern. The temporal evolution of the correlation between the teleconnection indices and river flow averaged for 22 yr periods showed how all atmospheric patterns evolve with a different trend structure -in such a way that the most prevalent pattern depends on the chosen period of time. Moreover, the time evolution of the correlation between the AO and river flow remains more stationary than for the rest of the indices, due to its larger scale. KEY WORDS: Teleconnection indices · Arctic Oscillation · Precipitation · River flow · Iberian Peninsula Resale or republication not permitted without written consent of the publisherClim Res 32: [63][64][65][66][67][68][69][70][71][72][73] 2006 respondence between precipitation and the dominant pattern of atmospheric-circulation variability in the North Atlantic, the North Atlantic Oscillation (NAO) (Wallace & Gutzler 1981). Thompson & Wallace (1998) introduced the hypothesis that the NAO is the local manifestation of a global pattern called the Arctic Oscillation (AO). This AO is the result of the coupling between the stratospheric polar vortex and tropospheric circulation. Previous research has found a strong correlation between the strength of the polar stratospheric vortex and geopotential anomalies in the middle troposphere in the Northern Hemisphere and that the structure is produced specifically with the NAO pattern. In a later study, Thompson & Wallace (2000) found that variation in the North Hemispheric polar vortex is stronger in wintertime, and that this is the season when the correlation with the NAO is larger. In this manner, the correlation between the NAO and rainfall or the NAO and river flow in winter could be considered as a consequence of the variations of a hemispheric variation mode (AO). In addition, Rodríguez-Puebla et al. (1998) identified 4 regional precipitation regimes on the IP associated with the following large-scale atmospheric patterns at the monthly time scale: NAO index; East Atlantic (EA); Southern Oscillation Index (SOI) and Scandinavia (SCAN...

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