Spanish European Monitoring and Evaluation Programme (EMEP) stations were selected to relate acid rain episodes with the meteorological structure that caused the rainfall during a 5-year period. A principal component analysis (PCA) was used to determine the origin of major ions (SO4(2-), NO3-, Cl-, Ca2+, K+, Mg2+, and Na+) in the rainwater. In addition, the meteorological origin of the rain was identified. Previous works suggested a relationship between acid rain and storm convective clouds. However, statistical analyses of pH values show that only the short-lived convective phenomena may cause acid rain in Spain. In fact, rain generated by fronts and that related to long-lived convective systems is neutral or even slightly basic. Results suggest that the acid rain might be related to the meteorological time scale instead of to the cloud type.

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At the eastern region of the North Atlantic Subtropical Gyre, the Canary Current (CC) connects the Azores Current with the North Equatorial Current (Hernández-Guerra et al., 2005;Pérez-Hernández et al., 2013) (Figure 1a). The first studies carried out in the region used historical hydrographic data to determine the existence of a seasonal change in the structure of the eastern subtropical gyre. These studies found a CC flowing closer to the African coast in summer and through the western islands in winter (Stramma & Isemer, 1988;Stramma & Müller, 1989). Using one hydrographic cruise per season carried out between Madeira and north of the Canary Islands (at 28-32°N) in and 1998, Machín et al. (2006 reported that the CC presents a seasonal cycle characterized by a southward mass transport of −1.7 ± 1.0 Sv in winter, −2.8 ± 1.0 Sv in spring, −2.9 ± 1.1 Sv in summer, and −4.5 ± 1.2 Sv in fall with a shift westward from spring to fall. Pérez-Hernández et al. (2013) confirmed that CC migrates west of the Canary Islands in fall and described a mass transport of −5.8 ± 0.2 Sv southward. In addition, both Fraile-Nuez and Hernández-Guerra (2006) using Argo data, and Mason et al. (2011)