The Summer NAO (SNAO) is defined as the first EOF of sea-level pressure over the area 40°N–70°N, 90°W–30°E over 1950–2010 (Folland et al, 2009, Bladé et al, 2011). It only is the dominant mode of variaibility in high summer (July-August) and should not be used outside of summer.

• Folland, C. K., J. Knight, H. W. Linderholm, D. Fereday, S. Ineson, and J. W. Hurrell (2009). The Summer North Atlantic Oscillation: Past, Present, and Future. J. Climate, 22, 1082–1103.
• -Bladé I., B. Liebmann, D. Fortuny, G. J. Oldenborgh. (2011). Observed and simulated impacts of the summer NAO in Europe: implications for projected drying in the Mediterranean region. Climate Dynamics. DOI:10.1007/s00382-011-1195-x