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Changes in streamflow of the Tigris and the Euphrates Rivers are shown to be associated with the North Atlantic Oscillation (NAO), a large-scale mode of natural climate variability which governs the path of Atlantic mid-latitude storm tracks and precipitation in the eastern Mediterranean. Composite indices of Turkish winter (December‐March, DJFM) temperature and precipitation are developed which capture the interannual‐decadal climate variability for the Tigris‐Euphrates headwater region, a significant source of freshwater for Turkey, Syria and Iraq. These indices are significantly correlated with the NAO, with 27% of the variance in precipitation accounted for by this natural mechanism. As evidenced by the recent widespread drought events of 1984, 1989 and 1990, the Tigris‐Euphrates streamflow also exhibits significant, 940% variability, associated with extrema. Copyright © 2000 Royal Meteorological Society.