A stacked water isotope record for Summit, central Greenland, has been established covering approximately the last 900 years. Measurements of Δ18O and ΔD in the GRIP deep ice core and the 230-m core, S93, allow the reconstruction of a millennial record of the deuterium excess in a near-annual resolution. A short period of particularly high values of the Deuterium Excess at the beginning of the fourteenth century may be associated with the medieval warm period (MWP). The Little Ice Age (LIA) might be represented by a 100-year period of very low excess values in the sixteenth and seventeenth centuries. Using a simple isotope model, Δ18O and deuterium excess are interpreted in terms of surface temperature variations over central Greenland and over the subtropical North Atlantic, Greenland's principal vapor source region. An estimated cooling of -0.7 ¿C of subtropical sea surface temperatures (SSTs) during the Little Ice Age and a warming of 0.6 ¿C during the short warm period in the medieval is in agreement with previous studies. Over periods of about 100 years, an antiphasing between gradually decreasing Δ18O and increasing deuterium excess is observed. Interannual-to-decadal-scale variability associated to the North Atlantic Oscillation may be responsible for this anti-phase relationship. An alternative explication is a North-South oscillation in North Atlantic sea surface temperatures associated with short-term changes in the thermohaline circulation. ¿ 2001 American Geophysical Union <