The climatologically important seasonal signal in ice core oxygen isotope records as altered by diffusion with time. Here we use the diffusion model of Whillans and Grootes <1985> to calculate the effects of isotopic diffusion in nine shallow ice cores recovered over a 150¿150 km grid centered on Summit, Greenland. Two different computational methods give similar results for the back-diffused Δ18O seasonal extrema. Although back diffusing the ice core Δ18O records improves the correlations between the individual ice core oxygen isotope and Greenland coastal temperature records; the correlations remain poor, particularly for the low-accumulation sites. Stacking the back-diffused isotopic records from several cores results in much higher correlations, with a strong winter linkage between west coast stations and central Greenland. These correlations are also significantly greater than the correlations between the stacked measured (diffused) isotopic profiles and coastal temperatures. Our results indicate that the use of ice core isotopic records to identify anomalous seasons requires that diffusion effects be accounted for. ¿ 2000 American Geophysical Union