Preliminary investigations show regional differences in atmospheric 14C activity archived in tree-ring cellulose may reflect oceanic and atmospheric shifts associated with ENSO (El Ni¿o-Southern Oscillation) events. Much of the shared variation between sites in Washington and Arizona can be attributed to global forcings such as anthropogenic 14C-depleted CO2 emissions and modulation of 14C production. The contrasted residual variation between the Washington and Arizona data sets correlates with the annual Southern Oscillation Index (SOI) of the previous year. The spatial differences between Δ14C data sets may provide a proxy indicator of ENSO phenomenon. ¿ American Geophysical Union 1993