The snow isotopic composition (Δ18O and ΔD) of two shallow cores from the high accumulation summit region of Law Dome, east Antarctica, was measured at monthly resolution over the 1980--1992 period. While the Δ18O or ΔD signals clearly reflect the local temperature cycle, the deuterium excess (d=ΔD-8Δ18O) is shifted with respect to Δ18O cycle by a 4 months lag. Interpretation of this phase lag is investigated using both an Atmospheric General Circulation Model (AGCM), which includes the water isotopic cycles, and a simple isotopic model, which better describes the microphysical processes within the cloud. Using this dual approach, we show that the seasonality of Δ18O and d at Law Dome summit results from a combination of the southern ocean temperature cycle (shifted by 2--3 months with respect to the local insolation) and seasonal moisture origin changes due to a strong contribution of the local ocean when ice free. Both approaches are consistent with a dominant temperate to subtropical moisture origin. We thus demonstrate from our present-day subseasonal study that the record of d in the Dome Summit South (DSS) deep ice core represents a potential tool for identifying changes in Southern Ocean temperatures and/or sea ice cover at the scale of the past thousand years. ¿ 2000 American Geophysical Union