Stable isotopes of water are important climatic tracers used to understand atmospheric moisture cycling and to reconstruct paleoclimate. The combined use of hydrogen and oxygen isotopes in water provides an additional parameter, deuterium excess (d), which might reflect ocean surface conditions in moisture source regions for precipitation. The d records from polar ice cores covering glacial-interglacial cycles were used to reconstruct ocean surface temperatures at the moisture source, enabling elimination of source effects from the conventional isotope thermometer. However, observations of the essential relationship between d in vapor and ocean surface conditions are very limited. To date, theoretical values predicted using simple and atmospheric general circulation models (GCM) have not been validated against the data. Here, we show the isotope ratios of atmospheric water vapor near the ocean surface in middle and high latitudes of the Southern Ocean. Our observations show that d negatively correlates with relative humidity (h) above the ocean and correlates with sea surface temperature (SST). Despite the fact that the GCMs would underestimate the absolute value of observed d, the observations and simulation results are consistent for slopes between d versus h and d versus SST, suggesting that d is a reliable index to h and SST over the ocean surface.