A major part of the North Atlantic Deep Water (NADW) enters the Atlantic Ocean across the Denmark Strait Sill between Iceland and Greenland. For monitoring the Global Thermohaline Circulation, the area is predestinated for long term observation of the densest NADW component, the Denmark Strait Overflow Water (DSOW). The goal of the SFB 460 program lead by the Institut f\"{u}r Meereskunde Kiel was to detect possible variations in the deep overflow across Denmark Strait sill by means of acoustic observations, both direct ADCP current measurements and bottom Pressure/ Inverted Echo Sounders (PIES). The observation array has been deployed at positions resulting from multilinear regression of simulated measurements in a high resolution overflow process model. Currently, 39 months of time series of the field experiment from 1999 to 2003 are available. During this period, the overflow, previously regarded as remarkably stable on time scale longer than a few weeks, has exhibited some interannual fluctuations concerning plume thickness, volume transport and temperature. Significance and possible correlations of these variations to external sources are investigated.