The upper ocean of the Rockall Trough exhibits coherent interannual variations in temperature and salinity over the past 26 years, with highs in the mid-1980s and late 1990s and lows in the late 1970s and early 1990s, and with ranges of ¿0.5¿C and ¿0.05 in salinity. The origins of the interannual changes are discussed, covering three potential influencing factors: the propagation of anomalies developed upstream of the basin, the effect of local air-sea interaction, and the result of changes of regional circulation bringing different water masses into the region. The changes in heat and freshwater content of the upper ocean are directly compared to observed variations in air-sea heat and freshwater fluxes over the period of the time series. It is shown that the role of the atmosphere in locally altering the oceanic properties, particularly salinity, is relatively small and insufficient to explain the changes. Two recent hydrographic surveys are analyzed to ascertain how the distribution of water masses to the south of the basin may influence the properties of the northern Rockall Trough upper ocean, and the results are reviewed in the context of historical analyses. It is found that the critical factor in determining the properties is the varying amount of relatively cool and fresh North Atlantic Current water mixing with the dominant water mass, the warm saline Eastern North Atlantic Central Water at the entrance to the basin. Variations of inflowing water masses are caused by east-west changes in the location of the subpolar front, and the relationship of these changes in regional circulation to the North Atlantic wind stress field is discussed.