Water masses that ventilate intermediate ocean depth, such as the North Atlantic Subtropical Mode Water (NASTMW), play important roles in various aspects of climate, but their basic properties and space-time variability need to be better quantified. Herein we examine the mean annual cycle and long-term variability of NASTMW for the 40-year period 1961--2000. Integrated NASTMW properties such as water mass volume, temperature, and heat content show a consistent annual cycle superimposed over a much larger interannual-to-decadal cycle that is strongly correlated to the North Atlantic Oscillation index. The 40-year record clearly shows the ocean's ability to integrate atmospheric forcing over several years and to exhibit an interannual memory of wintertime forcing by the atmosphere.