A newly developed ocean reanalysis dataset (1987--2004) has been used to study interannual variability in the temperature inversion that characterizes the upper layer of the subarctic North Pacific. A local minimum in temperature at the upper edge of the inversion exhibits a significant interannual variability. Conversely, fluctuations in the local temperature maximum (T max) found at the lower edge are relatively small. In the T max case, the variation in the depth of T max is significant and is closely related to ENSO variability. Our estimate of the adjoint solution shows that during periods when the T max is located at shallower (deeper) depths, the source water comes from both the Gulf of Alaska and the Kuroshio Extension region (only from the Kuroshio Extension region). These facts suggest that the depth of T max is an important indication for diagnostics of the interannual variability of water-mass property in the subarctic Pacific.