Interannual variability of the circulation in the northeast Pacific Ocean is explored through a joint analysis of expendable bathythermograph (XBT) and expendable conductivity-temperature-depth (XCTD) data, satellite altimetry, and output from a model that was constrained by ocean data. XBT temperature profiles with high spatial resolution are available in the eastern North Pacific along two repeated transects. These ship tracks, along with the coast of North America, define a closed "box" which is used to study the time-mean circulation and its variability on interannual timescales. Geostrophic velocities from XBT data are compared with geostrophic velocities from model output as well as the full model velocity fields. Correlations in variability on interannual timescales between transport in the subpolar gyre and in the subtropical gyre are present in both model output and data. The nature of the variability, and its relation to the changes of the strength of the North Pacific Current (NPC), which supplies the water for both gyres, are explored. Interannual variability in gyre transport is found to be related to both the bifurcation of the NPC, resulting in an anticorrelation in transport between the two gyres, and to variations in NPC strength, resulting in simultaneous changes in the two gyres. The dominant signal is found to be a long-term increase in the NPC, which results in a strengthening of the subtropical gyre. Possible connections with local-scale wind stress changes and with the El Ni¿o/Southern Oscillation phenomenon are also explored.