Time sequences of XBT observations (1966–79) on the great circle transect from Honolulu to San Francisco are used to investigate the source mechanisms of interannual baroclinic long waves in the eastern subtropical North Pacific. The character of baroclinic long waves is represented by interannual changes in dynamic thickness (100/500 db), computed from the thermal structure information, the latter interpolated to standard locations along the transect (100 km separation), and filtered to remove sub-annual fluctuations. Two source mechanisms are hypothesized to have been important to the generation of these waves, i.e., short-term climatic variability in sea level along the eastern boundary, associated with El Niño (e.g., Chelton and Davis, 1982), and short-term climatic variability in wind-stress curl over the northeast Pacific. The source mechanism involving sea level variability along the eastern boundary is observed to have dominated the wind-driven mechanism for the region south of 30°N, wher... Abstract Time sequences of XBT observations (1966–79) on the great circle transect from Honolulu to San Francisco are used to investigate the source mechanisms of interannual baroclinic long waves in the eastern subtropical North Pacific. The character of baroclinic long waves is represented by interannual changes in dynamic thickness (100/500 db), computed from the thermal structure information, the latter interpolated to standard locations along the transect (100 km separation), and filtered to remove sub-annual fluctuations. Two source mechanisms are hypothesized to have been important to the generation of these waves, i.e., short-term climatic variability in sea level along the eastern boundary, associated with El Niño (e.g., Chelton and Davis, 1982), and short-term climatic variability in wind-stress curl over the northeast Pacific. The source mechanism involving sea level variability along the eastern boundary is observed to have dominated the wind-driven mechanism for the region south of 30°N, wher...