Annual variation in temperature of the upper 400 m of fhe norfhtvestern tropical Pacifie is sfudied on the basis of 45,000 observations taken with MBTs, XBTS, and hydrographie casts. The original data are reduced to monthly averages on a 2.5 0 latitude by 5 0 longitude grid at standard depths. The mean annual variation is decomposed into annual and semiannual components by harmonie analysis. In the subtropical region north of 20 ○N, the largest annual variations occur in the upper 100 m. Annual phase increases with depth suggesting an obvious connection to variation in heat flux through the sea surface. In the tropics south of 15○N, the largest temperature variations occur within the depth range of the main thermocline. There, annual and semiannual phase are constant in the depth range of the main thermocline, consistent ruith a baroclinic response to atmospheric forcing. Dynamic height 0/200 db calculated from the vertical temperature integral is dominated by the surface thermohaline regime north of 20 ○N and the subsurface baroclinic response south of 15 ○N. Both processes play a role in the annual variation in strength of currents of the subtropics. Interannual, annual, and semiannual variabilify are compared, showing that a11 three have high vuriance in the region hetween the equator and 10 ○N. This suggests the hypothesis that modulation or interruption of the atmospheric processes maintaining the annual or semiannual variation may play a role in forcing the interannual variation.