The annual cycle of temperature in the subthermocline equatorial Pacific is studied using a new compilation of historical hydrographic profiles. The observations have several characteristics suggestive of a vertically propagating, first meridional mode (l=1) long-wavelength Rossby wave: phase lines that slope downward from east to west indicative of upward and westward phase propagation amplitude maxima parallel to phase lines, and nearly symmetric off-equatorial maxima of annual amplitude. Estimates of zonal wavenumber, vertical wavenumber, and the location of maxima of isotherm displacements are consistent with those of the l = 1 Rossby wave. A solution to a linear continuously stratified model, driven by a version of the observed annual wind field, confirms this interpretation. The solution is dominated by a vertically propagating, l = 1 Rossby wave. The wave is generated primarily by the westward-propagating component of the equatorial zonal wind field; it carries energy along WKB ray paths i... Abstract The annual cycle of temperature in the subthermocline equatorial Pacific is studied using a new compilation of historical hydrographic profiles. The observations have several characteristics suggestive of a vertically propagating, first meridional mode (l=1) long-wavelength Rossby wave: phase lines that slope downward from east to west indicative of upward and westward phase propagation amplitude maxima parallel to phase lines, and nearly symmetric off-equatorial maxima of annual amplitude. Estimates of zonal wavenumber, vertical wavenumber, and the location of maxima of isotherm displacements are consistent with those of the l = 1 Rossby wave. A solution to a linear continuously stratified model, driven by a version of the observed annual wind field, confirms this interpretation. The solution is dominated by a vertically propagating, l = 1 Rossby wave. The wave is generated primarily by the westward-propagating component of the equatorial zonal wind field; it carries energy along WKB ray paths i...