Tropical instability waves north and south of the equator play an important role in the transport of mass and heat in the tropical Pacific ocean. Using data from the TOPEX/Poseidon satellite and the Tropical Atmosphere-Ocean buoy array, we present important observational evidence that Rossby waves modify the phase of tropical instability waves, supporting a recent hypothesis on interactions between these waves. A marked increase in the correlation coefficient between the northern and southern instability wave fields when Rossby waves are present indicates an increased phase correspondence between the fields. There is also an associated amplification of the southern instability wave field. A comparison with buoy data indicates that the satellite is able to determine the phase of instability waves accurately. This application of satellite and in situ data marks a new approach in the identification and analysis of coupling mechanisms between important dynamical processes in the equatorial Pacific. ¿ 2000 American Geophysical Union