It is generally observed in models of the coupled tropical ocean-atmosphere system that the meridional scales for oscillations at interannual periods are larger than an oceanic equatorial Rossby radius of deformation. Using 9 years of the high-resolution optimum interpolation sea surface temperature (SST) product of the National Oceanic and Atmospheric Administration (NOAA)/National Centers for Environmental Prediction (NCEP), analyses are made on the frequency dependence of the observed meridional scales, with emphasis on the latitudinal structures in the central Pacific at 140 ¿W. On the relatively short intraseasonal and seasonal timescales the SST variations are found to occur over a meridional scale of the oceanic equatorial Rossby radius of deformation suggested by conventional equatorially trapped wave theory. In contrast to this, on the longer annual and interannual timescales the meridional scales are found to increase beyond the oceanic equatorial Rossby radius of deformation. A physical explanation for this meridional scale increase with decreasing frequency in the coupled tropical ocean-atmosphere system is discussed. ¿ 1998 American Geophysical Union