The association between local maxima in the geoid and subduction zones is examined. While it is well known that subduction zones are associated with broad local maxima in the geoid at spherical harmonic degrees 4--9, there is an impressive correlation between back arc geoid maxima at the 5000 km length scale (i.e., spherical harmonic degrees >9) and subduction zones with the exception of the Scotia and Caribbean arcs. Geoid maxima are also observed in the forearc regions of the Aleutian and Central American subduction zones. Numerical modeling of compressible convection with multiple phase transformations indicates that an increase in viscosity at or near the 660 km phase transformation is necessary to explain the pattern of geoid maxima associated with subduction zones. The addition of an endothermic phase transformation at 660 km depth does not provide sufficient resistance to the slab to eliminate the need for a viscosity increase at or near 660 km. Short-wavelength geoid and topography profiles might provide an additional constraint on the deformation at subduction zones; however, it may be necessary to incorporate geologic history into subduction calculations to further improve the results presented here.