Global gravity data and seismic inferences of lateral inhomogeneities in the density distribution of the Earth's mantle provide information on convection in the lower mantle. The data are interpreted in terms of a model of mantle convection with two layers separated by the interface corresponding to the 670 km seismic discontinuity. It is shown that the observations are in approximate agreement with theoretical deductions for a l=2 convection mode in the lower mantle.