On decadal time scales, there are three major physical processes in the Earth’s outer core that contribute to gravity variations: (i) the mass redistribution due to advection in the outer core, (ii) the mantle deformation in response to (i), and (iii) the (core) pressure loading on the core-mantle boundary. Except the last one, they cannot be evaluated from surface observations. In this paper we use MoSST core dynamics model and PREM model to understand the gravity anomalies from the three processes. Our numerical results show that, the gravity anomalies are comparable in magnitude, though that from the process (i) is in general the strongest. The gravity anomalies from the first two processes tend to offset each other (“mantle shielding”). Consequently the pressure loading effect contributes more to axisymmetric part of the net gravity variation, while the net effect from the first two processes is more important to non axisymmetric components.