The sensitivity of deep-ocean heat content (DOC) below 700 m to the surface freshwater flux (E-P-R) is studied using the MIT adjoint ocean general circulation model with idealized basins. The DOC sensitivity is positive over the entire Atlantic, but negative over the entire Pacific and Southern Ocean south of 50degS. The implication is that DOC increases (decreases) when E-P-R increases anywhere over the Atlantic (Pacific and Southern Ocean), and vice versa. Further study indicates that, for a positive anomaly of E-P-R, the increase of DOC in the Atlantic is associated with an increase of downward advective and diffusive heat fluxes. The increase of these heat fluxes may in turn be associated with the enhancement of downward vertical velocity and temperature stratification. The strengthening of meridional overturning circulation may also have contributed to the enhancement of temperature stratification. On the other hand, the negative sensitivity in the Pacific and Southern Ocean is associated with an increase of upward eddy heat flux owing to the Gent-McWilliams mixing, which is probably associated with the strengthening of meridional temperature gradient on the isopycnal surface owing to stronger temperature stratification. Assuming a 30\% error bar of the observed E-P-R, the DOC uncertainty based on the adjoint sensitivity is about 0.3degK, and is one of the major factors affecting the DOC uncertainty among the surface forcing and oceanic internal diffusivities.