The Meridional Overturning Circulation (MOC) plays an important part in the Earth's climate, but the mechanisms that determine MOC response to climate change remain unclear. In particular, the relative importance of the adiabatic and diabatic dynamics in MOC is still under debate. This study aims to explore the relationship between the air-sea density flux and isopycnal MOC, and examine the possibility of diagnosing the adiabatic component of MOC from the air-sea density flux. This is done here using the concept of the "push-pull" mode, which consists of the adiabatic push into the deep ocean in the Northern Hemisphere and pull out of the deep ocean in the Southern Hemisphere. The evolutions of the isopycnal MOC and the "push-pull" mode are qualitatively similar. The maximum streamfunctions of the "push-pull" modes and isopycnal MOC both decrease by 3-5 Sv during 100 years, and their decrease is very similar to each other in the deep layers. In particular, the slope of the downward linear trend in the maximum is about -5 Sv per 100 years in both the "push-pull" modes and isopycanl MOC at the equator. The decrease in actual isopycnal MOC is faster at heavier densities than that at lighter densities. The first EOF mode of eigenvectors of the "push-pull" mode explains less percentage of variance than in the case of the isopycnal MOC at the equator. The detection of the global changes in MOC from the surface fluxes alone is feasible, if the surface fluxes are measured with sufficient accuracy.