Thermohaline current in North Atlantic
Complementary to the points about salinity emphasised by TomO and S Verhoef, one more point to emphasise is the role of deep ocean convection. You may have in mind this picture of water sinking because it gets heavy, which is not quite wrong, but you get a more accurate picture if you see that the water column down to about 3000 meter deep is convectively mixing (mainly in winter) in localised areas of the  Greenland, Iceland, Norwegian and Labrador seas. Convection is what you get when dense water lies above lighter fluid, which is as unstable as an egg on its head. 

The consequence of this mixing is twofold. 

First it shapes the hydrological structure of the deep ocean and determines its organisation into distinct water masses (e.g.: http://www.soes.soton.ac.uk/teaching/courses/oa631/plot_20w.jpg ) 

Second it induces a strong cooling of the ocean water masses. 

Together these effects control the strength of the meridional overturning cell. 

If your are after an advanced and authoritative review on this, see http://onlinelibrary.wiley.com/doi/10.1002/2015RG000493/full, which is open access.