Subpolar Atlantic’s mixed-layer and mid-depth waters are expected to become less dense under global warming, slowing the Atlantic Meridional Overturning Circulation (AMOC). However, the downward heat flux can counterintuitively increase as warmer water sinks in a lighter environment, thereby enhancing heat sequestration.  Here using models and observation we show the profound heat budget changes under increasing CO₂: A lighter subpolar Atlantic causes increasing downward transport of heat, making it an important component of the mixed-layer budget under AMOC, in addition to its meridional heat transport.  It leads to other unexpected climate consequences, including invalidating the prevailing paradigm based on the dominance of AMOC’s meridional heat transport, whereby a stronger AMOC leads to a warmer North Atlantic. That relationship, applicable to preindustrial conditions, is now being disrupted when vertical heat transport becomes important. The variations of AMOC cycles and subpolar sea-surface temperature have now become out of phase.