A weakened Atlantic Meridional Overturning Circulation will affect future heat waves by mostly attenuating their severity

Atmospheric heat waves are expected to be more severe, frequent and persistent in the decades to come. Here, we sought to ascertain to what extent such a worsened scenario of this adverse phenomenon may be affected by a projected slowdown of the Atlantic Meridional Overturning Circulation (AMOC), thus accounting for an important gap in our knowledge of physical mechanisms related to heat waves – the role of large-scale climate variability pertaining to ocean circulation impacts. We conducted a comparison between two CCSM4 simulations (nominal forcings vs. a parallel sensitivity experiment that keeps the AMOC strength constant). Our findings reveal that a hampered northward heat transport due to the AMOC weakening causes robust, widespread and significant cooling in the Northern Hemisphere and parts of South America relative to the control simulation, whereas most of the Southern Hemisphere displays a warming signal, but in a much milder amplitude and typically lacking significance over land masses. Therefore, notwithstanding other potentially noxious effects of an abated AMOC on the climate system, it should act mostly as an attenuating factor in this regard, as future heat waves will generally not be as intense as they could be under a global warming scenario where the AMOC would keep its strength.