How Is Meridional Coherence Maintained in the Lower Limb of the Atlantic Meridional Overturning Circulation?

Despite a strong focus on latitudinal continuity of the Atlantic Meridional Overturning Circulation (AMOC) variability, transport continuity in different layers that constitute the AMOC lower limb has received considerably less attention. In this study, we investigate the transport connectivity of Upper North Atlantic Deep Water (UNADW) and Lower NADW (LNADW), with both defined by density. Using two ocean circulation models and an ocean reanalysis, we find that subpolar-originated transport anomalies, particularly for UNADW, do not propagate to the subtropics over a period of five decades. We also find that transports in both layers are linked to AMOC at subpolar latitudes, yet only LNADW transport shows linkage to AMOC in the subtropical gyre. Thus, latitudinal AMOC continuity is likely unrelated to transport continuity in any single layer, but rather a result of connection between subpolar-AMOC and subtropical-LNADW transport. An exception to this generalization is possible with strong LNADW transport events. Plain Language Summary The importance of studying the Atlantic Meridional Overturning Circulation (AMOC), characterized as northward flowing upper waters and southward flowing deep waters, lies in its significant impacts on climate. The production of deep waters in the subpolar/subarctic North Atlantic (i.e., North Atlantic Deep Water or NADW) has been assumed to create coherent NADW transport changes across a range of latitudes. Such latitudinal transport coherence is thought to create a latitudinal coherent AMOC. However, though AMOC transport coherence has been widely discussed, no study to date has explored the latitudinal coherence of NADW or its relationship with AMOC. This study aims to fill this gap. We find that transport anomalies of NADW since the 1960s, specified by different density class, do not propagate coherently from the subpolar gyre to the subtropical gyre, suggesting that latitudinal AMOC coherence is unlikely achieved through coherent transport of these deep waters. Our study challenges the paradigm that the subtropical AMOC is strongly impacted by deep water production in the subpolar gyre and highlights the importance of local forcing on AMOC variability. Both results have implications for the ocean's role in climate variability.