The role of subpolar deep water formation and Nordic Seas overflows in simulated multidecadal variability of the Atlantic overturning.

We investigate the respective role of variations in subpolar deep water formation and Nordic Seas overflows for the decadal to multidecadal variability of the Atlantic meridional overturning circulation (AMOC). This is done by analysing long (order of 1000 yr) control simulations with five coupled climate models as well as sensitivity experiments performed with one of the models, in which we suppress the variability of either subpolar deep water formation or Nordic Seas overflows. For all models, the maximum influence of variations in subpolar deep water formation is found at about 45° N, while the maximum influence of variations in Nordic Seas overflows is rather found at 55° N to 60° N. Regarding the two overflow branches, the influence of variations in the Denmark Strait overflow is, for all models, substantially larger than that of variations in the overflow across the Iceland-Scotland-Ridge. The influence of variations in subpolar deep water formation is, on multi-model average, larger than that of variations in the Denmark Strait overflow. This is true both at 45° N, where the maximum standard de viation of decadal to multidecadal AMOC variability is located for all but one model, and at the more classical latitude of 30° N. At 30° N, variations in subpolar deep water formation and Denmark Strait overflow explain, on multi-model average, about half and one third respectively of the decadal to multidecadal AMOC variance. [ABSTRACT FROM AUTHOR]