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

[摘要] We investigate the respective role of variations in subpolar deep waterformation and Nordic Seas overflows for the decadal to multidecadalvariability of the Atlantic meridional overturning circulation (AMOC). Thisis partly done by analysing long (order of 1000 years) control simulationswith five coupled climate models. For all models, the maximum influence ofvariations in subpolar deep water formation is found at about45° N, while the maximum influence of variations in Nordic Seasoverflows is rather found at 55to 60° N. Regardingthe two overflow branches, the influence of variations in the Denmark Straitoverflow is, for all models, substantially larger than that of variations inthe overflow across the Iceland–Scotland Ridge. The latter might, however, beunderestimated, as the models in general do not realistically simulate theflow path of the Iceland–Scotland overflow water south of theIceland–Scotland Ridge. The influence of variations in subpolar deep waterformation is, on multimodel average, larger than that of variations in theDenmark Strait overflow. This is true both at 45° N, where themaximum standard deviation of decadal to multidecadal AMOC variability islocated for all but one model, and at the more classical latitude of30° N. At 30° N, variations in subpolar deep waterformation and Denmark Strait overflow explain, on multimodel average, abouthalf and one-third respectively of the decadal to multidecadal AMOC variance.Apart from analysing multimodel control simulations, we have performedsensitivity experiments with one of the models, in which we suppress thevariability of either subpolar deep water formation or Nordic Seas overflows.The sensitivity experiments indicate that variations in subpolar deep waterformation and Nordic Seas overflows are not completely independent. Wefurther conclude from these experiments that the decadal to multidecadal AMOCvariability north of about 50° N is mainly related to variations inNordic Seas overflows. At 45° N and south of this latitude,variations in both subpolar deep water formation and Nordic Seas overflowscontribute to the AMOC variability, with neither of the processes being verydominant compared to the other.

[学科分类] 海洋学与技术