Upstream basin circulation of rotating, hydraulically controlled flows

The overflows of dense water from the Nordic Seas into the North Atlantic are a key element of the global meridional overturning circulation. The deep southward limb of the overturning is fed primarily by the overflows, and the transport of the deep limb is closely linked to the transport in the warmer northward currents in the upper ocean of the North Atlantic. The threat of potentially significant shifts in climate due to changes in the overturning motivates an improved understanding of the overflows and the associated upstream basin circulation. Despite enhanced recent efforts [1, 2], the source regions and pathways of the deep water masses upstream of the overflows remain uncertain. Observations of the Denmark Strait and Faroe Bank Channel, the two primary exit points of deep water from the Nordic Seas, indicate that the overflows are hydraulically controlled. Hydraulic control is qualitatively suggested by the characteristic spillage observed in the drawdown of downstream isopycnals (Figure 1), the lack of seasonality in overflow transport and the dependence of the overflow transport on upstream interfacial elevation. The hydraulic control has also been quantitively confirmed by the identification of control sections where the flow undergoes sub-critical to super-critical transitions [4]. A feature of hydraulically controlled flows is that the stratification in the basin upstream of the overflow sill or strait consists of a uniform dense layer overlaid by a dynamically inactive upper layer, as shown in Figure 1. It is therefore not unreasonable to model the upstream basin as a single fluid layer (or 1 2 layers with reduced gravity) governed by the shallow water equations. Here we focus on the nature of the upstream basin circulation in an idealised one layer model with circular basin geometry and varying source location, using a combination of laboratory and numerical experiments.