Controls on dense-water formation along the path of the North Atlantic subpolar gyre

The North Atlantic subpolar gyre (SPG) plays a fundamental role in the global climate system through the formation of dense North Atlantic Deep Water (NADW) as part of the Atlantic Meridional Overturning Circulation. Observations show pronounced decadal variability in SPG water mass properties; however, it remains unclear to what extent such thermohaline changes impact the formation of dense water. Here, we explore the mechanisms governing dense-water formation along the path of the SPG using Lagrangian water parcel trajectories in an eddy-rich ocean sea ice hindcast. We show that neither the rate of transformation of water parcels across density surfaces nor their thermohaline properties on arrival into the eastern SPG are rate-limiting factors governing dense-water formation. Instead, the total amount of dense water formed during transit around the SPG can be skilfully predicted based solely on the volume transport of light, upper-limb waters arriving into the eastern SPG via the branches of the North Atlantic Current. This relationship between upper-limb volume transport and dense-water formation emerges since the SPG boundary current is long enough for all upper-limb thermal anomalies to be damped during transit. Our findings emphasise the close relationship between the strength of the SPG and subpolar dense-water formation on multi-decadal timescales, such that a stronger SPG circulation following persistent positive phases of the North Atlantic Oscillation results in greater NADW formation along-stream. This underscores the importance of monitoring the state of the SPG for both decadal and longer-term climate predictions.