Seasonal Changes in Dense Water Production Do Not Cause Seasonal Cycle in Atlantic Overturning

The overturning circulation in the subpolar North Atlantic and Nordic seas is the product of a net densification by air–sea buoyancy forcing and mixing that transforms light Atlantic Water into dense North Atlantic Deep Water. Recent studies have shown that the magnitude of the time-mean overturning circulation as measured at the Overturning in the Subpolar North Atlantic Program (OSNAP) mooring array can be attributed to the water mass transformation by air–sea fluxes and mixing to the north of the array. However, connecting subpolar overturning variability on seasonal time scales to seasonal variations in water mass transformation is contentious. Here, we show in observations and a state estimate that seasonal variability in water mass transformation by air–sea buoyancy fluxes and mixing predominantly affects the storage of water in water mass classes within the subpolar basins, rather than driving seasonal changes in the overturning circulation at the OSNAP mooring array. Furthermore, we show that the seasonal formation and export of dense water from the Irminger and Labrador Seas cannot fully explain overturning seasonality at OSNAP. Instead, the seasonal cycle of overturning as measured at the OSNAP mooring array is the result of seasonal imbalances in the inflows and outflows of dense water across the array, linked to seasonal changes in the density field imprinted on the circulation of the subpolar gyre. This implies that intra-annual-to-seasonal dense water transport variability at OSNAP cannot be interpreted as an overturning circulation associated with the formation and export of dense water.