Ensemble Ocean Simulations in the North Atlantic: Exploring the Intrinsic Variability in Subtropical Mode Water Dynamics

The non-linear nature of the ocean dynamics motivates the use of ensemble ocean simulations to discriminate the intrinsic and extrinsic sources of oceanic variability. Separating the mean and eddy flows from ensemble statistics also gives access to their local and instantaneous interactions in the non-stationary and inhomogeneous ocean. We here take advantage of this idea to quantify the local/instantaneous roles of laminar and eddy fluxes in the seasonal cycle of the North Atlantic subtropical mode water (STMW) that is formed through ocean-atmosphere interaction and controls large-scale oceanic ventilation.

We employ an ensemble of 48 North Atlantic 1/12-degree ocean simulations, where all members are driven by the same atmospheric forcing after slight initial perturbations. We achieve a space/time-dependent mean-eddy flow separation by obtaining a residual-mean flow that represents the common oceanic response of all ensemble members to the atmosphere, and a set of residual eddies that reflect the ensemble dispersion. We characterise the STMW as a low Ertel potential vorticity (PV) pool and find that its PV budget is mostly controlled by the ensemble mean PV flux: the formation and erosion of the STMW is predominantly driven by the residual-mean flow. The contribution of eddy PV transport is secondary; this can be attributed to the low intrinsic variability within the PV pool, as captured by the residual eddies. 

Overall, our results show that ensemble ocean simulations are powerful to investigate inhomogeneous, non-stationary, nonlinear multiscale ocean dynamics, providing deeper insights into the life cycle of large-scale climate-relevant features like STMW.