On the Dynamics of the Subtropical Mode Water from an Ensemble Simulation Viewpoint

The ocean, as the largest carbon reservoir, plays a crucial role in regulating the global climate by absorbing atmospheric carbon dioxide and heat generated by carbon emissions. It achieves this by transferring water masses from the surface to the ocean interior. Among the various features influencing ocean circulation, subtropical mode water (STMW) in the North Atlantic has received significant attention due to its formation through direct ocean-atmosphere interactions and its substantial impact on the ventilation of the upper ocean. In this study, we explore the mechanisms underlying the seasonal cycle of STMW from a dynamical perspective.

We use an ensemble of 48 partially coupled North Atlantic Ocean simulations, with mesoscale-permitting resolution, and apply an ensemble-averaging approach to separate the mean flow (residual-mean) and eddy fluctuations, both of which are temporally and spatially dependent. We quantify STMW by analyzing the evolution of a pool of low Ertel potential vorticity (PV). Our results indicate that the annual cycle of STMW can be explained by the interactions of three primary transports: (1) the ensemble-mean flux, representing the large-scale Eulerian-mean flow shared by all ensemble members, (2) bolus eddy transport driven by strong baroclinic instabilities within the PV pool, and (3) residual eddy transport due to the non-linear fluctuations among the ensemble members. During the winter when STMW begins to form, the ensemble-mean flow plays a dominant role, deflating the PV pool by transporting low-PV water from the north into the pool. Meanwhile, bolus transport actively mixes the PVs within the pool along isopycnal surfaces, leading to a PV homogenization. As the season progresses, residual eddy transport begins to counteract the ensemble-mean flow, creating a balance that results in the inflation of the PV pool and, consequently, the erosion of STMW.