Eighteen Degree Water Dynamics Viewed from an Ensemble 

The subtropical mode water in the North Atlantic, often referred to as ‘eighteen-degree water’ (EDW), has been investigated based on observational and theoretical studies. We here discuss the mechanism of EDW by using an ensemble-based approach which offers the advantage of separating the eddy field from the mean flow without making implicit assumptions on the temporal or spatial scales of the eddies. We employ an ensemble of North Atlantic Ocean simulations partially coupled with the atmosphere at mesoscale permitting resolution (1/12°), and determine EDW as a pool of the Ertel potential vorticity (PV) lower than the surroundings. Our results suggest that the maintenance of EDW can be explained by the down-gradient eddy PV fluxes balancing the mean flow: the low PV in the formation region is transported by the eddy fluxes to the pool and mixes with the surrounding high PV.