The Ventilated Ocean

Abstract

Abstract Adiabatic theories of ocean circulation and density structure have a long tradition, from the concept of the ventilated thermocline to the notion that deep ocean ventilation is controlled by westerly winds over the Southern Ocean. This study explores these ideas using a recently developed Lagrangian ocean model (LOM), which simulates ocean motions by computing trajectories of water parcels. A unique feature of the LOM is its capacity to model ocean circulations in the adiabatic limit, in which water parcels exactly conserve their densities when they are not in contact with the ocean surface. The authors take advantage of this property of the LOM and consider the circulation and stratification that develop in an ocean with a fully adiabatic interior (with both isopycnal and diapycnal diffusivities set to zero). The ocean basin in the study mimics that of the Atlantic Ocean and includes a circumpolar channel. The model is forced by zonal wind stress and a density restoring at the surface. Despite the idealized geometry, the relatively coarse model resolution, and the lack of atmospheric coupling, the nondiffusive ocean maintains a density structure and meridional overturning that are broadly in line with those observed in the Atlantic Ocean. These are generated by just a handful of key water pathways, including shallow tropical cells described by ventilated thermocline theory; a deep overturning cell in which sinking North Atlantic Deep Water eventually upwells in the Southern Ocean before returning northward as Antarctic Intermediate Water; a Deacon cell that results from a topographically steered and corkscrewing circumpolar current; and weakly overturning Antarctic Bottom Water, which is effectively ventilated only in the Southern Hemisphere. The main conclusion of this study is that the adiabatic limit for the ocean interior provides the leading-order solution for ocean overturning and density structure, with tracer diffusion contributing first-order perturbations. Comparing nondiffusive and diffusive experiments helps to quantify the changes in stratification and circulation that result from adding a moderate amount of tracer diffusion in the ocean model, and these include an increase in the amplitude of the deep meridional overturning cell of several Sverdrups, a 10%–20% increase in Northern Hemispheric northward heat transport, a stronger stratification just below the main thermocline, and a more realistic bottom overturning cell.