On the formation, ventilation, and erosion of mode waters in the North Atlantic and Southern Oceans

The mean residence times, subduction rates, and formation rates of Subtropical Mode Water (STMW) and Subpolar Mode Water (SPMW) in the North Atlantic and Subantarctic Mode Water (SAMW) in the Southern Ocean are estimated by combining a model and observations of chlorofluorocarbon‐11 (CFC‐11) via Bayesian Model Averaging (BMA), a statistical technique that weights model estimates according to how close they agree with observations. Subduction rates are estimated in two different ways to investigate the non‐advective contribution to thermocline ventilation, which in turn are compared to formation rate estimates. One subduction rate estimate is based on entrainment/detrainment velocities and the other subduction rate estimate allows ventilation to be both an advective and diffusive process instead of a purely advective one by using transit‐time distributions (TTDs). It is found that the subduction of all three mode waters is mostly an advective process, but up to about one‐third of STMW subduction likely owes to non‐advective processes. Also, while the formation of STMW is mostly due to subduction, the formation of SPMW is mostly due to other processes. About half of the formation of SAMW is due to subduction and half is due to other processes. A combination of air‐sea flux, acting on relatively short timescales, and turbulent mixing, acting on a wide range of timescales, is likely the dominant SPMW erosion mechanism. Air‐sea flux is likely responsible for most STMW erosion, and turbulent mixing is likely responsible for most SAMW erosion. Key Points Diffusive processes can significantly contribute to subduction Some mode waters primarily form via subduction, some don't Mode waters do not primarily erode via entrainment back into the mixed layer