For this global simulation, we adopt a uniform definition related to a maximum density difference of 0.01 m 3/s between the surface and the last model level within the mixed layer. The exact volume of detrained surface water depends on the criterion used for the definition of the surface mixed layer. As mass is conserved in the ocean model and as the Lagrangian trajectory scheme respects this constraint, all the particles injected into the thermocline return to the mixed layer, without intercepting coastlines or topography, or being trapped in the interior ocean. The total number of particles we use is over several million as any of them explains only a fraction of the total transport, not to exceed a prescribed maximum value (here 10 −3 Sv per documented month, 1 Sv = 10 6 m 3/s). Therefore, the mass flux escaping this layer is described in two complementary ways: we document with particles the advective mass transfer to the interior ocean during each month of the climatological year, and we also describe the flux transferred to the interior ocean on the occasion of abrupt (monthly) shallowings of the surface mixed layer. The model offers a discrete representation (with successive monthly-mean states) of the time-varying surface mixed layer. The model is forced by mean seasonal atmospheric fluxes, obtained from the ECMWF 15-year (1979–1993) reanalyses and smoothed by a 11-day running mean, and its density field is strongly constrained on an observational dataset of temperature and salinity. The bottom topography and the coastlines are derived from a global atlas completed by values from the 5′ × 5′ ETOPO5 dataset. The resolution is 2° in the zonal direction, with a meridional grid interval varying from 0.5° at the equator to a maximum of 1.9° in the tropic, and 31 levels in the vertical with the highest resolution (10 m) in the upper 150 meters. Multiple 3D trajectory calculations are computed from the monthly archive of velocity and tracer fields of the OPA model, run in a global, non eddy-resolving mode. We investigate in this study the ventilation rate of the global ocean by a Lagrangian analysis of a numerical simulation strongly constrained on observed climatologies for both temperature and salinity. Ventilation achieved by convection is related to severe atmospheric conditions (as surface cooling, sea-ice formation or enhanced evaporation) and may occur over longer periods. Ventilation achieved by subduction is usually diagnosed with hydrographic data or model results related to a late winter mixed layer, calculating the annual-mean advective export of water at its bottom. As time scales usually proposed for ocean advection match the decadal period of various ocean-atmosphere feedbacks, ocean-atmosphere interactions may control remotely further changes in atmospheric climate variability, by means of anomalies being advected within the interior ocean. It usually occurs over large scale domains as subduction or through much more localized convection areas. Ventilation is the process by which water is transferred from the surface mixed layer to the interior ocean. Depending on the underlying dynamics, the pathways for these newly-formed water masses may restrict to the subtropical and equatorial thermocline or extend to interocean mass transfers. Water mass properties set by ocean–atmosphere interactions are carried away within the interior ocean by the ocean circulation.
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