Using chemical tracers to assess ocean models

Chemical tracers can be used to assess the simulated circulation in ocean m odels. Tracers that have been used in this context include tritium, chlorof luorocarbons, natural and bomb-produced radiocarbon, and to a lesser extent , oxygen, silicate, phosphate, isotopes of organic and inorganic carbon com pounds, and certain noble gases (e.g., helium and argon). This paper review s the use of chemical tracers in assessing the circulation and flow pattern s in global and regional ocean models. It will be shown that crucial inform ation can be derived from chemcial tracers that cannot be obtained from tem perature-salinity (T-S) alone. Tn fact, it turns out that a model with a go od representation of T-S can have significant errors in simulated circulati on, so checking a model's ability to capture chemical tracer patterns is vi tal. Natural chemical tracers such a?; isotopes of carbon, argon, and oxyge n are useful for examining the model representation of old water masses, su ch as North Pacific and Circumpolar Deep Water. Anthropogenic or transient tracers, such as tritium, chlorofluorocarbons, and bomb-produced C-14, are best suited fur analyzing model circulation over decadal timescales, such a s thermocline ventilation, the renewal of Antarctic Intermediate Water, and the ventilation pathways of North Atlantic Deep Water and Antarctic Bottom Water. Tracer model studies have helped to reveal inadequacies in the mode l representation of certain water mass formation processes, for example, co nvection, downslope flows, and deep ocean currents. They show how coarse mo dels can chronically exaggerate the spatial scales of open-ocean convection and deep currents while underestimating deep flow rates and diffusing down slope flows with excessive lateral mixing. Higher-resolution models typical ly only resolve thermocline ventilation because of shorter integration time s, and most resort to high-latitude T-S restoring to simulate reasonable in terior water mass characteristics. This can be seen to result in spuriously weak chemical tracer uptake at high latitudes due to suppressed convective overturn and vertical motion, Overall, the simulation of chemical tracers is strongly recommended in model assessment studies and as a tool for analy zing water mass mixing and transformation in ocean models. We argue that a cost-effective approach is to simulate natural radiocarbon to assess long-t imescale processes, and CFCs for decadal to interdecadal ocean ventilation.