Temporal and spatial variability of noble gas tracers in the North Pacific

We develop a numerical model of dissolved argon and neon in the global ocean as a tool to investigate the physical processes controlling their saturation states in the upper ocean of the North Pacific. The distribution of argon and neon is simulated using the time-varying, three-dimensional circulation fields determined by the Estimating the Circulation and Climate of the Oceans (ECCO) project from 1992 to 2008. The model is in overall agreement with limited observational data from the subpolar and subtropical North Pacific using a relatively low vertical diffusivity. Temporal variability in argon saturation is enhanced in the surface ocean, dominated by diffusive gas exchange coupled to air-sea heat fluxes. This variability in surface argon saturation is significantly correlated to the Southern Oscillation Index (El Nino) in the tropics and to the North Pacific Index in midlatitudes. Using sensitivity experiments, we find that the mean state of argon saturation in the ventilated thermocline is characterized by a mutual compensation between mixing-induced supersaturation and sea level pressure and heat-flux-induced undersaturation. Neon distributions exhibit a stronger influence from bubble-mediated gas fluxes that is partially compensated by the effect of sea level pressure variation. Our result demonstrates the important role of air-sea interaction and ocean mixing in controlling the mean state of the dissolved noble gases and highlights the importance of diffusive gas exchange coupled to air-sea heat fluxes in controlling temporal variability, with implications for using noble gas measurements to derive estimates of diapycnal diffusivity in the subtropical thermocline.