Modelling of the anthropogenic tritium transient and its decay product helium-3 in the Mediterranean Sea using a high-resolution regional model

Abstract

Abstract. This numerical study provides the first simulation of the anthropogenic tritium invasion and its decay product helium-3 ( 3 He) in the Mediterranean Sea. The simulation covers the entire tritium ( 3 H) transient generated by the atmospheric nuclear weapons tests performed in the 1950s and early 1960s and is run till 2011. Tritium, helium-3 and their derived age estimates are particularly suitable for studying intermediate and deep-water ventilation and spreading of water masses at intermediate/deep levels. The simulation is made using a high-resolution regional model NEMO (Nucleus for European Modelling of the Ocean), in a regional configuration for the Mediterranean Sea called MED12, forced at the surface with prescribed tritium evolution derived from observations. The simulation is compared to measurements of tritium and helium-3 performed along large-scale transects in the Mediterranean Sea during the last few decades on cruises of R/V Meteor: M5/6, M31/1, M44/4, M51/2, M84/3, and R/V Poseidon: 234. The results show that the input function used for the tritium generates a realistic distribution of the main hydrographic features of the Mediterranean Sea circulation. In the eastern basin, the results highlight the weak formation of Adriatic Deep Water in the model, which explains its weak contribution to the Eastern Mediterranean Deep Water (EMDW) in the Ionian sub-basin. It produces a realistic representation of the Eastern Mediterranean Transient (EMT) signal, simulating a deep-water formation in the Aegean subbasin at the beginning of 1993, with a realistic timing of deep-water renewal in the eastern basin.

Simulation of the mantle and crustal helium isotope signature in the Mediterranean Sea using a high-resolution regional circulation model

“…This model satisfactorily simulates the main structures of the thermohaline circulation of the Mediterranean Sea, with mechanisms having a realistic timescale compared to observations. In particular, tritium/helium-3 simulations (Ayache et al, 2015) have shown that the EMT signal from the Aegean sub-basin is realistically simulated, with its corresponding penetration of tracers into the deep water in early 1995. The strong convection event of winter 2005 and the 968 M Ayache et al: Simulation of the mantle and crustal helium isotope signature following years in the Gulf of Lion was satisfactorily captured as well.…”

“…Physical forcing fields are read daily and interpolated to give values for each 20 min time step. The same approach was used by Ayache et al (2015) to model the anthropogenic tritium invasion and by Palmiéri et al (2015) to simulate CFCs and anthropogenic carbon. This choice is justified by the fact that these tracers are passive.…”

“…Full details of the model and its parameterizations are described by Beuvier et al (2012a, b), Palmiéri et al (2015) and Ayache et al (2015).…”

“…Each component has a characteristic 3 He / 4 He value. The anthropogenic 3 He distribution due to the decay of bomb tritium has already been addressed by Ayache et al (2015). For this study, we ran two separate simulations, one for each helium component.…”

“…This model of the Mediterranean Sea has been used previously to study anthropogenic tritium and its decay product helium-3 (Ayache et al, 2015), the anthropogenic carbon uptake (Palmiéri et al, 2015), the transport through the Strait of Gibraltar (Soto-Navarro et al, 2014), as well as the Western Mediterranean Deep Water (WMDW) formation (Beuvier et al, 2012a) and the mixed-layer response under high-resolution air-sea forcings (Lebeaupin Brossier et al, 2011). This model satisfactorily simulates the main structures of the thermohaline circulation of the Mediterranean Sea, with mechanisms having a realistic timescale compared to observations.…”