Ventilation of the Mediterranean Sea constrained by multiple transient tracer measurements

Abstract

Abstract. Ventilation is the primary pathway for atmosphere-ocean boundary perturbations, such as temperature anomalies, to be relayed to the ocean interior. It is also a conduit for gas exchange between the interface of atmosphere and ocean. Thus it is a mechanism whereby, for instance, the ocean interior is oxygenated and enriched in anthropogenic carbon. The ventilation of the Mediterranean Sea is fast in comparison to the world ocean and has large temporal variability. Here we present transient tracer data from a field campaign in April 2011 that sampled a unique suite of transient tracers (SF 6 , 3 H and 3 He) in all major basins of the Mediterranean. We apply the transit time distribution (TTD) model to the data in order to constrain the mean age, the ratio of the advective / diffusive transport and the number of water masses significant for ventilation.We found that the eastern part of the eastern Mediterranean can be reasonably described with a one-dimensional inverse Gaussian TTD (IG-TTD), and thus constrained with two independent tracers. The ventilation of the Ionian Sea and the western Mediterranean can only be constrained by a linear combination of IG-TTDs. We approximate the ventilation with a one-dimensional, two inverse Gaussian TTD (2IG-TTD) for these areas and demonstrate a possibility of constraining a 2IG-TTD from the available transient tracer data. The deep water in the Ionian Sea has a mean age between 120 and 160 years and is therefore substantially older than the mean age of the Levantine Basin deep water (60-80 years). These results are in contrast to those expected by the higher transient tracer concentrations in the Ionian Sea deep water. This is partly due to deep water of Adriatic origin having more diffusive properties in transport and formation (i.e., a high ratio of diffusion over advection), compared to the deep water of Aegean Sea origin that still dominates the deep Levantine Basin deep water after the Eastern Mediterranean Transient (EMT) in the early 1990s. The tracer minimum zone (TMZ) in the intermediate of the Levantine Basin is the oldest water mass with a mean age up to 290 years. We also show that the deep western Mediterranean has contributed approximately 40 % of recently ventilated deep water from the Western Mediterranean Transition (WMT) event of the mid-2000s. The deep water has higher transient tracer concentrations than the mid-depth water, but the mean age is similar with values between 180 and 220 years.

Perspectives of transient tracer applications and limiting cases

“…The transient tracer structure and limiting factors of the IG-TTD are presented and possible solutions to the determined restrictions are shown, e.g., the use of 39 Ar data in deep water. This work is an extension of the work by Stöven and Tanhua (2014), with the main focus on the future scope of IG-TTD applications.…”

“…There are different methods described in the literature on how to constrain the IG-TTD, e.g., Waugh et al (2002Waugh et al ( , 2004 and Schneider et al (2012). Here we describe the method of Stöven and Tanhua (2014), which is based on intersections of mean age functions. In the first step, the mean age of both tracers is calculated for each discrete water sample and a range of / ratios as described in Sect.…”

“…Thus it is difficult to obtain a generally valid tritium input function without neglecting prominent regional factors. For example, distinct local influences on the surface tritium concentration can be found in the Mediterranean Sea, which is characterized by a high net evaporation, large river runoff, dilution by Atlantic water and an intricate ventilation pattern (Roether et al, 2013;Stöven and Tanhua, 2014). Due to the possible uncertainties, it is recommended to use the tritium input function independent isotope ratio of tritium and the helium-3 ( 3 He trit ) decay product for TTD applications.…”

“…A / ratio of 1.0 is considered the unity ratio, which has been applied to many tracer surveys (Schneider et al, 2014(Schneider et al, , 2010Tanhua et al, 2008;Waugh et al, 2006Waugh et al, , 2004Huhn et al, 2013). However, for more complex mixing structures, several approaches have been used to constrain the / ratio and thus the TTD-based mean age (e.g., Waugh et al, 2002;Schneider et al, 2012;Sonnerup et al, 2013;Stöven and Tanhua, 2014).…”

“…An IG-TTD can be empirically constrained with a transient tracer couple that provides reliable mean age results of water masses in the ocean (e.g., Waugh et al, 2002;Schneider et al, 2012;Sonnerup et al, 2013;Stöven and Tanhua, 2014).…”