Abstract
Abstract. Currently available transient tracers have different application ranges that are defined by their temporal input (chronological transient tracers) or their decay rate (radioactive transient tracers). Transient tracers range from tracers for highly ventilated water masses such as sulfur hexafluoride (SF6) through tritium (3H) and chlorofluorocarbons (CFCs) up to tracers for less ventilated deep ocean basins such as argon-39 (39Ar) and radiocarbon (14C). In this context, highly ventilated water masses are defined as water masses that have been in contact with the atmosphere during the last decade. Transient tracers can be used to empirically constrain the transit time distribution (TTD), which can often be approximated with an inverse Gaussian (IG) distribution. The IG-TTD provides information about ventilation and the advective/diffusive characteristics of a water parcel. Here we provide an overview of commonly used transient tracer couples and the corresponding application range of the IG-TTD by using the new concept of validity areas. CFC-12, CFC-11 and SF6 data from three different cruises in the South Atlantic Ocean and Southern Ocean as well as 39Ar data from the 1980s and early 1990s in the eastern Atlantic Ocean and the Weddell Sea are used to demonstrate this method. We found that the IG-TTD can be constrained along the Greenwich Meridian south to 46° S, which corresponds to the Subantarctic Front (SAF) denoting the application limit. The Antarctic Intermediate Water (AAIW) describes the limiting water layer in the vertical. Conspicuous high or lower ratios between the advective and diffusive components describe the transition between the validity area and the application limit of the IG-TTD model rather than describing the physical properties of the water parcel. The combination of 39Ar and CFC data places constraints on the IG-TTD in the deep water north of the SAF, but not beyond this limit.
Highlights
Ocean ventilation plays a major role in climate
We found that the inverse Gaussian (IG)-transit time distribution (TTD) can be constrained along the Greenwich Meridian south to 46◦ S, which corresponds to the Subantarctic Front (SAF) denoting the application limit
In the following we focus on data of less problematic transient tracers, namely SF6, CFC-12, CFC-11 and 39Ar, which have relatively well-defined input functions and represent different time ranges; i.e., they can be applied to well- and less well-ventilated water masses
Summary
Ocean ventilation plays a major role in climate. It represents transport processes from the ocean surface to the ocean’s interior, carrying dissolved gases, nutrients, and microorganisms, and soluble hazardous substances and other coastal and offshore pollutants (Schlosser et al, 1999). Oxygen supply by ventilation represents another field of interest, with focus on the economically important marine resources like fish and seafood. To this end, ocean ventilation models are an important part of describing and understanding the complex biogeochemical interactions in the ocean. The onedimensional inverse Gaussian transit time distribution (IGTTD) can be applied to field data of transient tracer surveys in the ocean (Waugh et al, 2003; Klatt et al, 2002). 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 39Ar 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
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