Ventilation of the Mediterranean Sea constrained by multiple transient tracer measurements

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 (SF6, CFC-12, 3H and 3He) 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

Changes in ventilation of the Mediterranean Sea during the past 25 year

Significant changes in the overturning circulation of the Mediterranean Sea has been observed during the last few decades, the most prominent phenomena being the Eastern Mediterranean Transient (EMT) in the early 1990s and the Western Mediterranean Transition (WMT) during the mid-2000s. During both of these events unusually large amounts of deep water were formed, and in the case of the EMT, the deep water formation area shifted from the Adriatic to the Aegean Sea. Here we synthesize a unique collection of transient tracer (CFC-12, SF6 and tritium) data from nine cruises conducted between 1987 and 2011 and use these data to determine temporal variability of Mediterranean ventilation. We also discuss biases and technical problems with transient tracer-based ages arising from their different input histories over time; particularly in the case of time-dependent ventilation. We observe a period of low ventilation in the deep eastern (Levantine) basin after it was ventilated by the EMT so that the age of the deep water is increasing with time. In the Ionian Sea, on the other hand, we see evidence of increased ventilation after year 2001, indicating the restarted deep water formation in the Adriatic Sea. This is also reflected in the increasing age of the Cretan Sea deep water and decreasing age of Adriatic Sea deep water since the end of the 1980s. In the western Mediterranean deep basin we see the massive input of recently ventilated waters during the WMT. This signal is not yet apparent in the Tyrrhenian Sea, where the ventilation seems to be fairly constant since the EMT. Also the western Alboran Sea does not show any temporal trends in ventilation

Ventilation of the Arctic Ocean and the Gulf of St. Lawrence

Ocean ventilation describes the physical process that transports surface waters into the ocean’s interior. The process thereby links surface waters with deep waters, contributing significantly to the global ocean conveyer belt and affecting global water mass circulation. This thesis analyzed transient tracer measurements, primarily focusing on the tracers CFC-12 and SF6, to examine ocean ventilation patterns. The investigation of ventilation patterns focused on two regions. First, the Arctic Ocean and the primary aims here were (1) to analyze ventilation patterns and their temporal variability, and (2) to evaluate the use of the ‘Medusa’-tracers to improve the ventilation analysis. Second, the Gulf of St. Lawrence, where the main objective was to analyze deep water ventilation and hereby predict changes in the composition of the water mass flowing into the Gulf. The examination of Arctic Ocean ventilation patterns at intermediate depths (250 – 1500 m) over the past three decades (1991 - 2021) revealed a significant temporal variability. The analysis identified a multidecadal variability in ventilation throughout this 30-year time period, showing reduced ventilation (higher mean ages) in 1991 and 2021 in comparison to 2005 and 2015. Additionally, it was observed that during the past 16 years (2005 until 2021) the ventilation slowed down on the Eurasian side of the Arctic Ocean, with increasing mean ages observed from 2005 over 2015 to 2021. Transient tracer measurements, were also utilized to analyze deep water ventilation in the Gulf of St. Lawrence. The data identified the presence of older water in the eastern part compared to the western edge of the Gulf, contrary to the expected flow pattern of the water. This underscored the increasing influence of older, less oxygenated NACW in recent times, which limits the influence of recently ventilated, oxygenated water coming from the Labrador Sea

Perspectives of transient tracer applications and limiting cases

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

Changing ventilation of the Mediterranean Sea studies with a suite of novel halogenated transient tracers

Oceanic transient tracers have been concerned for more than four decades due to their ability in visualizing and quantifying ocean ventilation and understanding the effects of changing climate. They trace pathways climate anomalies follow as they enter and move through the ocean and provide us with valuable time information. When such time information is interpreted depending on input function (time changing concentrations), they are chronological transient tracers, such as dichlorodifluoromethane (CFC-12) and sulfur hexafluoride (SF6). During the past ~15 years, the non-monotonous change of atmospheric history of CFC-12 limited its ability as an oceanic transient tracer for recently ventilated water masses, but it still works for deep waters. Therefore, we took the Mediterranean Sea as an example and investigated the recent changes in deep ventilation based on long-term observations of CFC-12 and SF6 in the first manuscript. Since a combination of multiple transient tracers can better interpret ocean ventilation, we looked for and evaluated potential novel transient tracers: hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) in the second and third manuscripts. The specific findings are described below. In the first study, highly variable deep ventilation in the Mediterranean Sea in time and space are reported based on a combination of observations of traditional chronological transient tracers, hydrographic properties and apparent oxygen utilization from 13 cruises conducted during 1987-2018. Spatially, both the Eastern and Western Mediterranean Deep Water (EMDW and WMDW) show a general west-to-east gradient of increasing salinity and potential temperature but decreasing oxygen and transient tracer concentrations. Temporally, stagnant and weak ventilation is found in most areas of the EMDW during the last decade in spite of prevailing ventilation in the Adriatic Deep Water between 2011 and 2016, which could be a result of the weakened Adriatic source intensity. In the Western Mediterranean Sea, enhanced ventilation after the Western Mediterranean Transition (WMT) event is observed, and slightly weakened ventilation after 2016 could be a combined influence from the Eastern (for the weakened Adriatic source intensity) and the Western (for the weakened influence from the WMT event) Mediterranean Sea. In the second and third studies, we explored and evaluated potential novel chronological transient tracers: chlorodifluoromethane (HCFC-22), 1,1-dichloro-1-fluoroethane (HCFC-141b), 1-chloro-1,1-difluoroethane (HCFC-142b), 1,1,1,2-tetrafluoroethane (HFC-134a), pentafluoroethane (HFC-125), fluoroform (HFC-23), carbon tetrafluoride (PFC-14, CF4) and hexafluoroethane (PFC-116) from four aspects: input function (including atmospheric history and historical surface saturation), seawater solubility, feasibility of measurement and stability in seawater. By comprehensive analysis and evaluation, the most promising oceanic transient tracers are HCFC-142b and HCFC-141b currently since they fulfil essential requirements by virtue of well-documented atmospheric history, established seawater solubility, feasible measurements and inertness in seawater. However, they will likely only work for the next few years/decades considering the restrictions on their production and consumption imposed by the Montreal Protocol and their (future) decreasing atmospheric mole fractions. The compounds that have the greatest potential as oceanic transient tracers in the future are PFC-14 and PFC-116 because of their high stability in seawater, the long and well-document atmospheric concentration histories and well-constructed seawater solubility functions. The challenge is how to measure them accurately due to their low solubility. For HFC-134a, we are not able to fully evaluate its potential as a tracer due to the inconclusive results, especially on its solubility and stability in seawater, but also with regard to potential analytical challenges. HFC-125, HFC-23, and HCFC-22 can no longer be considered because there are alternative tracers with similar input functions that are better suited as oceanic transient tracers. In total, this work helps us understand ocean ventilation in the Mediterranean Sea in the past ~30 years (with an emphasis on the recent changes) from the perspective of the traditional chronological transient tracers, as well as explored and evaluated the potential novel chronological transient tracers in the ocean. The outcome sets the base for further investigation of these alternative tracers in order to better interpreting ventilation in the global ocean and understanding the effects of climate change

Upwelling velocity and ventilation in the western South China Sea deduced from CFC-12 and SF6 observations

This study presents observations of the transient tracers CFC-12 and SF6 in the western South China Sea during the fall of 2015. A CFC-12 maximum was discovered in the western South China Sea at the subsurface layer (150–200 m), which could be traced back to the North Pacific Tropical Water. The transit time distribution approach was used to estimate the ventilation time in this area. The constrained Δ /Γ ratio of 0.5 was obtained using CFC-12/SF6 tracer pair. This ratio is lower than the empirical unit ratio of one as used for previous estimates. Waters in the northern region of the western South China Sea appear younger than waters in the southern region. The water mass corresponding to the salinity minimum has a mean age of ∼67 ± 16 years along the 15º N line (marked by the red dashed rectangle in Fig. 1), which increases to ∼76 ± 18 years along the 10º N line (blue dashed rectangle, Fig. 1). The higher mean ages indicate that the intermediate water was ventilated from the North Pacific, which is far distant from the South China Sea. The column inventory of Cant is ∼31.3 mol C m–2. Upwelling velocities of up to ∼55 × 10–5 m s–1 was computed using the tracer data, indicating that tracer-free water as yet not influenced by human perturbation could be carried to the upper layer by upwelling. Using the transit time distribution derived mean age with transient tracers provides a possible way to determine the ventilation time scale for the study area

Recent changes in deep ventilation of the Mediterranean Sea; evidence from long-term transient tracer observations

The Mediterranean Sea is a small region of the global ocean but with a very active overturning circulation that allows surface perturbations to be transported to the interior ocean. Understanding of ventilation is important for understanding and predicting climate change and its impact on ocean ecosystems. To quantify changes of deep ventilation, we investigated the spatiotemporal variability of transient tracers (i.e. CFC-12 and SF6) observations combined with temporal evolution of hydrographic and oxygen observations in the Mediterranean Sea from 13 cruises conducted during 1987-2018, with emphasize on the update from 2011 to 2018. Spatially, both the Eastern and Western Mediterranean Deep Water (EMDW and WMDW) show a general west-to-east gradient of increasing salinity and potential temperature but decreasing oxygen and transient tracer concentrations. Temporally, stagnant and weak ventilation is found in most areas of the EMDW during the last decade despite the prevailing ventilation in the Adriatic Deep Water between 2011 and 2016, which could be a result of the weakened Adriatic source intensity. The EMDW has been a mixture of the older Southern Aegean Sea dense waters formed during the Eastern Mediterranean Transient (EMT) event, and the more recent ventilated deep-water of the Adriatic origin. In the western Mediterranean basin, we found uplifting of old WMDW being replaced by the new deep-water from the Western Mediterranean Transition (WMT) event and uplifting of the new WMDW toward the Alboran Sea. The temporal variability revealed enhanced ventilation after the WMT event but slightly weakened ventilation after 2016, which could be a result of combined influences from the eastern (for the weakened Adriatic source intensity) and western (for the weakened influence from the WMT event) Mediterranean Sea. Additionally, the Mediterranean Sea is characterized by a Tracer Minimum Zone (TMZ) at mid-depth of the water column attributed to the rapid deep ventilation so that the TMZ is the slowest ventilated layer. This zone of weak ventilation stretches across the whole Mediterranean Sea from the Levantine basin into the western basin