Edited by: Nadia Lo Bue, National Earthquake Observatory (INGV), Italy
Reviewed by: Darryn Waugh, Johns Hopkins University, United States; Anthony Bosse, Aix-Marseille Université, France
This article was submitted to Physical Oceanography, a section of the journal Frontiers in Marine Science
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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.
Ocean ventilation is an important process in the Earth system that transports ocean surface properties, such as salinity, heat, CO2 and dissolved gases to the interior ocean (
In the EMed, the Adriatic Sea used to be the main DWF region prior to the late 1980s (
In the WMed, the Western Mediterranean Deep Water (WMDW) forms in the northwestern Mediterranean, mainly in the Gulf of Lions. Open-ocean deep convection, combined with occasional dense shelf water cascading, is the major contributor to the thermohaline circulation and ventilation in the source regions (
In the Tyrrhenian Sea, the situation is a little bit different. Beneath the LIW, the transitional EMDW (tEMDW, a mixture of LIW and EMDW from the EMed) mixes with the water in the Tyrrhenian Sea and settles between 600 and 1500 m (
As the area separating the EMed and WMed, the Strait of Sicily is composed of the surface Modified Atlantic Water (MAW) flowing eastward, the lower LIW and bottom tEMDW flowing westward (
Transient tracers have been used to understand ventilation and circulation processes, and determine water mass characteristics in the Mediterranean Sea in multiple studies. For example, the vertical and spatial distributions of transient tracers have been described in the EMed by
The primary goal of this study is to investigate the temporal and spatial variability (with an emphasis on the recent changes) in deep and intermediate ventilation of the Mediterranean Sea based on transient tracer (CFC-12 and SF6) observations. To the end, we address the spatiotemporal distributions of transient tracer (CFC-12 and SF6) concentrations as well as the temporal evolution of potential temperature-salinity (Θ–S) diagrams and depth-profiles of CFC-12 and SF6 concentrations, salinity, potential temperature, potential density and apparent oxygen utilization in pressure (CFC-12/SF6/S/Θ/σΘ/AOU vs. P) between 1987 and 2018.
Key meta-data for the Mediterranean Sea cruises used in this study.
Year | Cruise | Research vessel | Cruise period | W/E. Med | CFC-12/SF6a | References |
1987 | M5/6 | Meteor | 1987.08.18-09.24 | W, E | CFC-12 | |
1995 | M31/1 | Meteor | 1994.12.30-1995.03.22 | W, E | CFC-12 | |
1997 | Ura2 | Uranis | 1997.08.30-09.08 | E | CFC-12 | |
1997 | P234 | Poseidon | 1997.10.23-11.10 | W | CFC-12, CFC-11 | |
1998 | Aegaeo98 | Aegaeo | 1998.10.14-10.19 | E | CFC-12 | |
1999 | Ura7 | Uranis | 1999.02.11-02.17 | E | CFC-12 | |
1999 | M44/4 | Meteor | 1999.04.10-05.16 | W, E | CFC-12 | |
2001 | M51/2 | Meteor | 2001.10.18-11.11 | W, E | CFC-12 | |
2011 | M84/3 | Meteor | 2011.04.05-04.28 | W, E | CFC-12, SF6 | |
2016 | ESAW2 | Bios Dva | 2016.04.05-04.10 | E | CFC-12, SF6 | |
2016 | CRELEV2016 | Aegaeo | 2016.06.02-06.10 | E | CFC-12, SF6 | |
2016 | TALPro2016 | Angeles Alvarino | 2016.08.18-08.29 | W | CFC-12, SF6 | |
2018 | MSM72 | Maria S. Merian | 2018.03.02-04.03 | W, E | CFC-12, SF6 |
The Transit Time Distribution (TTD) model describes the propagation of tracer boundary conditions from the ocean surface into the interior based on the Green’s function (
We are also using Apparent Oxygen Utilization (AOU) as a measure of ventilation. We use AOU rather than oxygen since the dependencies of S and Θ on oxygen saturation is already factored into the concept, which is similar to the case for concentrations of CFC-12 and SF6 in ppt rather than in pmol kg−1. The AOU is different from the transient tracers in that the input function is constant so that a change in AOU can be directly related to a change in ventilation, or oxygen consumption rate.
We start by discussing the spatiotemporal distribution of ventilation in the Mediterranean Sea by analyzing CFC-12 and SF6 concentration sections at roughly the same locations in the EMed and WMed separately. As a second step, we consider the temporal variability of ventilation by comparing the structures of potential temperature and salinity (Θ–S diagrams), as well as depth-profiles of transient tracer concentrations (CFC-12 and SF6), salinity (S), potential temperature (Θ in°C), potential density (σΘ in kg m–3 referenced to 0 dbar pressure) and apparent oxygen utilization (AOU in μmol kg–1) for stations within each box in
The age of a water parcel is defined as the time elapsed since it left the mixed layer where it was in contact with the atmosphere before transported into the ocean interior. The concept of tracer age does not consider turbulent mixing in the ocean interior, which is unrealistic but it provides a framework to compare the ventilation time-scale at one location over time. The atmospheric temporal evolutions of CFC-12 and SF6 overlay when the atmospheric records of SF6 are shifted back by 14 years (
For this study, we were able to find locations where CFC-12 and SF6 were measured in 2001 and 2016, respectively, in the northern Cretan Passage and the Tyrrhenian Sea. The averaged tracer ages were calculated by first interpolating the individual profiles to standard depths and then by taking the arithmetic mean of the interpolated profiles (
We show vertical sections of CFC-12 concentrations for 1987, 1995, 1999, 2001, 2011, and 2018, and SF6 concentrations for 2011 and 2018 to illustrate the spatial evolution of tracer distributions during the last ∼30 years in the Eastern Mediterranean Sea (
Vertical sections of CFC-12 concentration (in ppt) in the Eastern Mediterranean Sea (see inset map for station locations) in
This data set represents the only pre-EMT transient tracer observation for the Mediterranean Sea (
The Meteor cruise in 1995 presents the first comprehensive transient tracer observations after the EMT event (
The double core TMZs still existed at the time, but with some changes. In the Ionian Sea, the CFC-12 concentration increased from ∼90 ppt in 1995 to ∼120 ppt in 1999/2001 (
The spatial distribution of CFC-12 concentrations in 2011 (
Although the 2018 cruise was unable to sample the Levantine basin, there is evidence that the extent of the Adriatic-derived water in the EMDW in the western Ionian Sea expanded eastward and upward from 2011 to 2018 (
We have a time-series of properties from 1987 to 2016 in the Adriatic Sea (
Temporal variability of the Southern Adriatic Sea illustrated with
The Adriatic intermediate water (
Similar to
Above the CDW, a characteristic Θ–S inversion at 300–1300 m depth (
In this area, we examined clusters of stations from the western and eastern parts of the basin south of Otranto Strait. For the deep layer (below ∼1200 m) in the north-western Ionian Sea (
Similar to
For the intermediate layer (
In the western and central Ionian Sea (
Similar to
In the western Ionian intermediate water (
We have a time-series of 6 occupations from 1987 to 2018 in the eastern Ionian Sea (
Similar to
Weak ventilation was found in the eastern Ionian intermediate water from 1987 to 1995 as seen by the decreasing CFC-12 concentrations and slightly increasing AOU (
We have a time-series of 8 occupations from 1987 to 2018 in the northern Cretan Passage (
Similar to
From the perspective of the tracer age difference (
Here we present two areas in the Levantine basin, east of Crete and west of Cyprus (
Similar to
Increasing CFC-12 concentrations in the EMDW in the Levantine basin below ∼1800 m from 1987 to 2011 indicates strong ventilation between 1987 and 1999 and relatively slow ventilation after that (
The Levantine (especially the central Levantine) intermediate water (
The Strait of Sicily is the relatively shallow area connecting the western and eastern Mediterranean basins. In the deep-water layer (below ∼600 m), the concentration of CFC-12 shows a generally increasing trend after a small decrease between 1987 and 1995 (
Similar to
Sections of the vertical distribution of CFC-12 concentrations in the WMed for 1995, 1997, 2001, 2011, and 2018, and SF6 concentrations for 2018 are presented in
Vertical sections of CFC-12/CFC-11 concentration (in ppt) in the Western Mediterranean Sea (see inset maps for station locations) in
Although the Tyrrhenian Sea is influenced by both the EMed and WMed, no Θ–S inversions are observed in the Tyrrhenian Deep Water (TDW) during the last three decades (
Similar to
The CFC-12 concentrations increased between 1987 and 1997 in the deep-water layer (below ∼1500 m) in the Tyrrhenian Sea (
In the Tyrrhenian intermediate water (
From the perspective of the tracer age difference (
In the northern part of the Western Mediterranean Sea, i.e., Gulf of Lions and Liguro-Provençal basin, we have only observations in two years (1997 and 2016), so that only limited information on the temporal evolution can be made. Increased CFC-12 concentration, salinity, Θ and σΘ are observed in the water layer below ∼300 m (
While similar CFC-12 concentrations in 1995, 1997, and 2001 at deep and intermediate depths of the central Algerian basin (
Similar to
In the Algerian intermediate water (
In the water layer below ∼500 m in the Alboran Sea (
We have compared transient tracer observations from 1987 to 2018 (CFC-12/11 and SF6) in the Mediterranean Sea, mainly focusing on the layers below the Levantine Intermediate Water (LIW), in order to characterize the temporal evolution of ventilation. Here we discuss trends and variability of the ventilation patterns in the Mediterranean Sea based on the combined observations of transient tracers, salinity, potential temperature, potential density and apparent oxygen utilization described in the previous section. We start with a discussion of the slowly ventilated TMZ and then discuss trends in deep water ventilation for the different basins. During recent decades, the influences of the EMT and WMT events have led to bottom and deep-water renewal that has modified the TMZ and bottom water ventilation patterns.
The Mediterranean Sea is one of few places in the global ocean with a pronounced TMZ at mid-depth of the water column attributed to rapid ventilation in the deep waters so that the TMZ corresponds to the slowest ventilated layer. The TMZ in the contemporary Mediterranean Sea is presented from the Levantine basin to the Alboran Sea, although with a break in the Strait of Sicily. The dominating water mass of the TMZ is the Transitional Mediterranean Water (TMW) in the EMed and the transitional EMDW (tEMDW) in the WMed. The depth of the TMZ has shallowed in both the western and eastern basins before the EMT and WMT ventilation episodes but has been deepening after these events. For the eastern basin, the TMZ shallowed from 1987 to the 1990s and then deepened up to the 2010s. In the western basin, this shift is not so obvious although the TMZ deepened from 1987 to 1995, shallowed to the mid-2000s (not shown) and then deepened slightly to the 2010s followed by slow upward motion (
We start with the Adriatic Sea as a major source region of deep waters in the Eastern Mediterranean basin. Here we observe no ventilation of the deep-water (below 600 m) from 1987 to 1999, but with strong ventilation between 1999 and 2011 that continued in the period 2011–2016. Even though there was no ventilation of the deep water in the Adriatic Sea up to 1999, as seen by constant CFC-12 and increased AOU, it got slightly saltier and warmer. The trend for the intermediate layer (200–600 m) is similar, but with a pronounced decrease in transient tracers between 1987 and 1995 and then increased ventilation observed in 1999, indicating how changes in this layer are different from the deep layer. The extremely cold winter in 2012 (
The intensified ventilation in the Adriatic Sea influenced the overflow through the Strait of Otranto sill into the Ionian Sea where the EMDW is formed from the AdDW as it mixes with the remnant deep water from the Aegean source. For the other important deep water source of the EMed, the Southern Aegean Sea (i.e., the Cretan Sea), a clear trend is observed with well-ventilated waters in 1995, where after the concentrations remained essentially constant up to 1998, although with considerable variability in the data, and slightly higher concentrations in 2011 and 2018.
The Adriatic and Aegean sources meet in the Ionian Sea, and the increase in CFC-12 concentrations between 1987 and 1995 in the EMDW is larger in the east, which is coincident with the east-to-west gradient of the influence of the dominant Aegean source in the Ionian Sea at that time. This is also illustrated by the amplitude of Θ–S inversion related to the Aegean source, which decreases from the eastern to the western Ionian Sea, as well as from the eastern to the north-western Ionian Sea. In 1999, the influence of the Aegean source was weaker in the eastern and central Ionian deep water but stronger in the western Ionian deep water compared to those in 1995, which describes the delayed influence of the Aegean source to the western Ionian Sea. Subsequently, the amplitude of Θ–S inversions related to the Aegean source became smaller in the whole Ionian Sea and the extent of the reversal decreased from 2001 to 2018. In 2018, the Θ–S inversions created by the influence of the Aegean source became very small and even invisible. With the influence of the Aegean became weaker and found predominantly at shallower depths (
The water from the Adriatic Sea spreads eastward from the Ionian Sea toward the Cretan Passage, as indicated by the Θ–S inversions (
It is worth noting that the change of physical properties (such as salinity, Θ and σΘ) in the northern Cretan Passage deep water after 1995 is significant while the change of CFC-12 concentrations during the same time is, in practice, small. We see no evidence of new DWF in (the surrounding of) the Aegean/Cretan Sea since the EMT event so that the new Adriatic-originated water could reach into the bottom layer of the Levantine basin, where it was detected in 2011 (
Although the spatial distribution of CFC-12 concentrations in the EMed in 2018 is different from that in 1987, similar CFC-12 water column gradients (bottom-to-intermediate) were found in 1987, 2011 and 2018 in the western Ionian Sea. However, the distribution of CFC-12 in 2018 is closer to the one in 1987 than that of 2011. Similarly, the distribution pattern of SF6 in 2018 is closer to that of CFC-12 concentrations in 1987 than that of SF6 concentrations in 2011. This shows a trend of water mass distributions toward a pre-EMT state that is more articulated in 2018 compared to 2011. However, the hydrographic properties are still far away from the pre-EMT condition in 2018, although the transient tracers distribution support a relaxation toward pre-EMT conditions.
The Strait of Sicily plays a significant role in the water exchanges between EMed and WMed. Due to few transient tracer data, additional Θ–S diagrams in 1985, 1986, 1992, 1997, 1998, and 2003 (
The Gulf of Lions is the main source region for deep water in the WMed, but we have only two repeats of transient tracers in this region. The increased transient tracer concentrations indicate intense ventilation from 1997 to 2016. When combined with more data from the CTD and mooring in 1987, 1988, 1993, 1999 (
The evolution of properties in the deep waters of the Gulf of Lions is comparable to those in the adjacent Algero-Provençal basin, the main basin in the WMed. The WMed deep water is characterized by nearly constant CFC-12 concentrations, i.e., nearly stagnant ventilation, between 1995 and 2001 followed by enhanced ventilation up to 2016 and possibly slow ventilation during the last few years of the time-series up to 2018. The recent slow ventilation could be attributed to the weakened influence of the WMT event that started in winter 2004/05. The Θ–S inversions that are tell-tale of the WMT event were found in 2011, 2016 and 2018 in the central Algerian basin and 2011 and 2018 in the western Algerian basin (
There is no direct deep ventilation in the Tyrrhenian Sea but the signal of ventilation is imported by advected water masses. Similar to the Algerian basin, the Tyrrhenian Deep Water (TDW) is characterized by signs of intense ventilation during the 2011–2016 period followed by a possible slowdown of the ventilation after 2016. The increased transient tracer concentrations of the bottom layer around the Sardinia Channel indicate the overflow of well-ventilated WMDW from the WMT event into the deep layer of the Tyrrhenian Sea. Another sign is the weakened intensity of the EMed influence in the intermediate layer (tEMDW and LIW). The TMZ is less-developed in the Tyrrhenian Sea than those in the western basin (
As the shallow sea connected the Algerian basin with the Atlantic Ocean, the Alboran Sea is characterized by well-ventilated deep water with signs of increased ventilation between 1997 and 2018, although such ventilation signal is imported by advected water masses.
The Mediterranean Sea is one of the best-ventilated bodies of water in the global ocean and is as such characterized by high transient tracer concentrations in the deep layer below a zone of lower tracer concentrations in the intermediate layer, the Tracer Minimum Zone (TMZ). This zone of weak ventilation stretches across the whole Mediterranean Sea from the Levantine basin into the western basin. In this study, we report on spatiotemporal variability of deep and intermediate water ventilation of the Mediterranean Sea using a 30+ year time-series of transient tracer and hydrographic observations. During this period, the effects of two “events” dominate the variability of ventilation, the Eastern Mediterranean Transition (EMT) and the Western Mediterranean Transition (WMT).
We have summarized the results and conclusions of this study and depicted the Mediterranean deep and intermediate overturning circulation in the key convective areas over time (
Schematic figures of the Mediterranean deep overturning circulation
The combination of two (or more) transient tracers (e.g., CFC-12 and SF6) can better constrain ventilation. In particular, considering the decreasing CFC-12 and increasing SF6 atmospheric concentrations, the ability of CFC-12 alone in interpreting ventilation in the Mediterranean Sea is decreasing, while SF6 is able to deliver information of ventilation and changes in ventilation. The complicated and variable ventilation of the Mediterranean Sea would benefit from an expanding suite of transient tracers. For instance, does a range of halogenated CFC replacement compound constitute possible additional tracers (
Cruise data in
PL performed the data processing, contributed figures and tables, and wrote the manuscript. TT conducted the sampling from cruises ESAW2, CRELEV2016, TALPro2016, and MSM72 and supported the writing process. Both authors contributed to the article and approved the submitted version.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We acknowledge the great support by the scientists and crew from expeditions M5/6, M31/1, Ura2, P234, Aegaeo98, Ura7, M44/4, M51/2, M84/3, ESAW2, CRELEV2016, TALPro2016, and MSM72. We thank Dr. Tim Stöven for his instructions on some MATLAB scripts. We gratefully thank support through the scholarship program from the China Scholarship Council (CSC). We also thank reviewers for their valuable suggestions.
The Supplementary Material for this article can be found online at: