By reducing water density and increasing vertical stratification, global warming is expected to impede ventilation mechanisms in the world ocean and regional seas with potential consequences for the oxygenation of the subsurface layer . On a global scale, the reduction in ventilation processes constitutes a larger contribution to marine deoxygenation than the warming-induced reduction in oxygen solubility . While the reduction in ventilation mechanisms is often evidenced, it remains challenging to determine whether such changes are the signal of natural variability or rather bear witness to a significant regime change attributed to global warming .

The Black Sea provides a miniature global ocean framework where processes of global interest occur at a scale more amenable to investigation. Its deep basin is permanently stratified, and the ventilation of the subsurface layer relies in substantial parts on the convective transport of cold, oxygen-rich water formed each winter at the surface. Between 1955 and 2015, the Black Sea oxygen inventory declined by 40 % , which echoes the significant deoxygenation trend that affected the world ocean over a similar period .

The permanent stratification of the Black Sea results from two external inflows . The saline Mediterranean inflow enters the Black Sea by the lower part of the Bosporus Strait, the sole and narrow opening of the Black Sea towards the global ocean. The greatest part of the terrestrial freshwater inflow enters the Black Sea on its northwestern shelf. The contrast in density (salinity) between these two inflows maintains a permanent stratification in the open basin (halocline) that prevents ventilation of the deep layers. This lack of ventilation induces the permanent anoxic conditions that characterize 90 % of the Black Sea waters. Between the oxic and anoxic (euxinic) layers, a suboxic zone, where both dissolved oxygen and hydrogen sulfide are below reliable detection limits , is maintained by biogeochemical processes .

Just above the main halocline, the ventilation of the Black Sea subsurface waters (∼ 50–100 m), is ensured by convective circulation. It proceeds from the sinking of surface waters, made colder and denser by loss of heat towards the atmosphere in wintertime . A similar ventilation process is observed, for instance, in the Mediterranean Gulf of Lion e.g.,. In the Black Sea, however, the dense oxygenated waters never reach the deepest parts, as their sinking is restrained at an intermediate depth by the permanent halocline. The accumulation of cold waters at an intermediate depth forms the so-called Cold Intermediate Layer (CIL). The process of CIL formation thus provides an annual ventilating mechanism that structures the vertical distribution of oxygen and, by extension, that of nutrients and living components of the ecosystem .

The semi-enclosed character of the Black Sea, combined with the fact that ventilation is limited to the upper ∼ 100 m, makes it highly sensitive to variations in external forcing. In particular, variations in atmospheric conditions (e.g., air temperature, wind curl) result in pronounced and relatively fast inter-annual alterations of the Black Sea physical structure .

While several studies have evidenced a warming trend in the Black Sea surface temperature , showed that the Black Sea intermediate waters present an even stronger warming trend. This difference between the surface and subsurface temperature trends can be explained by the fact that the CIL dynamics buffers the atmospheric warming trends and minimizes its signature in sea surface temperature .

The inter-annual variability in CIL formation can be explained for the most part on the basis of winter air temperature anomalies , although intensity of the basin-wide cyclonic circulation , the freshwater budget and the intensity of short-term meso-scale intrusions also play a role . An extensive description of the CIL dynamics, detailing the contributions of and variability in the mechanisms mentioned above was recently provided by . One aspect is particularly relevant to our study: in wintertime, the deepening of the mixed layer and the uplifting of isopycnals in the basin center (as the cyclonic circulation intensifies) expose subsurface waters to atmospheric cooling. If a well-formed CIL was present during the previous year, subsurface waters exposed to atmospheric cooling will already be cold, which increases the amount of newly formed CIL waters . Due to this positive feedback and to the accumulation of CIL waters formed during successive years, the inter-annual CIL dynamics is better described when winter air temperature anomalies are accumulated over 3 to 4 years, rather than considered on a year-to-year basis , which is in agreement with the 5 years upper estimate provided by for the residence time within the CIL layer.

Given this non-linear context, there are reasons to suspect that global warming, by increasing the average air temperature around which annual fluctuations occur, may induce a persistent shift in the regime of the Black Sea subsurface ventilation. Indeed, used Argo float data (2005–2018) to highlight a recent constriction of the CIL layer, following a trend leading to conditions where the CIL, as a layer colder than the underlying waters, would no longer exist. The authors further indicate implications for the Black Sea thermo-haline properties, as this recent weakening of the CIL layer goes hand in hand with increasing trends in surface and subsurface salinity, indicative of diapycnal mixing at the basis of the former CIL layer.

Here, we combine different data sources to analyze the variability in the Black Sea intermediate-layer ventilation over the last 65 years and, in particular, investigate the existence of a statistically significant shift in the CIL formation regime, in regard to the variability observed over this period. The hypothesis of a significant regime shift is tested against the more traditional linear and periodic interpretation of the observed trends e.g.,, as the consequence for Black Sea ventilation and the future of the Black Sea oxygenation status in particular are drastically different.

Indeed, evidenced a clear relationship between oxygen conditions in the lower part of the CIL layer and the temperature in that layer which is directly related to inter-annual variations in the CIL formation intensity. This relationship explains a large part of the inter-annual fluctuations in oxygen concentration in that layer, which occur at a timescale of a few years. Those fluctuations are superimposed on the larger-scale change in oxygenation state that is attributed to an increase in the primary production induced by the eutrophication phase of the late 1970s.

Our analysis thus aims to expand on these investigations and in particular to focus on the annual convective ventilation as a component of the complex Black Sea deoxygenation dynamics , in the context of the recent warming trend affecting the Black Sea .

Section  details the datasets considered to characterize the Black Sea CIL and oxygenation dynamics and the method of regime shift analysis. In Sect. , we analyze the long-term CIL dynamics through the lens of regime shift analysis. In Sect. , we use outputs from a three-dimensional hydrodynamic model and recent Argo records to relate CIL formation rates to changes in the Black Sea oxygenation conditions. In Sect. , we discuss those results in the frame of larger timescales, while we conclude in Sect. .

The intra-annual resolution provided by the 3D model and Argo time series (Fig. ) suggests that the partial CIL renewal, which was taking place systematically each year before 2009, has now become occasional. Here we focus on period P4, better detailed in our datasets, to characterize the CIL formation as a basin-wide ventilation process and its relationship with changes in oxygen saturation at different σ levels.

Basin-wide CIL formation and destruction rates were computed from the synoptic 3D model outputs, as differences between weekly C values (Fig. ). The seasonal sequence depicts CIL formation peaks from late December to March, typically reaching C formation rates of 5, 10 and 1 MJ m-2 d-1 for the period P1–P3, P2 and P4, respectively (Fig. a–c). The CIL cold content is then eroded at different rates before, during and at the end of the thermocline season, with a damped erosion rate through the thermocline season between 0 and about 1 MJ m-2 d-1. CIL formation processes have been described extensively in the past e.g.,, in more detail than is allowed by the integrated perspective adopted here. This integrated point of view, however, serves to point out the striking quasi-absence of CIL formation peaks for the years 2001, 2007, 2009, 2010, 2013 and 2014 (Fig. d). In fact, during the period of Argo oxygen sampling (2011–2020), only 2012 and 2017 depict important CIL formation events, while minor CIL formation events are shown for 2015 and 2016.

Oxygen saturation in this period varies in concordance with CIL formation up to σ layers of about 16.0 kg m-3 (Fig. ). Large increases can be observed from December to March in the years 2012, 2015, 2016 and 2017 when CIL formation is significant, which denotes the impact of convective ventilation. The narrow interquartile ranges depicted in Fig.  denote the efficiency of the isopycnal diffusion of oxygen: the amount of oxygen imported with the newly formed CIL waters is distributed horizontally and contributes to increasing the average oxygen saturation of a given σ layer.

While major CIL formation events in 2012 and 2017 induced a significant increase in oxygen saturation through the whole oxygenated water column, the minor events in 2015 and 2016 seem to have had a limited penetration depth. For instance, oxygen saturation at 14.6 kg m-3 only stagnates during 2015 and 2016 as compared to 2014, while oxygen saturation at 15.1 kg m-3 keeps decreasing during these 2 years, indicating that the amount of oxygen brought to this layer during minor ventilation events is not sufficient to counterbalance the biogeochemical oxygen consumption terms (i.e., respiration and oxidation of reduced substances diffusing upwards).

Following our attempt to summarize large datasets and to characterize a basin-scale annual oxygenation rate, we computed for each layer an annual oxygenation index as the difference between the median oxygen saturation in November between successive years. The rationale behind this approach is that CIL formation typically extends from December to March (Fig. a–c).

In order to obtain a general indication of the (pycnal) penetration depth of the convective ventilation associated with CIL formation, we assessed the Pearson correlation coefficient between this annual oxygenation index and a first-order assessment of annual CIL formation, obtained as the annual difference in the C composite time series. The correlation between oxygenation and CIL formation is high near the surface and decreases continuously as deeper pycnal levels are considered. Those correlations remain significant (p<0.1) until σ=15.4 kg m-3 (Fig. ).

The regime shift paradigm describes an abrupt and significant change in the observable outcome of a non-linear system, as could result from a threshold in this system response to external forcing. In contrast, a periodic model supposes either a linear response to periodically varying external forcing or an oscillation resulting from internal dynamics. It is our hypothesis, supported by the quantitative analysis presented in this study, that the regime shift model should be favored for interpreting the recent evolution of the Black Sea CIL dynamics. The prerequisite for the regime shift analysis was first to issue an unified, synoptic metric to characterize inter-annual variations in the CIL content. The consistency between the different data sources used to construct this metric demonstrates the robustness of this metric. To our knowledge, no multi-source comparison has been previously achieved over such an extended period. Note that some dependencies exist among certain sources, as discussed explicitly in Table .

Although we acknowledge that the statistical advantage (AIC) of the regime shift description is subtle, we consider that it deserves further consideration as this difference in interpretation is fundamental in regards to the expected consequences on the Black Sea oxygenation status and in particular the threat to Black Sea marine populations, whose ecological adaptation (and rate of exploitation) has been built upon a ventilation regime and consequent biogeochemical balance that may no longer prevail.

Indeed, it appears that the intermittency of significant CIL formation events characterizes the new ventilation regime: the ventilation of the Black Sea intermediate layers does not occur each year anymore but is occasionally absent for 1 or 2 consecutive years. Moreover, major CIL formation events, which bear the potential for deeper oxygenation, appear significantly less frequently.

The extent to which the current regime differs from the previous ventilation regimes is clearly illustrated on a T–S diagram: in situ measurements from period P4 are commonly found in a range of the T–S diagram (T>8.35 ∘C and σ within 14.5–15 kg m-3) that was extremely rare in previous periods (see density contours on Fig. a–d; two-dimensional density estimates were obtained with the R function MASS:kde2d).

As indicated by , this trend may lead to the disappearance of a characteristic layer of the Black Sea, which constituted a major component of its thermo-haline structure and constrained the exchanges between surface, subsurface and intermediate layers. In particular, the authors highlight surface and subsurface salinity trends that indicate recent occurrences of diapycnal mixing at the lower base of the intermediate layer, where waters are characterized by a strong reduction potential due to the presence of reduced iron and manganese species, ammonium, and finally hydrogen sulfide .

On a decadal timescale, the average oxygen signature of a given isopycnal layer within the CIL depends on the frequency of CIL formation events of sufficient intensity (Sect. ), which is in line with the ventilation dynamics described by for the upper pycnocline. Although inter-annual fluctuations in the CIL formation rate still occur, the regime shift analysis specifically describes a reduction in the average CIL cold content, which appears to be associated with a reduction in the frequency of significant ventilation events (Fig. ) and therefore a potential decrease in the oxygen saturation signature in the lower part of the CIL.

Importantly, this reduction may also affect deeper layers of the Black Sea. Indeed, the mid-pycnocline (σ between about 14.6 and 16 kg m-3) is formed by the two end-member mixing lines between the CIL layer and the Bosporus inflow , which proceeds from the entrainment of CIL waters by the Bosporus inflow and subsequent lateral ventilation . Considering the characteristic residence time for the upper (about 5 years; ) and intermediate (9–15 years; ) pycnocline, it is appropriate to consider such temporal averages to characterize the oxygen signature of the CIL member composing the mixture of pycnocline waters.

Displaying historical oxygen saturation data (1955–2020) on the T–S diagram (Fig. e) indeed shows generally deeper oxygenation during high-CIL regimes than during low-CIL regimes. For instance, oxygen saturation in the density range σ=14.8–15.2 kg m-3 lies in the range of 30 %–70 % in the T–S region that is characteristic of P2, while it only reaches 10 %–50 % in the T–S region characteristic of P4. This indicates that the analysis linking oxygenation and CIL formation for the recent period (Sect. ) can be extended to larger timescales by considering changes in the frequency of significant CIL formation events. Thus, the depth up to which the reduction in CIL formation may impact on the biogeochemical balance of the Black Sea (by affecting the oxygenation level) will depend on the period over which the current ventilation regime will continue.

Indeed, it is important to highlight that the current regime may not necessarily be a new steady regime – even though it has been identified as such by the change point analysis – but that it could be part of a transient downwards trend that started in the mid-1990s. The reason why it appeared important to us to oppose non-linear regime dynamics with smooth linear and sine trend models is that the recent CIL dynamics, when depicted by the regime change approach, is not slow-trending but suggests a step change towards a new phenology for the intermediate Black Sea. Only future observations may now confirm or invalidate the relevance of the proposed regime shift paradigm.

Beyond the changes in convective ventilation highlighted above, it thus appears as a lead priority to assess the biogeochemical consequences of this new thermo-haline dynamics of the Black Sea. In particular, the influence of CIL formation on the biogeochemical components of the oxygen budget should be addressed in more detail, asking for instance how the presence or absence of CIL formation influences planktonic growth, trophic interactions and organic-matter respiration rates. We voluntarily adopted here a wide integrative point of view so as to highlight the large-scale relevance of the identified regime shift to Black Sea oxygenation. However, we still consider that a detailed assessment of all components of the oxygen budget, i.e., ventilating processes and biogeochemical terms, is required in order to infer the future evolution of the Black Sea oxygenation status e.g.,.

Although there are clear indications of a long-term warming trend in the Black Sea , it remains a delicate task to strictly dissociate the contributions of global warming from those of regional atmospheric oscillations . One such assessment in the neighboring Mediterranean Sea concluded that global warming trend and regional oscillation contributed 42 % and 58 % of the recent regional sea surface temperature trend (1985–2009), respectively. While a corresponding assessment will have to be routinely reevaluated for the Black Sea as the time series expands, it may conservatively be considered that global warming has made a significant contribution to warming winters in the Black Sea. This contribution is expected to increase in the next decades .

We have analyzed the variability in the Black Sea CIL formation over the last 65 years and investigated the existence of regime shifts in this dynamics. For this purpose, we have produced a composite time series of the CIL cold content (C) that is considered a proxy for the intensity of the convective ventilation resulting from the formation of dense oxygenated waters. This composite time series is built from four different data products issued from observations and modeling so as to optimize its temporal extent in regards to preceding studies e.g.,. The consistency between those products and in particular the close correspondence between observational and mechanistic predictive time series supports the reliability of the composite series and its adequacy to describe the evolution of the Black Sea subsurface convective ventilation during the last 65 years.

The composite time series was analyzed to detect different regimes, corresponding to periods characterized by significantly distinct averages. We identified three main regimes that have existed over last 65 years: (1) a standard regime prevailing during 1955–1984 and 1999–2008 that is consistent with the full-period average C, (2) a cold regime (high C, 1985–1998) which has been previously documented (see references in Sect. 3), and (3) a warm regime (low C, 2009–2019) which is characterized by the intermittency of the annual partial CIL renewal. Statistical considerations indicate that the abrupt shift can not adequately be described by a combination of long-term linear and periodic trends. However, monitoring the future evolution of the CIL is necessary to confirm that this abrupt-change description should be favored over that of a transient dynamics.

The synoptic CIL formation rates provided by the 3D hydrodynamic model and the detailed description of oxygenation conditions provided by BGC-Argo floats allowed us to detail the role of CIL formation in oxygenating the upper part of the Black Sea intermediate layers through convective ventilation (i.e., σ of between 14.4 and 15.4 kg m-3). Given that cold winter air temperature is the leading driver of CIL formation ; given that CIL formation constitutes a dominant ventilation mechanism for the Black Sea intermediate layer; and assuming that oxygen conditions constitute an environmental structuring factor affecting the ecosystem organization, its vigor and its resilience , this shift in the Black Sea ventilation regime may be associated with global warming and is expected to affect its biogeochemical balance and to threaten marine populations adapted to previously prevailing ventilation regimes.

To understand how global warming impacts marine deoxygenation dynamics is a worldwide concern. The relatively fast and clear response that stems from the specific Black Sea geomorphology makes it a natural laboratory to study this dependency and related phenomena, although the specificity of this morphology also limits the direct transposition of Black Sea results to the global ocean. Here, we showed that non-linear dynamics and feedbacks in ventilation mechanisms resulted in a significant shift in the average ventilation regime, in response to rising air temperature. Since the temporal extent of low-oxygen conditions is critical for ecosystems, we stress the importance of assessing the potential for similar ventilation regime shifts in other oxygen-deficient basins.