Mediterrán étrend

[Berniece Domnick]

TheMediterranean is a semi-enclosed strongly evaporative basin (i.e. evaporation greatly exceeds precipitation and river runoff). As a consequence inflowing low salinity Atlantic surface waters (i.e. about 36.2 pss at the Strait of Gibraltar) are transformed into very saline Mediterranean intermediate and deep waters (with a deep water salinity of about 38.4 and 38.7 pss in western and eastern sub-basins, respectively). The density contrast between the highly saline Mediterranean Water and the relatively fresh Atlantic Water (AW) drives an inverse estuarine circulation in the Mediterranean. Mediterranean water of high salinity exits into the Atlantic, below the AW layer, and the net freshwater transport at the Gibraltar strait balances the freshwater loss through the surface.

Geography and bathymetry map of the Mediterranean Sea. Main areas of dense water formation (black boxes) and dense water masses discussed in the text are also depicted. The Sicily Strait separates EMED and WMED sub-basins as defined in this study.

Long-term salinity increasing trends in the Mediterranean are consistent with observed long-term changes in the various components of the water cycle, namely a long-term increase in E driven by surface warming over the last few decades (e.g. Mariotti et al. 2002; Romanou et al. 2010; Skliris et al. 2012) combined with a decrease in P from late 1960s to mid-1990s mainly associated with the North Atlantic Oscillation (NAO) multi-decadal variability (Krahmann and Schott 1998; Tsimplis and Josey 2001; Mariotti et al. 2002; Mariotti 2010) and a drastic reduction of river runoff (R) attributed both to climate change and damming of major Mediterranean rivers since the 1960s (Rohling and Bryden 1992; Skliris and Lascaratos 2004; Skliris et al. 2007; Ludwig et al. 2009). There are no sufficient current velocity and salinity profile observations to estimate long-term water/salt transport variations at the Gibraltar and Dardanelles straits in order to assess their contribution in the salt and freshwater budgets of the basin. Observations at the Gibraltar Strait show a salinification of the Mediterranean outflow over the last few decades (Millot et al. 2006). On the other hand, AW can also be an important source of the Mediterranean salinity trend (Somot et al. 2016). Recent observational studies showed a large salinity increase in the Atlantic surface waters adjacent to the Gibraltar Strait over the 2000s, suggesting a significant contribution to the salinification of the Mediterranean Sea at shorter timescales (Millot 2007; Soto-Navaro et al. 2012).

Both datasets are using high-level quality checked salinity data. Although there is a well-known problem with XBT failing rate issue in the older MEDATLAS dataset (which is corrected in the more recent En4 database used here) Vargas-Yanez et al. (2010) showed that the omission of XBT data from the time series resulted in no significant change in the long-term temperature trends throughout the Mediterranean basin while salinity trends remained unchanged.

Salinity observations at individual locations are not appropriate to infer water cycle changes since surface signals are rapidly advected by ocean surface currents. However by integrating 3-D salinity changes over the ocean one may filter out the rearrangement of saline and fresh waters from ocean currents. By quantifying the increase of anomalously salty and fresh waters i.e. the amplification of the mean 3-D salinity pattern, one may infer changes in the water cycle.

Here we assess water cycle change in the Mediterranean Sea through observed 3-D salinity changes using two different methods. In the first method the net evaporation change is inferred from the Mediterranean volume-averaged salinity change and net salt flux changes at the straits connecting the Mediterranean basin with the adjacent basins, using an approximate salt conservation equation:

In the second approach water cycle changes are linked to changes in the volumetric distribution in salinity coordinates using a new thermodynamic method developed by Zika et al. (2015), based on the water mass transformation theory (Walin 1977, 1982), as explained below.

If we ignore the salt mixing term (MIX) the peak value in FWT distribution in S space provides an estimate of the change of the water cycle amplitude. In regions of net evaporation such as the Mediterranean Sea a negative peak denotes the total increase of net evaporation (i.e. total accumulated freshwater volume removed from the basin) and a positive peak denotes the total decrease of net evaporation (total accumulated freshwater volume added in the basin). Equation (5) generally stands for an enclosed system i.e. the global ocean or an enclosed oceanic basin. In the case of a semi-enclosed basin such as the Mediterranean Sea, the FWT term also includes the freshwater displacement due to changes in the transports at the straits.

Annual timeseries of total volume-averaged salinity show important decadal variability, especially in the upper layer, characterised by roughly three distinct periods: (a) 1950s to late 1960s with no significant trend, (b) early 1970s to mid-1990 s with a moderate increasing salinity trend, and (c) from mid-1990 s onwards with a much stronger increasing salinity trend (Fig. 6d).

In addition, one may argue that the increased salinification of the basin may reduce stratification through the density increase, thus further enhancing diffusive fluxes. Salinification and warming trends over the second half of the twentieth century are shown to be almost density compensated in both the WMDW (Bethoux and Gentili 1999) and in the MOW within the Northeast Atlantic subtropical region (Potter and Lozier 2004), suggesting no significant long-term changes in Mediterranean Sea stratification before the 2000s. However large salinity-driven density increases are observed in the WMDW over the last 15 years mainly associated with the WMT (Schroeder et al. 2016; Somot et al. 2016; Houpert et al. 2016).

Increased salt influx by AW in the ocean reanalysis is also consistent with both our water mass transformation analysis and salinity observations around the Gibraltar strait. There is strong evidence of a long-term salinification of the upper layer of the eastern part of subtropical North Atlantic (Skliris et al. 2014b) which feeds the AW inflow in the Mediterranean Sea, whilst recent studies show a strong inter-annual salinification of the surface Atlantic waters adjacent to the Gibraltar Strait during the 2000s (Millot 2007; Soto-Navaro et al. 2012). Concerning the Dardanelles BSW inflow there even less observational evidence (as compared to Gibraltar Strait) to assess long-term changes. Stanev and Peneva (2002) investigating the long-term variations of Bosphorus transport showed an increasing trend over the second half of the twentieth century, indicating that the Aegean Sea was dominated by higher outflow from the Black Sea contributing to a long-term dilution of this regional basin. This is consistent with both the two salinity datasets used here showing a small but statistically significant freshening in the upper layer of the Northern Aegean Sea.

The ocean reanalysis applied here shows that the long-term salt outflux increase through the lower layer at the Gibraltar Strait is almost balanced by the salt influx increase in the upper layer resulting in a very small net salt outflux increase with the equivalent basin salt content increase being about only 5% of the actual basin salt content increase in En4. This indicates that net salt flux change at the Gibraltar Strait has a very small contribution in changing the Mediterranean salinity with respect to the local water cycle change.

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Phytoplankton and chlorophyll concentration as a proxy for phytoplankton respond rapidly to changes in environmental conditions, such as light, temperature, nutrients and mixing (Colella et al. 2016). The character of the response depends on the nature of the change drivers, and ranges from seasonal cycles to decadal oscillations (Basterretxea et al. 2018). Therefore, it is of critical importance to monitor chlorophyll concentration at multiple temporal and spatial scales, in order to be able to separate potential long-term climate signals from natural variability in the short term. In particular, phytoplankton in the Mediterranean Sea is known to respond to climate variability associated with the North Atlantic Oscillation (NAO) and El Nio Southern Oscillation (ENSO) (Basterretxea et al. 2018, Colella et al. 2016).

In the Mediterranean Sea, the trend average for the 1997-2022 period is slightly negative (-0.690.69% per year) confirm the results obtained from previous release (1997-2021). This result is in contrast with the analysis of Sathyendranath et al. (2018) that reveals an increasing trend in chlorophyll concentration in all the European Seas. Starting from 2010-2011, except for 2018-2019, the decrease of chlorophyll concentrations is quite evident in the deseasonalized timeseries (in green), and in the maxima of the observations (grey line), starting from 2015. This attenuation of chlorophyll values of the last decade, results in an overall negative trend for the Mediterranean Sea.

The volume is divided in four sections, dedicated to migration trends, risks, development and governance. The volume features contributions from different IOM offices, as well as from other international organisations, research institutions and civil society organisations.

Sea-level change is one of the most concerning climate change and global warming consequences, especially impacting coastal societies and environments. The spatial and temporal variability of sea level is neither linear nor globally uniform, especially in semi-enclosed basins such as the Mediterranean Sea, which is considered a hot spot regarding expected impacts related to climate change. This study investigates sea-level trends and their variability over the Mediterranean Sea from 1993 to 2019. We use gridded sea-level anomaly products from satellite altimetry for the total observed sea level, whereas ocean temperature and salinity profiles from reanalysis were used to compute the thermosteric and halosteric effects, respectively, and the steric component of the sea level. We perform a statistical change point detection to assess the spatial and temporal significance of each trend change. The linear trend provides a clear indication of the non-steric effects as the dominant drivers over the entire period at the Mediterranean Sea scale, except for the Levantine and Aegean sub-basins, where the steric component explains the majority of the sea-level trend. The main changes in sea-level trends are detected around 1997, 2006, 2010, and 2016, associated with Northern Ionian Gyre reversal episodes, which changed the thermohaline properties and water mass redistribution over the sub-basins.

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