Abstract
The internal variability of the thermohaline circulation of the Mediterranean Sea is examined under contrasting extreme thermal and mass atmospheric forcing conditions. Three millennium-long numerical simulation experiments were performed under: (a) the current climatology, (b) a strong buoyancy forcing (SBF) scenario due to cold and dry conditions resembling the Younger Dryas event, and (c) a weak buoyancy forcing (WBF) scenario due to S1a sapropel deposition-like conditions (warm and wet). To isolate the inherent variability of the system, independent of interannual atmospheric forcing variability, the latter was defined as a perpetual year pertinent to each experiment. Self-diagnosed heat and salt fluxes, consistent to sea-surface characteristics of the above periods, forced three millenium-long, relaxation-free numerical experiments. These simulations were preceded by initial spin-up periods. The inherent spatiotemporal variability of the Mediterranean Sea was analyzed using the empirical orthogonal function (EOF) and spectral analysis on the simulated density fields. Our results revealed that the Mediterranean Sea exhibits high sensitivity to climatic conditions, allowing its circulation to change from anti-estuarine (for the SBF scenario, leading to a buoyancy loss to the atmosphere) to estuarine (for the WBF scenario, corresponding to a buoyancy gain from the atmosphere). In all three experiments, the interannual and decennial variabilities dominate in upper layers, and the decennial variability dominates in the Gibraltar and Sicily Straits. Under current climatic conditions the first two EOF modes express only 60% of the density variability in the deep layers. This contribution exceeds 90% under more extreme conditions. Moreover, the first EOF modes correspond to a basin-wide in-phase variability of the deep layers under the reference and WBF conditions. During SBF conditions the first modes reveal a vertical buoyancy exchange between upper and deeper layers. The second EOF mode of deep waters under both extreme scenarios showed that the western and eastern basins exchange buoyancy in decennial (for the cold/dry) and interdecennial (for the warm/humid) timescales. The residence time of the Eastern Mediterranean deep water was diagnosed to be centennial, semicentennial, and intercentennial for the cases of current period, SBF, and WBF, respectively.
Highlights
The Mediterranean Sea is characterized as a concentration basin (Nielsen, 1912; Sverdrup et al, 1942)
The mean millennial overturning circulation of the Mediterranean Sea for each numerical experiment reveals that both the basins behave as concentration basins in the reference and strong buoyancy forcing (SBF) experiments, whereas the eastern Mediterranean clearly becomes a dilution basin under the weak buoyancy forcing (WBF) conditions
The SBF simulation (Figure 4B) produces more dense water in the eastern basin compared to the reference experiment (Figure 4A), whereas the results in the western basin are rather surprising, due to the production of dense water extending over a larger area of the basin; most of the produced water does not reach the maximum depths of the basin, possibly due to a higher stratification in the deeper layers
Summary
The Mediterranean Sea is characterized as a concentration basin (Nielsen, 1912; Sverdrup et al, 1942). In the past the Mediterranean basin occasionally exhibited periods of dramatic reduction of deep-water ventilation, which can be identified in the sedimentary record by the presence of sapropels. These dark organic-rich layers are found throughout the Eastern Mediterranean, the Tyrrhenian Sea, and some parts of the Western Mediterranean and demonstrate wet and warm climatic conditions dominating the basin. The last sapropel deposition S1 was recorded during the Holocene Climatic Optimum (∼10–6 ka BP) Both S1 sapropel sublayers (S1a and S1b) correspond to the warmest and most humid Holocene intervals for the Mediterranean, respectively (Gogou et al, 2007; Triantaphyllou et al, 2009a,b, 2016; Geraga et al, 2010; Triantaphyllou, 2014)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.