Open Ocean Convection and Deep Water Formation Revisited in the Mediterranean, Labrador, Greenland and Weddell Seas

Abstract. 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.

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Changes in Mediterranean circulation and water characteristics due to restriction of the Atlantic connection: a high-resolution ocean model

Climate of the Past ◽

10.5194/cp-11-233-2015 ◽

2015 ◽

Vol 11 (2) ◽

pp. 233-251 ◽

Cited By ~ 6

Author(s):

R. P. M. Topper ◽

P. Th. Meijer

Keyword(s):

Deep Water ◽

Ocean Model ◽

Messinian Salinity Crisis ◽

Deep Water Formation ◽

Western Basin ◽

Eastern Basin ◽

Water Characteristics ◽

Water Formation ◽

Sill Depth ◽

The Mediterranean

Abstract. A high-resolution parallel ocean model is set up to examine how the sill depth of the Atlantic connection affects circulation and water characteristics in the Mediterranean Basin. An analysis of the model performance, comparing model results with observations of the present-day Mediterranean, demonstrates its ability to reproduce observed water characteristics and circulation (including deep water formation). A series of experiments with different sill depths in the Atlantic–Mediterranean connection is used to assess the sensitivity of Mediterranean circulation and water characteristics to sill depth. Basin-averaged water salinity and, to a lesser degree, temperature rise when the sill depth is shallower and exchange with the Atlantic is lower. Lateral and interbasinal differences in the Mediterranean are, however, largely unchanged. The strength of the upper overturning cell in the western basin is proportional to the magnitude of the exchange with the Atlantic, and hence to sill depth. Overturning in the eastern basin and deep water formation in both basins, on the contrary, are little affected by the sill depth. The model results are used to interpret the sedimentary record of the Late Miocene preceding and during the Messinian Salinity Crisis. In the western basin, a correlation exists between sill depth and rate of refreshment of deep water. On the other hand, because sill depth has little effect on the overturning and deep water formation in the eastern basin, the model results do not support the notion that restriction of the Atlantic–Mediterranean connection may cause lower oxygenation of deep water in the eastern basin. However, this discrepancy may be due to simplifications in the surface forcing and the use of a bathymetry different from that in the Late Miocene. We also tentatively conclude that blocked outflow, as found in experiments with a sill depth &leq;10 m, is a plausible scenario for the second stage of the Messinian Salinity Crisis during which halite was rapidly accumulated in the Mediterranean. With the model setup and experiments, a basis has been established for future work on the sensitivity of Mediterranean circulation to changes in (palaeo-)bathymetry and external forcings.

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The Mediterranean is getting saltier

Ocean Science Discussions ◽

10.5194/osd-11-735-2014 ◽

2014 ◽

Vol 11 (1) ◽

pp. 735-752 ◽

Cited By ~ 2

Author(s):

M. Borghini ◽

H. Bryden ◽

K. Schroeder ◽

S. Sparnocchia ◽

A. Vetrano

Keyword(s):

Deep Water ◽

Mediterranean Basin ◽

Western Mediterranean ◽

Intermediate Water ◽

Deep Water Formation ◽

The Past ◽

Deep Waters ◽

Western Mediterranean Basin ◽

Water Formation ◽

The Mediterranean

Abstract. The deep waters of the Mediterranean Sea have been getting saltier and warmer for at least the past 40 yr at rates of about 0.015 and 0.04 °C per decade. Here we show that two processes contribute to these increases in temperature and salinity. On interannual time scales, deep water formation events in severe winters transmit increasingly salty intermediate waters into the deep water. The second process is a steady downward flux of heat and salt through the halocline-thermocline that connects the Levantine Intermediate Water with the deep water. We illustrate these two processes with observations from repeat surveys of the western Mediterranean basin we have made over the past 10 yr.

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Was there any deep water formation in the Mediterranean Sea before 8000 years BP?

Deep Sea Research Part A Oceanographic Research Papers ◽

10.1016/0198-0149(87)90089-6 ◽

1987 ◽

Vol 34 (2) ◽

pp. 309-312 ◽

Cited By ~ 3

Author(s):

J. Poutiers

Keyword(s):

Mediterranean Sea ◽

Deep Water ◽

Deep Water Formation ◽

Water Formation ◽

The Mediterranean

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Levantine Intermediate and Levantine Deep Water Formation: An Argo Float Study from 2001 to 2017

Water ◽

10.3390/w11091781 ◽

2019 ◽

Vol 11 (9) ◽

pp. 1781 ◽

Cited By ~ 1

Author(s):

Elisabeth Kubin ◽

Pierre-Marie Poulain ◽

Elena Mauri ◽

Milena Menna ◽

Giulio Notarstefano

Keyword(s):

Deep Water ◽

Eastern Mediterranean ◽

Potential Temperature ◽

Sea Surface ◽

Intermediate Water ◽

Deep Water Formation ◽

Argo Float ◽

Levantine Sea ◽

Water Formation ◽

The Mediterranean

Levantine intermediate water (LIW) is formed in the Levantine Sea (Eastern Mediterranean) and spreads throughout the Mediterranean at intermediate depths, following the general circulation. The LIW, characterized by high salinity and relatively high temperatures, is one of the main contributors of the Mediterranean Overturning Circulation and influences the mechanisms of deep water formation in the Western and Eastern Mediterranean sub-basins. In this study, the LIW and Levantine deep water (LDW) formation processes are investigated using Argo float data from 2001 to 2017 in the Northwestern Levantine Sea (NWLS), the larger area around Rhodes Gyre (RG). To find pronounced events of LIW and LDW formation, more than 800 Argo profiles were analyzed visually. Events of LIW and LDW formation captured by the Argo float data are compared to buoyancy, heat and freshwater fluxes, sea surface height (SSH), and sea surface temperature (SST). All pronounced events (with a mixed layer depth (MLD) deeper than 250 m) of dense water formation were characterized by low surface temperatures and strongly negative SSH. The formation of intermediate water with typical LIW characteristics (potential temperature > 15 °C, salinity > 39 psu) occurred mainly along the Northern coastline, while LDW formation (13.7 °C < potential temperature < 14.5 °C, 38.8 psu < salinity < 38.9 psu) occurred during strong convection events within temporary and strongly depressed mesoscale eddies in the center of RG. This study reveals and confirms the important contribution of boundary currents in ventilating the interior ocean and therefore underlines the need to rethink the drivers and contributors of the thermohaline circulation of the Mediterranean Sea.

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Was there any deep water formation in the Mediterranean Sea before 8000 years BP?

Deep Sea Research Part B Oceanographic Literature Review ◽

10.1016/0198-0254(87)90203-2 ◽

1987 ◽

Vol 34 (9) ◽

pp. 761

Keyword(s):

Mediterranean Sea ◽

Deep Water ◽

Deep Water Formation ◽

Water Formation ◽

The Mediterranean

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“Climate shift of the Atlantic Meridional Overturning Circulation (AMOC) in Reanalyses (ORAS5): possible causes, and sources of uncertainty”

10.5194/egusphere-egu2020-3461 ◽

2020 ◽

Author(s):

Vincenzo de Toma ◽

Chunxue Yang ◽

Vincenzo Artale

Keyword(s):

Deep Water ◽

Gulf Stream ◽

Volume Transport ◽

Heat Fluxes ◽

Labrador Sea ◽

Deep Water Formation ◽

Preliminary Results ◽

Water Formation ◽

Gibraltar Strait ◽

The Mediterranean

We present preliminary results and insights from the analysis of the ensemble of&#160; Oceanic Reanalysis System 5 (ORAS5), produced by the European Center for Medium Weather Forecast (ECMWF), which reconstruct ocean&#8217;s past history from 1979 to 2018, with monthly means temporal and spatial resolution of 0.25&#176; and 75 vertical levels.We focused on the AMOC, which can be considered as one of the main drivers of&#160; the Earth&#8217;s Climate System, and we observed that the strength at 26.5&#176;N presents a shift in the mean of about 5 Sverdrup in the period 1995-2000 which can be considered as a climate tipping point.&#160;&#160;We aim to investigate the causes of this reduction and propose three mechanisms responsible for the observed AMOC volume transport reduction: the Gulf Stream Separation path, changes of the Mediterranean Outflow Water (MOW) and the North Atlantic Deep Water (NADW) formation processes in the Labrador Sea respectively.&#160;The Gulf Stream Separation path is investigated by visualizing the barotropic stream function averaged over two periods, before and after the 1995-2000. In particular it is possible to detect a shift in the direction of the barotropic currents, which is enhanced further by seasonal climatology analysis. In the first period (greater volume transport), patterns are more intense, and the Gulf Stream reach higher latitudes, allowing for a more vigorous deep water formation in the Labrador Sea than in the second period.&#160;&#160;Moreover, we observe the AMOC volume transport reduction at 26.5&#176;N accompanied with a reduction in the heat fluxes over the Labrador Sea. We think this reduction of heat fluxes has a cascade effect on horizontal averages for temperature, salinity, and potential density profiles, which are manifestations of less deep water production in the Labrador Sea, that can ultimately drive the AMOC weakening.&#160; Finally, the Mediterranean Sea has experienced, in the last decades, a general warming trend, in particular of deep water temperatures since the mid-1980s. It is well known that this warming induce a large variability in the hydrological characteristics of the MOW becoming more likely one key factor driving the AMOC variability observed in ORAS5. In fact, there&#8217;s a larger ensemble spread in both the temperature and salinity climatological profiles at 40&#176;N, i. e. in correspondence of the Gibraltar Strait and Gulf of Cadiz.This analysis highlights the high sensitivity of the MOW to perturbations producing the different ensemble members of ORAS5.Our hypothesis is that the nonlinear interaction between these three mechanisms&#160; could have a complex feedback on the AMOC variability.&#160;In conclusion, our preliminary results brought out the relevance of the deep water formation process in the Labrador Sea, the MOW and the Gulf Stream path as the main sources of the AMOC variability and stability. Besides,&#160; our analysis points out the need for further studies, e. g. increasing resolution at the Straits (like Gibraltar Strait), investigating correlations with the variability of the subpolar gyre and developing conceptual studies, using Intermediate Complexity Models interpreted under the lens of Dynamical System Theory and Statistical Mechanics.

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Changes in Mediterranean circulation and water characteristics due to restriction of the Atlantic connection: a high-resolution parallel ocean model

Climate of the Past Discussions ◽

10.5194/cpd-10-2979-2014 ◽

2014 ◽

Vol 10 (4) ◽

pp. 2979-3026

Author(s):

R. P. M. Topper ◽

P. Th. Meijer

Keyword(s):

Deep Water ◽

Ocean Model ◽

Messinian Salinity Crisis ◽

Deep Water Formation ◽

Western Basin ◽

Eastern Basin ◽

Water Characteristics ◽

Water Formation ◽

Sill Depth ◽

The Mediterranean

Abstract. A high-resolution parallel ocean model is set up to examine how the sill depth of the Atlantic connection affects circulation and water characteristics in the Mediterranean Basin. An analysis of the model performance, comparing model results with observations on the present-day Mediterranean, demonstrates its ability to reproduce observed water characteristics and circulation (including deep water formation). A series of experiments with different sill depths in the Atlantic–Mediterranean connection is used to assess the sensitivity of Mediterranean circulation and water characteristics to sill depth. Basin-averaged water salinity and, to a lesser degree, temperature rise when the sill depth is less and exchange with the Atlantic is lower. Lateral and interbasinal differences in the Mediterranean are, however, largely unchanged. The strength of the upper overturning cell in the western basin is proportional to the magnitude of the exchange with the Atlantic, and hence to sill depth. Overturning in the eastern basin and deep water formation in both basins, on the contrary, are little affected by the sill depth. The model results are used to interpret the sedimentary record of the Late Miocene preceding and during the Messinian Salinity Crisis. In the western basin a correlation exists between sill depth and rate of refreshment of deep water. On the other hand, because sill depth has little effect on the overturning and deep water formation in the eastern basin, the model results do not support the notion that restriction of the Atlantic–Mediterranean connection may cause lower oxygenation of deep water in the eastern basin. However, this discrepancy may be due to simplifications in the surface forcing and the use of a bathymetry different from that in the Late Miocene. We also tentatively conclude that blocked outflow, as found in experiments with a sill depth &leq;10 m, is a plausible scenario for the second stage of the Messinian Salinity Crisis during which halite was rapidly accumulated in the Mediterranean. With the model setup and experiments, a basis has been established for future work on the sensitivity of Mediterranean circulation to changes in (palaeo-)bathymetry and external forcings.

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

Ocean Science Discussions ◽

10.5194/osd-10-1405-2013 ◽

2013 ◽

Vol 10 (4) ◽

pp. 1405-1445 ◽

Cited By ~ 3

Author(s):

A. Schneider ◽

T. Tanhua ◽

W. Roether ◽

R. Steinfeldt

Keyword(s):

Mediterranean Sea ◽

Deep Water ◽

Adriatic Sea ◽

Western Mediterranean ◽

Tyrrhenian Sea ◽

Deep Water Formation ◽

Overturning Circulation ◽

Water Formation ◽

The Mediterranean ◽

Transient Tracer

Abstract. The Mediterranean Sea has a fast overturning circulation and the deep water masses are well ventilated in comparison to the deep waters of the world ocean. Significant changes in the overturning circulation 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 Transit (WMT) near the mid of the decade following. 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. This variability is important to understand and to monitor, because ventilation is the main process to propagate surface perturbations, such as uptake of anthropogenic CO2, into the ocean interior. 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 stagnation 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.

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