Wolfgang Roether

Recent Changes in Eastern Mediterranean Deep Waters

Results from a recent hydrographic survey show that an influx of Aegean Sea water has replaced 20 percent of the deep and bottom waters of the eastern Mediterranean. Previously, the only source of such waters was the Adriatic Sea, and the waters of the eastern Mediterranean were in near-steady state. The flux changed the water characteristics and displaced older waters upward. Its cause was increasing Aegean Sea salinity, resulting from changes in either the circulation pattern or the large-scale freshwater balance. Current deepwater studies may be affected by the intrusion, but effects might be found also at shallower depths and over a larger region.

The large deep water transient in the Eastern Mediterranean

The recent changes in the thermohaline circulation of the Eastern Mediteranean caused by a transition from a system with a single source of deep water in the Adriatic to one with an additional source in the Aegean are described and assessed in detail. The name Cretan Sea Overflow Water (CSOW) is proposed for the new deep water mass. CSOW is warmer (θ>13.6°C) and more saline (S>38.80) than the previously dominating Eastern Mediterranean Deep Water (EMDW), causing temperatures and salinities to rise towards the bottom. All major water masses of the Eastern Mediterranean, including the Levantine Intermediate Water (LIW), have been strongly affected by the change. The stronger inflow into the bottom layer caused by the discharge of CSOW into the Ionian and Levantine Basins induced compensatory flows further up in the water column, affecting the circulation at intermediate depth. In the northeastern Ionian Sea the saline intermediate layer consisting of Levantine Intermediate Water and Cretan Intermediate Water (CIW) is found to be less pronounced. The layer thickness has been reduced by factor of about two, concurrently with a reduction of the maximum salinity, reducing advection of saline waters into the Adriatic. As a consequence, a salinity decrease is observed in the Adriatic Deep Water. Outside the Aegean the upwelling of mid-depth waters reaches depths shallow enough so that these waters are advected into the Aegean and form a mid-depth salinity-minimum layer. Notable changes have been found in the nutrient distributions. On the basin-scale the nutrient levels in the upper water column have been elevated by the uplifting of nutrient-rich deeper waters. Nutrient-rich water is now found closer to the euphotic zone than previously, which might induce enhanced biological activity. The observed salinity redistribution, i.e. decreasing values in the upper 500–1400 m and increasing values in the bottom layer, suggests that at least part of the transition is due to an internal redistribution of salt. An initiation of the event by a local enhancement of salinity in the Aegean through a strong change in the fresh water flux is conceivable and is supported by observations.

Recent changes in deep water formation and spreading in the eastern Mediterranean Sea : a review

Abstract Observations of the last decade testify that the characteristics of the deep thermohaline circulation in the Eastern Mediterranean Sea have changed thoroughly. The source of the most dense waters of the basin has moved from the Adriatic to the Aegean Sea. This new source has proved to be very efficient since the estimated formation rate for the period 1989–95 was more than 1 Sv, about three times more than the dense water formation rate of the Adriatic Sea. These new waters with hydrological characteristics, that are warmer and more saline, have replaced almost 20% of the older deep waters of the basin, and have uplifted the deep isopycnals by about 500 m. This major event can be attributed to important meteorological anomalies in the Eastern Mediterranean and to changes of circulation patterns. The extended dry period of 1988–93 and the exceptionally cold winters of 1987 and 1992–93 created favorable conditions for increased formation of dense water in the Aegean. Furthermore, changes in the circulation patterns in the intermediate water range (Levantine Intermediate Water LIW and Cretan Intermediate Water), themselves possibly linked to meteorological anomalies, appear to have played an important role in the redistribution of salt. As a result of an interruption to exchanges between the Ionian and Levantine Basin, the salinities in the latter started to rise, high salinity waters were diverted into the Aegean [ Malanotte-Rizzoli, P., Manca, B. B., Ribera dAcala, M., & Theocharis, A. (1998). The Eastern Mediterranean in the 80s and in the 90s: the big transition emerged from the POEM-BC observational evidence. Rapport du Commission International de la Mer Medittanee, 35, 174–175] and the westward transport of LIW was reduced. An additional effect of the deep water discharge from the Aegean and the resulting uplifting of mid-depth waters was to lower salinities in the LIW layer. This effect is most strongly felt in the Ionian Sea. A 3-D primitive equation numerical model for the Eastern Mediterranean with a 20 km grid size is used to simulate the observed changes and understand the basic mechanisms which caused them. Under appropriate atmospheric forcing the model successfully reproduces the main characteristics of the transient. These results indicate that the observed changes can be, at least partially, explained as a response of the Eastern Mediterranean, and more specifically of the Aegean, to atmospheric forcing variability.

Eastern Mediterranean deep water renewal on the basis of chlorofluoromethane and tritium data

Abstract Simultaneous chlorofluoromethane (CFM) and tritium data obtained in the eastern Mediterranean during the POEM survey of the F/S Meteor in 1987, supported by tritium data taken in 1978, indicate that the deep water in the eastern Mediterranean below about 1200 m deep forms a coherent thermohaline convective system. The data are interpreted by means of a conceptual numerical model of the renewal, recirculation and mixing of the waters below about 1000 m depth. The model gives a replacement of the waters below 1400 m depth, by waters from above 1000 m depth, converted by way of the Adriatic, of 2.9 ± 0.9 10 5 m 3 s −1 . This rate corresponds to between about 15 and 40% of the total water exchange through the Straits of Sicily. Relative to their respective surface water time histories, much less tritium than CFM Freon 12 (F 12) is present in the deep water. This is interpreted as a consequence of the different modes of oceanic input for the two tracers. Similar tritium deficits are predicted to be a common phenomenon in ocean waters, and are believed to provide a useful oceanographic tool. In the present context, it is the tracer combination that provides the constraint on the convective deep water renewal. Furthermore, the tritium data suggest that the majority of the water converted in the deep water formation process (in the southern Adriatic) was stored beforehand as intermediate-depth water for several years. The F 12 data require that most of this water subsequently came into convective contact with the air-water interface for more than a month.

Chlorofluoromethane and oxygen in the Eastern Mediterranean

CFM-12 (chlorofluoromethane, CCl2F2) data from the 1987 expedition of the F.S. Meteor in the Eastern Mediterranean are presented and discussed in the context of simultaneous oxygen and hydrographic measurements. Taken together, these data strongly suggest that the Adriatic is the only substantial source of bottom and deep water for the Eastern Mediterranean, in that no signatures of Aegean-derived water are found in the Levantine and Ionian Basins below about 1200 m depth. On the basis of a CFM-12 budget, the time-averaged inflow rate of Adriatic deep water into the Ionian is estimated to be 0.3 ± 0.1 Sv. The lowest CFM-12 and oxygen concentrations are found in the deep water between 2800 and 1200 m depth. This water mass is considered the oldest water in the Eastern Mediterranean and appears to be renewed by upwelling of bottom water. A lens of high CFM-12 water centered near 700 m depth is observed at a single station (Sta. 756) south of Crete. According to multi-parameter water analysis this water originates in the Aegean (about 300 m depth). The form of the θ-S relation and the fact that the anomalous water was only observed at one station suggest that inflow of Aegean water occurs in the form of isolated lenses. The salinity distribution in 700 m depth, together with the anomaly at Sta. 756, indicates that water from the Aegean spreads out in the western Levantine and the Ionian between about 500 and 1200 m depth. We propose to denote this water mass Cretan Intermediate Water. Atlantic Water has CFM-12 concentrations in equilibrium with 1987 atmospheric concentrations everywhere in the Eastern Mediterranean, and oversaturations for both CFM-12 and oxygen are found in the seasonal thermocline above it. In contrast, Levantine Intermediate Water (LIW) is undersaturated for both oxygen and CFM-12, even in the presumed formation areas in the northeast Levantine.

Evolution and status of the Eastern Mediterranean Transient (1997–1999)

Abstract The Eastern Mediterranean Transient (EMT) was the major climatic event in the circulation and water mass properties of the Mediterranean in the last century. In this paper, we describe the present status of the EMT and its evolution since 1995 using hydrological and tracer data from 1997 to 1999. Few but important changes have been observed in the circulation pattern. The intrusion of the Asia Minor Current (AMC) that carries the saline Surface Levantine Waters into the Aegean has been reduced compared to the picture of late 1980s. This means that one of the mechanisms that increased the salt content of the Aegean during the peak of the EMT is no longer present. The Modified Atlantic Water (MAW) signal that has been weakened in the Levantine Basin during the early stages of the EMT has also been re-enhanced. The Aegean still functions as a source of deep (Cretan Deep Water, CDW) and intermediate waters (Cretan Intermediate Water, CIW) for the Eastern Mediterranean, although with modified characteristics. The most important changes in the thermohaline structure of the Cretan Sea (southern Aegean Sea) are the weakening of the signal of the old Mediterranean mid-depth waters and the modification of the properties of the CDW both leading to a reduced stratification. The outflowing CDW is no longer dense enough to reach the bottom of the adjacent basins, but ventilates layers between 1500 and 2500 m. Only the deep eastern Straits of the Cretan Arc are still active in the discharge of CDW, while at the western Strait (Antikithira), the density of the outflowing water was reduced significantly. The intermediate water CIW formed in the Aegean is characterized as a shallow CFC-12, temperature and salinity maximum layer, and differs much from the “old” CIW formed before the EMT, which was found in the layer below the Levantine Intermediate Water (LIW). The new CIW extends into the Ionian Basin through Antikithira Strait. It has lately been observed to enter the Adriatic, where its high salinity is expected to re-establish deep-water formation in this basin. The spreading of the CDW that had been deposited in the Cretan Passage in the first phase of the EMT has progressed further. The entire bottom layer of the Levantine Basin is now covered by the CDW. In the Ionian, the CDW has reached the Straits of Sicily and Otranto. Similar pathways in the Ionian are followed by the new shallower outflow of the CDW.

Short-circuiting of the overturning circulation in the Antarctic Circumpolar Current

The oceanic overturning circulation has a central role in the Earth’s climate system and in biogeochemical cycling, as it transports heat, carbon and nutrients around the globe and regulates their storage in the deep ocean. Mixing processes in the Antarctic Circumpolar Current are key to this circulation, because they control the rate at which water sinking at high latitudes returns to the surface in the Southern Ocean. Yet estimates of the rates of these processes and of the upwelling that they induce are poorly constrained by observations. Here we take advantage of a natural tracer-release experiment—an injection of mantle helium from hydrothermal vents into the Circumpolar Current near Drake Passage—to measure the rates of mixing and upwelling in the current’s intermediate layers over a sector that spans nearly one-tenth of its circumpolar path. Dispersion of the tracer reveals rapid upwelling along density surfaces and intense mixing across density surfaces, both occurring at rates that are an order of magnitude greater than rates implicit in models of the average Southern Ocean overturning. These findings support the view that deep-water pathways along and across density surfaces intensify and intertwine as the Antarctic Circumpolar Current flows over complex ocean-floor topography, giving rise to a short circuit of the overturning circulation in these regions.

Is the Adriatic returning to dominate the production of Eastern Mediterranean Deep Water

Since the Aegean took over the deep water production of the Eastern Mediterranean at the end of the 1980s, the proficiency of the Adriatic as a formation site has been under question. The salt supply in the intermediate water enabling the Adriatic to produce dense water was diminished because of a salinity decrease by upwelling mid-depth waters. Tracer data indeed indicate that the deep layer in the Adriatic has not been ventilated for most of the 1990s. The data presented also show that the dilution of the intermediate water reached a peak in 1995, after which more ventilated and saline waters were added. The recent increase of salt supply to the Adriatic by an extremely saline intermediate water mass supplied from the Aegean, establishes the preconditioning required to resume dense water production in the Adriatic.

Property distributions and transient-tracer ages in Levantine Intermediate Water in the Eastern Mediterranean

We present distributions of chlorofluorocarbons (CFCs) and ages derived from them, of carbontetrachloride, and of hydrographic properties, in Levantine Intermediate Water (LIW) in the Eastern Mediterranean. The data originate from surveys of F/S METEOR in 1987 and 1995, which bracket the profound changes that have occurred in the Eastern Mediterranean deep waters, due to bottom water formation from Aegean Sea overflow and related enhanced upwelling (Roether, W., Manca, B.B., Klein, B., Bregant, D., Georgopoulos, D., Beitzel, V., Kovacevich, V., Luchetta, A., 1996a. Recent changes in Eastern Mediterranean deep waters. Science, 271, pp. 333–335). As a framework for an interpretation, classical knowledge on LIW is summarized. A density horizon of σθ=29.05 is selected to characterize LIW, for which salinities and temperatures in 1995 were still similar to classical values. A principal result derived from the CFC-age distributions is that the enhanced upwelling of deep waters has been continuous up into the LIW layer. Newly formed LIW in both surveys is found to be distributed over an extended region which includes the Cretan Sea. The lowest CFC ages in LIW, amounting to several years, are found in this region. Smaller but significant apparent CFC ages are present in the mixed layer in a winter situation (1995). The CFC data are compatible with a formation of LIW by open-ocean convection. Outcropping of the isopycnal typical of young LIW was observed in the Aegean Sea in 1995, while to the east and southeast of the Rhodes Gyre no evidence of a major recent LIW formation was found. The CFC age distributions give an upper limit for the apparent travel time of LIW up to the Strait of Sicily of about 8 years. CCl4 is found to be chemically unstable in the Eastern Mediterranean (chemical lifetime in LIW <5 years), but this feature allows us to use this tracer as a low-life age marker. The present work can serve as a basis for future data evaluation by Mediterranean circulation models.

Oxygen consumption in the Eastern Mediterranean

Abstract Rates of oxygen consumption are determined by fitting simulated oxygen concentrations to observations from a medium-resolution survey of the Eastern Mediterranean in 1987 (METEOR cruise M5/6). The simulations are obtained with a previously described two-dimensional kinematic model of the sea, which is newly calibrated using concurrent hydrographic and tracer data, and with oxygen consumption as a function of depth being parameterized following previous work. The consumption rate is obtained as R(z)=22 (z/100) −2 +0.31 μmol/(kg yr) for z m , and R =0.53 μmol/(kg yr) for z ⩾1000 m . For the waters below about 700xa0m depth, the uncertainty in R is approximately ±35%. Significant error contributions arise from the oxygen concentration of the waters newly supplied to the deep waters, and from possible deviations from a steady state in circulation and in oxygen cycling. The upper ocean rates are rather more uncertain, but they are compatible with rates from the literature. The deduced deep-water oxygen consumption rate is considerably higher than the rates found in previous deep-ocean work. Such rather high rates, which possibly are related to the comparatively high temperatures of the deep waters, have repercussions in various contexts, e.g. in the assessment of environmental conditions in the past that led to the formation of sapropel layers. The updated circulation model yields a deep-water renewal rate for the Eastern Mediterranean only moderately different from a previous value. The rate actually replenishing the deep regime amounts to 5.1×10 5 xa0m 3 /s (±20%), of which 2.8×10 5 xa0m 3 /s (±30%) are recirculated deep water. Convective renewal of the deep regime (>1200xa0m depth) by the combined addition of surface and intermediate waters requires 150xa0years (±30%).