Birgit Klein

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.

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.

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.

Interbasin deep water exchange in the western Mediterranean

Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean, The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins [1988]. There, an outflow of water from the Tyrrhenian Sea into the Algero Provencal Basin was postulated in the depth range 700-1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero-Provencal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero-Provencal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600-1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv=10(6) m(3) s(-1)), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero-Provencal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero-Provencal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior

Identification of diapycnal mixing through optimum multiparameter analysis: 2. Evidence for unidirectional diapycnal mixing in the front between North and South Atlantic Central Water

Optimum multiparameter (OMP) analysis is used to analyze mixing in the central water boundary of the tropical North Atlantic Ocean. Diapycnal mixing is found to be prevalent in the frontal region. OMP analysis shows that the mixing is unidirectional (South Atlantic Central Water is always mixed upward into North Atlantic Central Water) but cannot identify the process responsible for the observed diapycnal mixing. A histogram of stability ratios Rρ for all mixing lines shows Rρ values between unity and the value found in the parent water masses. It is suggested that this may indicate competition between isopycnal mixing and double diffusion. Double diffusive fluxes are likely to make a recognizable and significant contribution to diapycnal mixing between the Central Waters.

Apparent loss of CFC‐113 in the upper ocean

We report and evaluate data of the chlorofluorocarbon CFC-113, in comparison with concurrent data for CFC-12, from cruises into the temperate North Atlantic, the tropical western Pacific, the Eastern Mediterranean, and the Weddell Sea. We consistently find CFC-113 deficiencies in the warmer upper waters, which we interpret as CFC-113 depletion at rates of the order of 3% per year, with possibly accelerated rates in the mixed layer or near the surface. These results severely limit a tracer utility of CFC-113 in the warm upper waters of the ocean. Tracer applications in the deep waters of the ocean should be much less affected, since CFC-113 loss in such waters appears to be far slower. This also holds for the Eastern Mediterranean, despite its rather warm deep waters. For the Weddell Sea, the data indicate a CFC-113 deficiency (∼20%) in the shelf waters, which converts into a similar deficiency in newly formed deep and bottom water. Such CFC-113 deficiency has to be accounted for in Southern Ocean tracer work. Future work is proposed to study CFC-113 loss in the ocean in additional detail and to elucidate the loss mechanism(s), on which our data give little clue.

Can the weak surface currents of the Cape Verde frontal zone be measured with altimetry

Three data types are compared in the low-current-velocity regime in the southeastern North Atlantic, between 12-degrees-N and 30-degrees-N, 29-degrees-W and 18-degrees-W: Geosat altimetric sea level and derived surface geostrophic velocities, shallow current meter velocities, and dynamic heights derived from hydrographic data from cruises 4, 6, and 9 of the research vessel Meteor. The four current meter daily time series, at depths around 200 m, were smoothed over 1 month; the altimetric geostrophic velocities were computed from sea surface slopes over 142 km every 17 days. The correlation coefficients between the current meter and altimetric geostrophic velocities range between 0.64 and 0.90 for the moorings near 29-degrees-N but between 0.32 and 0.71 for the two around 21-degrees-N; the associated rms discrepancies between the two measurement types range between 1.5 and 4.4 cm/s, which is 49% to 127% of the rms of the respective current meter time series. Dynamic heights relative to 1950 dbar for the months of November 1986 (d(M4)), November 1987 (d(M6)), and February 1989 (d(M9)) were computed from Meteor cruises 4, 6, and 9. Both dynamic heights and altimetric heights (h(M4), h(M6), h(M9)) were averaged over 1-degrees boxes for the duration of each cruise. Differences d(M4) - d(M6) and d(M9) - d(M6) were computed only at bins where at least one station from both cruises existed, Assuming that dynamic heights d in dynamic centimeters are equivalent to sea level h in centimeters, the standard deviation sigma of the differences ((h(M4) - h(M6)) - (d(M4) - d(M6))) and corresponding M9 - M6 values was 2.1 cm. This value (squared) is only 13% of the (5.8 cm)2 variance of the dynamic height differences and is indistinguishable from the 2.7- to 5.6-cm natural variability of sea level in the area expected between the times when the ship and the satellite sampled the ocean. The areally averaged discrepancy for M9 - M6 was only 0.7 cm, but the corresponding value for M4 - M6 was 5.2 cm. A systematic difference between the water vapor corrections used before and after July 1987 is responsible for the M4 - M6 difference. The average M4 - M6 discrepancy is only 0.1 cm using the Fleet Numerical Oceanography Center correction, with a standard deviation of 3.1 cm. In spite of the underlying differences in sampling and physics, including unknown barotropic components not included in our hydrographic dynamic heights, and in data errors, including water vapor, ionospheric, and orbital effects on the altimetry, consistent interannual changes of the mean sea level from the independently obtained altimetric and hydrographic data sets are obtained, and correlated seasonal changes in surface currents are observed with both altimetry and current meters.