One day hydrographic cruises to deepest part of the Dead Sea
    The main aim of these cruises is to monitor seasonal and interannual changes of the thermohaline structure of the Dead Sea. Our intention is to carry out at least four cruises per year. Sometimes we were not able to stay with this intention and interval between cruises was more than three month. During each cruise we obtain vertical distribution of water temperature and quasi salinity measured on several levels. Each profile includes about 16-20 levels. Unfortunately we have only 4 Niskin bottles dedicated to the Dead Sea hydrography. Therefore data for one profile is obtained by several casts. Water temperature is measured by CTD SBE-19 and is controlled by reversed digital thermometers (SIS ). Water from each Niskin bottle is sampled in hermetic bottles. We store water samples in thermostat with temperature about 32°C to avoid salt crystal precipitation. Water density is measured in IOLR laboratory by “Anton Paar” densimeter DMA 5000 . Below you can select charts with vertical temperature and quasi-salinity profiles from our database.
IOLR Dead Sea Hydrographic Database:     

 

The Dead Sea hydrography from 1992 to 1999

by

I. Gertman, D. A. Anati, A. Hecht, J. Bishop, Y. Tsehtik.

  Introduction.

The systematic monitoring of the Dead Sea hydrography began two decades ago, in 1977, and its findings and main results and conclusions were made public throughout the years by several articles and review papers, which appeared in scientific literature (Anati, 1997). These cover, fairly reliably, the years 1977-1991 but are less detailed concerning the post-1991 period. Thus, it became necessary to update the users of the Dead Sea hydrographic information, particularly those microbiologists, geologists, geochemists and climatologists whose research is closely connected with thermohaline structure of the lake, on the findings of these later years. The present paper is dedicated to them.

 Basic hydrological features.

        The Dead Sea surface level has been dropping since 1977 at an average rate of about 60 cm per year. In the time sequence of this surface level trend, there are two conspicuous events in the opposite sense: the surface level rises of winter 1980 and winter 1992, both following large amounts of fresh-water flows originating from flash floods and from  Jordan river runoff. Apart from these two special events, the Dead Sea surface level has been dropping at a rapid rate, namely 80 cm per year, with a variability of less then 2%. The level changes between 1992 and 2000 are shown in Fig. 1. The large surface level rise of 1992, about two meters, brought about a meromictic period which persisted for four years, the same duration as the previous meromictic period which lasted from 1980 to 1984. The onset of the meromictic period is marked in Fig. 1 in mid-January 1992, that is, while the surface level was still on the descent (Fig. 1). This may seem contradictory, but, as explained below, entirely conforms with the evolution of the thermohaline structure of the Dead Sea.

        The terms “meromictic” and holomictic” were coined by Hutchinson (1957). He defined a meromictic lake as one in which part of the deep sea water is stabilized, as opposed to a holomictic lake, i.e. one which can go freely through circulation periods. The Dead Sea has been a meromictic lake for several centuries, until the historic overturn of December 1979 (Steinhorn and Gad, 1983), which started a new and short holomictic period. With that event, the Dead Sea entered a new phase in which it switched from one regime to the other. A more precise definition of these two regimes, meromictic and holomictic, became necessary and was reformulated as follows.

With reference to a lake, a period is meromictic if it is longer then one seasonal cycle; it begins with the onset of stable stratification and terminates with the first overturn. A period is holomictic if it is non-meromictic, and occurs between two meromictic periods.

By this definition a holomictic period is assumed finite. It always begins with an overturn and always terminates with the onset of stable stratification. As an illustration, consider the typical spring-summer stratification of the Dead Sea. If it comes after a winter overturn (that is, if the winter column has been totally mixed during the previous winter) it cannot be classified either as meromictic or holomictic until a whole seasonal cycle has elapsed. On the other hand, a spring-summer stratification that comes after a meromictic winter is always meromictic. Also, by the above new definition, the 1979-80 holomictic period turns out to be considerably shorter (approximately two months) than it was previously (Anati et all, 1987) considered to be.

 The total stability of a lake is the energy required (by winds, currents shear, waves, etc.) to mix the whole water column.

                              ,

where A(z) is horizontal area at depth z, ρ is the water density and h is the maximal depth of the lake. Its changes in the Dead Sea since 1992 well illustrate  (Fig. 1) the main difference between a meromictic period, during which stability never vanished, and a holomictic period, during which, at least once a year, the whole water column was entirely mixed and its stability reduces to zero.

Concerning the onset of the meromictic period marked in Fig. 1: in March 1992 the Dead Sea was found to be stably stratified, as one can see from the total stability changes. Therefore, by the above definition, the meromictic regime had already been operative for some time. Its onset is estimated to have occurred around mid-January 1992.

Analysis of seasonal and long-term variability of the Dead Sea hydrography in the report is based on two long-term databases. The first one consist of deep water observations at the deepest point of the Dead Sea, 15 km off En Gedi in a North-East direction (EG320). The measurements were conducted by the Hebrew University in Jerusalem (Anati, 1997) and, since 1988 by the Israel National Institute of Oceanography, at relatively equal time intervals of about two months. The second database consists of upper layer water temperature observations conducted from the Dead Sea meteorological platform of the Israel National Institute of Oceanography since 1992, 4 km off En Gedi in a South-East direction (EG100) (Hecht et all, 1992). The temperature data were measured with a thermistor chain, recording at a 20-min. time interval. 

The upper layer. 

The sharp increase of the total stability of the Dead Sea in 1992-1993 was defined by a large amount of incoming fresh water during an extremely rainy winter, 1991-1992  (Beyth et all, 1993). The thickness of the newly formed upper layer with relatively low salinity was not more than 20 m (see Fig. 4 from Beyth at all, 1993), but its presence completely changed the hydrological regime of the Dead Sea for the following three years.

It should be noted that for salinity description we traditionally (Anati, 1999) use a density anomaly from 1000 kg/m³ for a water temperature of 25 °C. In this paper we will use the term “quasi-salinity”, taking into account that changes in seawater density attached to a constant temperature are identical to changes in seawater salinity.

In summer, 1991 the quasi-salinity of the Dead Sea had a maximum in the surface layer of about 1236 kg/m³. During the following winter the surface layer quasi-salinity dropped to 1164 kg/m³ (Beyth at al., 1993). Setting up an upper layer with salinity lower than water beneath this layer increased effects of buoyancy in the upper layer and erected an obstacle to processes of vertical turbulent mixing. During summer, 1992 the depth of the seasonal thermocline was about 10 m (Fig. 2). After winter of 1991-1992, evaporation exceeded precipitation and runoff. Therefore, the positive vertical gradient of salinity in the upper layer was worn out. The depth of the summer thermocline increased during the first three years: in summer, 1993 it was 12-15 m, in summer, 1994 it was 18-20 m and in summer, 1995 it reached the relatively stable present position of about 25-30 m.

During the winter of 1992-1993 the upper layer water temperature dropped to 16-17 ?C. This was colder than water beneath the upper layer by about 5-6 ?C. Nevertheless, development of the density convection stopped during the winter at a depth of about 15-20 m, due to a positive gradient of salinity in the upper layer. Overcooling of the upper layer, in comparison with deep water, was also observed in winter, 1993-1994 and in winter, 1994-1995. This appearance is typical for meromictic seas (for example the Black Sea) because winter cooling influences only on a relative small volume of the upper layer located above a positive halocline.

The monthly averaged February temperature of the Dead Sea surface layer had a strong positive trend from 16.5?C to 22.9?C for the period 1992-1999 (Fig. 2). For the same period, the monthly averaged August temperature of the surface layer fluctuated near a mean value of 33.7?C with a standard deviation 0.5?C. The winter water temperature positive trend is defined solely by the long term wearing out of the positive halocline and by the resulting increase in the volume of sea water masses mixed by winter convection. This volume increased by 9-10 times from, about 8 km³ (winter, 1992-1993) to a magnitude of the entire Dead Sea volume, about 700 km³ (since winter, 1995-1996). The volume of the summer upper mixed layer also increased during the same period, but by not more than in 2-3 times, from about 6 km³ (summer, 1992) to about 16 km³ (summer, 1996). Therefore, the August temperature of the meromictic upper layer evidenced no significant long-term changes.

During the same period, the salinity of the meromictic upper layer increased dramatically and, in summer 1995, it overshot the salinity of the deep water. And in autumn, 1995 density convection mixed the sea volume entirely. Thus, in November 1995 the latest meromictic period of the Dead Sea was terminated.

Since the onset of the new holomictic period long-term tendencies in the upper layer parameters became less evident. Every holomictic winter the processes of density convection completely destroy stratification. Increase of summer salt storage, attributed to fresh water budget deficit, is not now limited mainly to the upper layer volume, but rather is distributed throughout the entire Dead Sea volume.

Seasonal changes of the holomictic upper layer will be analyzed below by using data collected in 1998 and 1999, anomalously hot and arid years. Fig. 3 shows an evolution of water temperature and quasi-salinity for 1 m depth level and for 30 m depth level. During summer, the seasonal thermocline is constantly above the 30 m level. During autumn, the thermocline deepens below this level. Intensification of thermal fluctuation during this period shows the intermittent character of the mixing processes.

A stable thermocline is usually formed during the last week of March. Afterwards, all salt increase, caused by evaporation, is concentrated above the thermocline. A maximum of quasi-salinity (238-239 kg/m³) is reached at the same time that the maximum upper layer temperature (34-35°C) is reached. The thickness of the upper layer during July-August is about 25-28 m (Fig. 4). In the second half of August the upper layer temperature diminishes. (Fig. 5 shows details of daily changes in the upper layer stratification for 10-20 August 1998.) During sunlight hours there is a strong signal of heating caused by insolation. For the nighttime period, even a slight cooling is sufficient to mix the upper layer due to the strong destabilizing halocline.

The Dead Sea upper layer cooling process in the holomictic period has two phases. The first one with a rate of about 0.1°C per day and the second one with a rate of 0.01°C per day. During the first period only the upper layer, above the seasonal thermocline, is affected by cooling. When the thermocline deepens past 30-meters of depth, about 30 days are necessary to entirely mix the Dead Sea volume. After this the surface temperature cooling rate lessens to 1/10th of what it was. Now, negative heat flux via the sea surface leads to intensive convection penetrating to the bottom and serves to cool the entire Dead Sea volume. The change of the phases is related to the date of the first overturn of the Dead Sea. The date may be easily defined from the curve of changes of the Dead Sea surface temperature with time (Fig. 3). So the first overturn in 1998 took place approximately in mid-December and, in 1999, it occurred about two weeks earlier at the start of December.

The deep water.

The different signatures of meromictic and holomictic regimes will be demonstrated through  changes in the thermohaline properties of the deep water. Fig. 6 shows the evolution of the temperature and quasi-salinity in the deep water mass (depth more than 100 m) since 1992.

During the meromictic period, 1992-1995, the deep water shows no seasonality, the temperature remains almost constant. This is not surprising since the deep layer was then sealed from atmospheric or other influences.  In comparison, during the previous meromictic period of 1980-1984 (Anati et all, 1987) the deep water’s temperature remained within a range of 23.15±0.03°C.

During the holomictic periods, on the other hand, there is seasonality of about 0.5°C (1°C peak to peak, Fig. 6), but the trend is opposite to that of similar upper layers of lakes in the northern hemisphere: the lower temperatures occur in summer - then, the deep water warming up begins due to a vertical turbulent mixing with the upper (warmer) layer, which ends abruptly with the event of yearly overturn around December, thus attaining the highest temperatures – at the beginning of the coldest season.

Throughout the meromictic period, the deep layer of the Dead Sea was sealed and thus, could not have been diluted with fresh water. Nevertheless, a seemingly exponential decrease of salinity is observed (Fig. 6). This salinity decrease of sealed Dead Sea waters is a known phenomenon (Gavrieli, 1997), which has been attributed to halite precipitation from supersaturated brines; the amounts of halite losses have been compared to independent estimates of salt deposition and found to fit in the scheme. Just before the big 1992 floods the Dead Sea brine was indeed supersaturated and massive salt crystal formations were documented by several means (Gavrieli, 1997). After the winter 1992 floods and the onset of the 1992-1995 meromictic period, the same mechanism was again operative; the deep sealed brine began approaching its steady-state saturation line “from above”, thus decreasing in salt concentration. This salt relaxation process, consisting mainly of salt crystals sinking, is much slower than that found in laboratory experiments on salt crystal formation (Anati, 1993).

Also the post-1995 holomictic period deep water was also sealed, but only seasonally. For the first three years 1996-1998 of the holomictic regime,  the trend for the first half of each calendar year is one of decreasing salt concentration (Fig. 6) due to winter convective mixing with the incoming upper layer fresh water. However, as soon as summer double-diffusive mixing with the upper (saltier) layer begins, the rate of salinity increase in the deep water is strong enough to overshoot the continuing halite loss, and the overall yearly trend is, therefore, one of increasing salinity. In 1999, after a hot and droughty summer, there was no deep water salinity decrease during the also dry winter period.  

The seasonal quasi-salinity changes at the beginning of the holomictic period were approximately 0.25 kg/m³ (0.5 kg/m³ peak to peak). It should be noted that amplitudes of seasonal fluctuation of parameters of the deep water mass could not be estimated with sufficient accuracy because our measurements are carried out irregularly and sometimes over large intervals of time. For example, we did not conducted observations during autumn, 1999 due to lack of an available vessel. There is no doubt that, in the beginning of December 1999, the Dead Sea deep water mass had maxima both in temperature and salinity.

References.

 Anati, D. A., 1999, The salinity of hypersaline brines: Concepts and misconceptions: International Journal of Salt Lake Research, v. 8, p. 55-70.

 Anati, D. A., 1997, The hydrography of a hypersaline lake: In book “The Dead Sea: the lake and its setting”, edited by Tina M. Niemi, Zvi Ben-Avraham, Joel R. Gat. Oxford University Press, Oxford, p. 89-103.

 Anati, D. A., Stiller, M., Shasha, S. and Gat, J. R., 1987, Changes in the thermo-haline structure of the Dead Sea: 1979-1984: Earth and Planetary Science Letters, v. 57, p. 2,191-2,196.

 Anati. D., A., 1993, How much salt precipitates from the brines of a hypersaline lake? The Dead Sea as a case study: Geochimica et Cosmochimica Acta, v. 57, p. 2191-2196.

 Beyth, M., Gavrieli, I., Anati, D. and Katz, O., 1993. Effects of the December 1991-May 1992 floods on the Dead Sea vertical structure: Israel Journal of Earth Science, v. 41, p. 45-48.

 Gavrieli, I., 1997, Halite deposition from the Dead Sea: 1960-1993: In book “The Dead Sea: the lake and its setting”, edited by Tina M. Niemi, Zvi Ben-Avraham, Joel R. Gat. Oxford University Press, Oxford, p. 161-170.

Hecht, A., Bishop, J. and Brokman, G., 1992, The investigation of air-sea interaction processes on the Dead Sea: IOLR report No. H14/92, 24 pp.

 Hutchinson, G.E., 1957. A treatise on limnology. Willey, New York, 1015 pp.

 Stenhorn, I., and Gat, J. R., 1983, The Dead Sea: Scientific American, v. 249, p. 102-109.