Studying physical process in the modern Red Sea has enabled us to
interpret a recod of climate change going back 500,000 years
The Red Sea is a long narrow basin extending over 2000km from Perim at 12.5°N to Suez at 30°N. Like the Mediterranean Sea, the Red Sea is almost enclosed having just one connection to the global ocean.
The Bab el Mandab connects the Red Sea to the global ocean via the Gulf of Aden. The narrowest section is at Perim Island where the width is about 20km. But the shallowest sill over which the deep water must pass is located west of Hannish Island 150 km north-north west of Perim. The sill has a depth of 137m and is located in a triangular shaped channel about 12km wide flanked by broad shelves with depths of the order of 50m.
Strong evaporation, of about 2 m per year, over the Red Sea drives an exchange flow in the strait where relatively fresh water enters the basin in the upper layers, above an outflow of dense salty water.
A record of global sea-level change
At the time of the last glacial maximum sea-level was 120m lower than it is today and the Hannish Sill was just 20m deep. Consequently the exchange between the Red Sea and the global ocean is very sensitive to changes in sea-level during the glacial cycle. A recent study at the SOC (Siddall et al 2003, see also Sirocko 2003) has exploited this sensitivity to derive an estimate of global sea-level during the last 500,000 years from the analysis of sediment cores taken in the Red Sea (Figure 2). A model of the exchange through the strait was used to determine the concentration of oxygen isotopes in the upper layers of the Red Sea as a function of sea-level.
Modelling the exchange flow
Despite many similarities between the Bab el Mandab and the Strait of Gibraltar, there are some striking differences. Most notable of these is the seasonal variability of the flow in the Bab el Mandab, a consequence of the prevailing monsoon climate. During the winter season (October to May) the flow can be characterised as a two-layer exchange: warmer, fresher water flows into the Red Sea in the surface layer and cooler, saltier water flows into the Gulf of Aden in the lower layer (Figure 3a). A similar two-layer exchange is observed at Gibraltar. Whilst there is some seasonal variation in the outflow from the Mediterranean, the basic pattern of a two-layer exchange persists throughout the year. The summer regime in the Bab el Mandab is very different: the high salinity outflow is almost arrested, the flow in the surface layer is reversed, and intermediate water from the Gulf of Aden flows in to the Red Sea between the two out-flowing layers (Figure 3b). Smeed (2000) developed a three-layer model of hydraulically controlled exchange flows that demonstrated how the seasonal variability could be explained by the monsoon driven upwelling of cold water in the Gulf of Aden. The model was refined by Siddall et al (2002) who showed that the model agreed quantitatively with observations of the exchange fluxes made by Murray & Johns (1997). This model was used by Siddall et al (2003) to determine the conditions in the Red Sea during the last glacial cycle.
Future work
In the next stage of this work we propose to use a numerical Ocean General Circulation Model (OGCM) to model the circulation of the Red-Sea at times of reduced sea-level. The model will be validated by comparison with observations from the modern Red Sea. In particular we will examine how well the model represents a number of key physical processes such as the exchange flow through the strait and the transport of buoyancy within the basin. The model will then be used to simulate the circulation during times of reduced sea-level, and the results used to interpret data from Red Sea sediment cores. A primary goal will be to develop an accurate, high resolution record of global sea-level during the last 70,000 years.
References
Murray, S. P. & W. Johns (1997). Direct observations of seasonal
exchange through the Bab el Mandab Strait. Geophysical Research
Letters, 24(21), 2557-2560.
Siddall, M., D.A. Smeed, S. Matthiesen, & E.J. Rohling, (2002).
Modelling the seasonal cycle of the exchange flow in Bab El Mandab
(Red Sea). Deep-Sea Research Part I-Oceanographic Research Papers,
49(9), 1551-1569.
Siddall M., E.J. Rohling, A. Almogi-Labin, Ch. Hemleben, D. Meischner,
I. Schmelzer, & D.A. Smeed (2003). Sea-level fluctuations during
the last glacial cycle. Nature, 423, 583-588.
Sirocko, F. (2003). Global change: Ups and downs in the Red Sea.
Nature, 423, 813-814.
Smeed, D. A. (2000). Hydraulic control of three-layer exchange flows:
Application to the Bah al Mandab. Journal of Physical Oceanography,
30(10), 2574-2588.