Red Sea Numerical Modelling

Principal Investigator :	Michael Siccha
External collaborators :	Prof. Hezi Gildor, Hebrew University of Jerusalem, Israel
	Dr. Eli Biton, National Institute of Oceanography, Haifa, Israel

Background

The Red Sea is, due to its unique geographic and topographic setting, of particular interest to oceanographers and paleoclimatologists. The combination of high evaporation rates and limited exchange of water with the open ocean causes an unparalleled amplification of the sea level signal in the oxygen isotopes in the sediment archives of the Red Sea [1]⁠, effectively allowing the reconstruction of sea level from carbonates in sediment samples. Regarding longer time scales, the limited exchange of the Red Sea with the open ocean led to extremely saline conditions with aplanktonic intervals during times of sea level lowstands in the last and penultimate glacial [2,3]⁠. On the other hand the Red Sea is also highly sensitive to alterations in atmospheric forcings during times of sea level high-stands in the interglacials [4,5,6].

Oceanography

The Red Sea is a desert-enclosed, narrow basin of ~2000 km length, with an average width of ~300 km. A deep trench of up to 2500 m depth runs along the axis of the otherwise shallow sea (average depth ~500 m). The only connection of the basin to the open ocean is through the Strait of Bab al Mandab in the south, that links the Red Sea with the Gulf of Aden and the Indian Ocean. The critical point for inter ocean exchange [7], Hanish Sill, located ~130 km northward of the conspicuous Perim Narrows in the Bab al Mandab Straits has a depth of only 137 m.

Annual cycle of Sea Suface Salinity in the Red Sea
Note the inflow of water along the Saudi Arabian Coast during the winter months und the circular patches resulting from the formation of eddies in the central Red Sea.

Precipitation over the Red Sea is with an average of 0.12 m/a very low and often neglected. It is however both spatially and seasonally concentrated; the southern part of the Red Sea below 18°N receives about 2/3 of the total annual precipitation during the months July and August alone, when the African tropical rain belt (ITCZ) reaches its northernmost position and its extensions penetrate into the Red Sea region. Freshwater run-off occurs through Wadis (ephemeral streams) draining seasonal rainfall from the slopes of the bordering mountain ranges and is only sparsely quantified.In relation to the Indian monsoon the winds over the Red Sea have two seasonal modes of asymmetric duration. During the Indian winter monsoon season, from October to April, NNW winds prevail over the entire Red Sea. Wind direction over the southern Red Sea is reversed during the Indian summer monsoon season, from May to September, when NE winds from the Indian ocean become entrained in the topography and blow from SE over the southern Red Sea, unto to a convergence zone at approximately 18°N.

Deep water formation in the northern Red Sea
Isosurfaces of potential density, illustrating the formation of deep water in the northern Red Sea and adjoining gulfs during the months January to March.

The circulation of the Red Sea is anti-estuarine and primarily driven by the evaporation and secondarily by the seasonal winds [8,9]⁠.The high evaporation rate of the Red Sea has been topic of many studies and most recent estimates put the net fresh water flux into the range of -2.06±0.22 m/a [10]⁠. The main part of the Red Sea deep water, residing below 300 m depth with a uniform temperature and salinity of 21.7 °C and 40.6 psu is replenished convectively during the winter months in the northernmost Red Sea with contributions of outflow waters from the adjoining Gulfs.

Exchange flow at the critical point of Hanish Sill has in reference to the Red Sea wind field two seasonal modes. For the largest part of the year the exchange is two-layered, with an influent surface layer and effluent bottom layer. The Indian summer monsoon winds force this two-layered flow into a three layered exchange flow during the months June/July to August/September. The layer of influent water is displaced to intermediate depths of ~100 to 50 m between a thin wind-driven layer of effluent water at the surface and an, in volume reduced, layer of effluent deep water at the bottom [7]⁠.

Publications

We just published a study about marine isotopic stage 5e. The altered conditions of forcing parameters on the Red Sea circulation system during this time are in some cases opposing each other, making the resultant effect difficult to predict. The conditions we modelled for MIS 5e were unexpected but agree nicely with the sparse information obtained from proxies in the sediment record.

Outlook

We plan to further improve the model in various aspects. The most straightforward improvement will be an increase in resolution. We will also incorporate realistic run-off into the model, which will hopefully allow us to reduce the amount of modifications neccessary to archive a three layer exchange flow in the current model configuration. Furthermore we will include submodels for oxygen isotopes and productivity.

References

[1] M. Siddall, D. A. Smeed, C. Hemleben, E. J. Rohling, I. Schmelzer, and W. R. Peltier, “Understanding the Red Sea response to sea level,” Earth and Planetary Science Letters, vol. 225, no. 3–4, pp. 421–434, 2004.
[2] M. Fenton, S. Geiselhart, E. J. Rohling, and C. Hemleben, “Aplanktonic zones in the Red Sea,” Marine Micropaleontology, vol. 40, no. 3, pp. 277–294, 2000.
[3] E. J. Rohling, “Review and new aspects concerning the formation of eastern Mediterranean sapropels ,” Marine Geology, vol. 122, no. 1–2, pp. 1–28, 1994.
[4] E. Biton, H. Gildor, G. Trommer, M. Siccha, M. Kucera, M. T. J. van der Meer, and S. Schouten, “Sensitivity of Red Sea circulation to monsoonal variability during the Holocene: An integrated data and modeling study,” Paleoceanography, vol. 25, no. 4, Nov. 2010.
[5] G. Trommer, M. Siccha, E. J. Rohling, K. Grant, M. T. J. van der Meer, S. Schouten, U. Baranowski, and M. Kucera, “Sensitivity of Red Sea circulation to sea level and insolation forcing during the last interglacial,” Climate of the Past, vol. 7, no. 3, pp. 941–955, 2011.
[6] G. Trommer, M. Siccha, E. J. Rohling, K. Grant, M. T. J. van der Meer, S. Schouten, C. Hemleben, and M. Kucera, “Millennial-scale variability in Red Sea circulation in response to Holocene insolation forcing,” Paleoceanography, vol. 25, 2010.
[7] D. A. Smeed, “Exchange through the Bab el Mandab,” Deep-Sea Research Part II-Topical Studies in Oceanography, vol. 51, no. 4–5, pp. 455–474, 2004.
[8] G. Eshel, M. A. Cane, and M. B. Blumenthal, “Modes of subsurface, intermediate, and deep water renewal in the Red Sea,” Journal of Geophysical Research, vol. 99, no. C8, pp. 15941–15952, 1994.
[9] E. Tragou and C. Garrett, “The shallow thermohaline circulation of the Red Sea,” Deep-Sea Research Part I - Oceanographic Research Papers, vol. 44, no. 8, pp. 1355–1376, 1997.
[10] S. S. Sofianos, W. E. Johns, and S. P. Murray, “Heat and freshwater budgets in the Red Sea from direct observations at Bab el Mandeb,” Deep-Sea Research Part II-Topical Studies in Oceanography, vol. 49, no. 7–8, pp. 1323–1340, 2002.