On the sources, formation, and circulation of Red Sea deep water

Abstract

Red Sea Deep Water is renewed from sources in the far north of the Red Sea. Upwelling of deep water throughout the basin maintains the sharp pycnocline with its base near the depth of the sill at Bab el Mandeb. Before now, deep water renewal was thought to occur mainly by means of convective deep water formation in the north which renews the bottom water and gives rise to a southward‐flowing bottom current. Carbon‐14 and 3He data from the GEOSECS Indian Ocean Expedition lead to the identification of an additional, volumetrically larger, mode of northern deep water formation in which new deep water is injected immediately beneath the pycnocline. This mode of deep water formation (to be called the isopycnal mode) also gives rise to a southward‐flowing current, in this case at the top of the deep water mass. GEOSECS data also demonstrate the existence at intermediate depths in the deep water of a return flow to the north. The surface history in the nuclear era of the Δ14C in the deep water source region was reconstructed by direct radiocarbon measurements on annual growth bands of corals. Calculation then shows that new deep water forms in the convective mode at a rate of 0.05±0.01 Sverdrup (Sv) (expressed as if the flow were continuous) and is due principally to processes occurring in the mouth of the Gulf of Suez. The injection beneath the pycnocline of new deep water formed in the isopycnal mode is 0.11±0.02 Sv. This new deep water appears to originate mainly in the extreme open northern Red Sea. The Gulf of Aqaba makes a modest but significant contribution to both modes. The total deep water renewal rate is about 0.16 Sv, yielding a bulk deep water residence time of about 36 years. Oxygen consumption in the deep water shows approximately exponential decrease of consumption rate with depth. As expected on geological and geophysical grounds, hydrographically significant fluxes of heat and salt from the sea floor into the Red Sea deep water are discernible in the hydrography. Helium‐3 flux from the sea floor into the deep water is about 69 1 STP/year.