On the mean flow in the Tiran Strait in winter

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Abstract
Direct observations with 12 moored current meters of the velocity field in the Tiran Strait confirmed the presence of a well-developed 2-layered gravitational convection circulation between the Red Sea and the Gulf of Aqaba. Measurements indicate a level of no motion at 100 m in the deep (270 m) Enterprise Passage; the shallower Grafton Passage (80 m) is completely in the upper layer. Characteristic speeds are 30 cm/s and 60 cm/s in the upper and lower layers, respectively. Integration over the channel area indicates an average discharge in each layer of .apprx. 29,000 m3/s for Feb. 1982. Evidence is presented that the volume balance is quasi-steady over a 2-3 day time scale. Using a monthly evaporation rate, volume and salt conservation equations predict a discharge within 10-15% of observed values. The Enterprise Passage shows clear evidence of reaching the critical flow state of Stommel and Farmer (1952), and Bryden and Stommel (1983). Periodic episodes of critical flow are controlled by the spring-neap tidal current cycle. During neap tides, when turbulent mixing is at a minimum, stratification strengthens and gravitational convection spins up. The strong northerly winds (10-15 m/s) exert surprisingly little effect on the mean flow. It is anticipated that higher summer evaporation rates will increase the duration and frequency of critical flow.

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