Cross Flow

 Last spring, a small research boat made an unusual trip – from one side to the other of the northwestern-most tip of the Red Sea. On this crossing (of the Gulf of Eilat or the Gulf of Aqaba, depending on one's map) were researchers from Jordan, Israel and the U.S. who had recently joined forces to study how water flows and mixes in the unique body of water that lies between Jordan and Israel.

Dr. Hezi Gildor of the Weizmann Institute's Environmental Sciences and Energy Research Department; Dr. Riyad Manasrah of the Marine Science Station in Aqaba, Jordan; Prof. Amatzia Genin of the Interuniversity Institute for Marine Sciences and the Hebrew University of Jerusalem, Eilat; and Dr. Stephen Monismith of Stanford University are conducting this research through the NATO Science for Peace and Security Program. Their efforts should greatly improve scientists' understanding of water currents and circulation. But the group has an immediate, practical goal as well: A detailed understanding of water movement in the Gulf can help the environmental agencies on both sides (which already cooperate to protect its unique ecology) plan a response to spills or prevent pollution from spreading.

Recent research by Gildor and Dr. Erick Fredj of the Jerusalem College of Technology has already revealed a surprise: Floating material such as oil might remain near the spill site for an extended period of time, rather than dispersing throughout the surface area of water. Using data collected from two on-shore high-frequency radar stations, Gildor created a computer map of the currents. He then added evenly spaced "particles" to a computer water-flow simulation to see where they would go. The calculation, which showed the particles moving with the currents over several days, revealed that some of the particles tended to move closer together, forming large clumps; at the same time, barriers created by the current separated particle clusters and prevented them from dispersing or mixing with other clusters. Large bodies of water don't normally lend themselves to experiments, but a set of aerial photos taken soon after a rare winter flood provided evidence for the accuracy of the model: The images show well-defined brown stains in the blue water – silt that had washed down from the nearby desert mountains into the Gulf and collected in the areas predicted in the model. In addition to the two radar stations on the Israeli side, a third is now set to go online on the Jordanian side, which will greatly increase the data available to the scientists.

"The Gulf of Eilat," says Gildor, "offers scientists an exceptional research opportunity. Although it is relatively small, it is also quite deep, and many types of ocean phenomena take place in its waters. Because of its limited size and the fact that it's almost entirely surrounded by land, detailed measurements can be obtained at a higher resolution than is possible in the open ocean. Also, there's the added advantage of being close to shore."

One such phenomenon is usually found only in places that are much harder to study, such as the waters off the Antarctic coast. Called a density current, it takes place when cold air from a nearby land mass cools the top layer of ocean, making it denser and heavier than the water below. This layer then sinks, creating a vertical current. Although density currents are confined to narrow belts of sea near land, they are important drivers of the global ocean currents that, in turn, affect global climate patterns. The Gulf of Eilat, although it is much closer to the equator than other areas that experience this phenomenon, has all the right conditions for density currents: On the one hand, the shallow strait at the entrance to the Red Sea prevents the deep, cold water of the outer ocean from flowing into the Gulf. On the other hand, the Gulf water is surrounded by desert, where atmospheric temperatures can drop to near freezing on winter nights, thus cooling the surface water. Gildor and his research team found that pulses of density current regularly occur off Eilat's shore in wintertime, and they used their observations to create a high-resolution computer model of these flows.

The Gulf is an invaluable natural laboratory – one that Gildor is turning into an important basis for improving ocean modeling – and collaboration between scientists on both sides is crucial to conducting research in its waters. Gildor: "There's no physical line down the middle of the Gulf, and its water doesn't recognize political borders. To really understand it, we need to be able to study this body of water as a whole."   
 

Dr. Hezi Gildor's research is supported by the Sussman Family Center for the Study of Environmental Sciences; and the estate of Sanford Kaplan. Dr. Gildor is the incumbent of the Rowland Schaefer Career Development Chair in Perpetuity.