Science Frontiers

Meddies � A New Sound in Oceanography

In the Atlantic Ocean, researchers have discovered a phenomenon that they have named “Meddies.” meddies are basically discs of water that originate from the Mediterranean and develop a rotation as they move into the Atlantic Ocean. The discs grow to immense sizes and create portions of the Atlantic Ocean with varying salinities and temperatures. The increased salinity of the discs is a result of the discs being from the Mediterranean, where surface water evaporates quickly to leave extremely salty water. Once the meddies hit the Atlantic Ocean, their movement speed decreases and they begin to mix with the surrounding ocean water. Due to the enormous size and temperature differences of the meddies, researchers have predicted a change in the ocean's heat exchange and the earth's climate.

Although meddies were discovered nearly three decades ago, researchers were unable to conduct efficient research due to cost and reliability issues. But now, researchers have found that an acoustic tool used by the oil industry can be used to take “rapid, high-resolution snapshots of the meddies.” Because of the feasibility of this new tool, the window of opportunity to research meddies has increased. The tool analyzes the density of the water using a towed array of microphones which receive sounds waves that originate from air guns then bounce off the seafloor. The differences in time between sounds waves allows researchers to determine the density of the water through which the sound waves traveled.

The time differences had been noted by the oil industry as they searched for oil deposits, but the data had been discarded as noise. The use of the tool in oceanography was first noted in 2003 when a team led by W. Steven Holbrook of the University of Wyoming discovered distinct areas of density boundaries from the acoustic data. The scientific community accepts the use of changes in density as valid indicators of changes in temperature and salinity. The acoustic data allows researchers to model the interactions between ocean fronts using the unique data for each ocean current.

Ever since researchers have recognized the use of the acoustic data in oceanography, they have taken old data from the oil industry and created experiments. One team led by Valenti Sallarès studied data from a 1993 survey near Spain and reported in the June 14 Geophysical Research Letters that it had imaged “three meddies in unprecedented detail.” From the data, researchers could see intricate details of mixing that revealed characteristics of the water. Oceanographers have had a positive response to the findings with Raymond Schmitt saying “At first blush, it's just exciting for people to be able to see these things.” Despite the progress in data being collected, oceanographers are still unsure about how to interpret the data of meddies and other ocean-mix areas.

Although researchers have been able to collect the data necessary to deem the tool useful, the oceanography community has yet to put the data into wide use. A large reason for the lack of application is because a “reliable conversion” between seismic and traditional oceanographic measurements has not yet been made. Seismic data reports the places where the speed of sound changes whereas oceanographic data contains a direct measurement of the water conditions. Sallarès hopes that the quantification of the seismic data from the mixing processes will allow a unification of the two sets of data.

The introduction of this new tool has produced a new realm of research deemed “seismic oceanography.” Holbrook has led his own research surveys into the topics and said that the discipline needs to “'produce exciting and useful quantitative results' so that oceanographers can view it as 'a critical enhancement of their toolbox, rather than a curiosity.'”