Magazine article Oceanus

The Hunt for 18[degrees] Water: Oceanographers Examine "Mode Waters" That Save the Signals of Past Winters

Magazine article Oceanus

The Hunt for 18[degrees] Water: Oceanographers Examine "Mode Waters" That Save the Signals of Past Winters

Article excerpt

Terry Joyce is looking for Val Worthington's water.

In 1959, Woods Hole oceanographer Valentine Worthington gave a name and identity to a long-observed but poorly understood phenomenon of the North Atlantic Ocean. Analyzing data from as far back as the H.M.S. Challenger expedition of the 1870s, Valentine described how the interior of the Sargasso Sea contained distinct parcels of water with remarkably constant salinity, density, and temperature--roughly 18[degrees]C (64[degrees]F). To Worthington, the appropriate name for this quirky mass was simple and straightforward: 18[degrees] water.

In the early 1970s, Worthington persuaded colleagues and funding agencies to mount an expedition to study 18[degrees] water. lie saw connections between these peculiar water masses and the circulation of the entire North Atlantic, as well as the weather above it. But when he finally went to sea in 1974 and 1975, he couldn't find what he was looking for.

Researchers now know that 18[degrees] water is produced by a critical energy transfer between warm Gulf Stream water and the cold winter atmosphere. Unfortunately for Worthington, he went to sea in the midst of a warm winter, an ebb time in the assembly line of production of 18[degrees] water.

Decades later, his successors in the Physical Oceanography Department at Woods Hole Oceanographic Institution and from eight other oceanographic institutions have launched a far-reaching program to examine the formation and evolution of Worthington's famous water and how it might influence North Atlantic climate. The CLIVAR Mode Water Dynamics Experiment (CLIMODE) began its own series of expeditions in November 2005, and this time researchers seem to be finding what their predecessor was looking for.

During winter, chilly winds blow from the Arctic and North American interiors out to sea, where the warm Gulf Stream rides east over the Sargasso Sea. The cool, dry winds pull heat and moisture from the Stream and carry them off to Europe and North Africa. The cooler, salty water left behind forms a layer on top of the ocean that can extend 1,300 feet (400 meters) deep.

Spring and summer heat eventually warms the surface again, but the chilled waters formed in winter do not dissipate. They are denser than warm waters, so they sink--not to the seafloor, but to the middle of the sea. This layer of water hangs suspended between warm surface waters and even colder deep waters, wedged like butter cream in the midst of a layer cake.

Oceanographers call 18[degrees] water and other water masses like it "mode water," a statistical term for a mass of water that has homogeneous temperature and other characteristics. Mode waters are like pools within the deeper and wider pool of world oceans, and they form in the middle latitudes all over the world. (The Kuroshio Current in the northwest Pacific and the Agulhas Current in the southwest Indian Ocean are also famous for mode-water creation.) Once they form and sink, layers of mode water remain relatively intact for several years and are swept around by the ocean's circulation.

As Worthington wrote in 1959, "the 18[degrees] water is of more than usual interest" because it can often be found hundreds of miles to the south and west of where it is formed, "in places where the winter surface temperature is far higher than 18[degrees] and there is no possibility of it being formed locally." In fact, Worthington's 18[degrees] water can be found as far south as the Caribbean. …

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