Alexander Dallas Bache, second superintendent of the U.S. Coast Survey. Courtesy NOAA Photo Library.

From an early age, Franklin was curious about the Gulf Stream. Franklin passed on his excitement for Gulf Stream research to some of his relatives. Franklin’s grandnephew, Jonathan Williams Jr. studied the temperature of the current at different depths as he sailed with his great uncle across the Atlantic. But, it was Franklin’s great grandson, Alexander Dallas Bache, who made significant progress in the study of the Gulf Stream. In 1845, as the second Superintendent of the US Coast Survey (what later became the US Coast Guard), Bache ordered a complete scientific survey of the Gulf Stream. It included a study of physical, chemical, geological and biological oceanography. Bache wanted to know if the current's temperature varied at different depths. To find out, he requested a systematic study of the temperature of the Gulf Stream stating, "Make, then, as many cross sections of the Stream as convenient and as the investigation may show to be necessary." From this study, Bache concluded that the Gulf Stream is divided into bands of warm and cold water. Bache assumed, although he had no scientific data to prove it, that there was a cold current running underneath the Gulf Stream. He may have gotten this idea from his grandfather, Franklin, who had described the Gulf as a warm river floating over a colder one.

Today, satellite images and deep water measurements of ocean temperatures prove that Franklin and Bache were correct: the Gulf Stream is a warm water current flowing over cold ocean water.

This chart of the Gulf Stream, compiled by the Coast Survey in 1860, was based on a series of systematic studies begun in 1845 by Alexander Dallas Bache. Courtesy NOAA Photo Library.

Think about what this means. If cold water sinks and warmer water rises, then ocean water must always be moving. In fact, this is true. The moving streams of water are called density-driven currents, and they occur in all bodies of salt water. These currents are not caused by wind but by differences in density. In the ocean the density of the water is created not only by its temperature, but also by its salinity. Thus, in order to understand density-driven currents, we need to understand both the temperature and salinity of the ocean waters. In this activity, you will learn more about density as you investigate why some water sinks and some floats. You will also learn what this sinking and floating has to do with ocean circulation.

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