By Jeannette Bedard
In the ocean, the surface is easy to observe. From antiquity, seafarers strove to interpret the surface they saw, knowing that understanding the ocean was the key to their survival (1). Underneath that deceptive, hard-to-navigate surface lurks miles of water that are close to impossible to observe directly and tricky to measure.
Around 1592, Galileo lowered his gaze from the heavens to a desktop curio. It was a hand-blown glass ornament filled with alcohol. On warmer days, alcohol rose higher in the glass (1). Galileo realized the trinket before him could be standardized with gradations added to measure temperature. This instrument, the thermoscope, was the first step toward thermometers and measurement of deep ocean temperatures.
Thermometers capable of measuring deep ocean temperatures were developed in the 19th century and quickly a curious fact emerged: water at the bottom of the ocean at the equator was only a few degrees above freezing, the same temperature found below the polar ice—approximately 35 degrees Fahrenheit (2 to 3 degrees Celsius). Since warmth from the sun falls unevenly on earth’s surface and heats the equator more than the poles, how can the deep equatorial and polar water temperature be so similar?
Let’s pretend the ocean is like an aquarium full of water. If you happen to have some food dye and an empty aquarium, fill it up and try this at home. At one end, a lamp can act as the sun and warm the water, add a little bit of food dye to the surface water. The warm water is lighter so it will spread out over the surface of the aquarium. If you waited long enough, the surface warm water would warm the water below it and eventually the entire aquarium’s temperature would rise. At the other end of the aquarium, carefully add a few ice cubes and a couple drops of a different color dye. The ice cools the water around it, and because cold water is heavier than warm water, it sinks. The cold water spreads out along the bottom of the aquarium creating its own layer. This bottom layer would stay cold, even where the surface was being heated.
Although the oceans are infinitely more complex than an aquarium, uneven heating of the surface creates the same effect. Warm equatorial water spreads out, pushing towards the poles, while cold water created in the icy oceans surrounding Antarctica and in the North Pacific sinks to the bottom before slowly creeping towards the equator.
You can see there is an opportunity for detective work here, because the temperature of a parcel of water is defined (mostly) by where it was last at the surface. By measuring the temperature of any parcel of water away from the surface, oceanographers can make a reasonable guess where that water originated.
(1) McConnell, A. (1981), Historical Instruments in Oceanography. 51pp, Science Museum, London, UK
(2) Pinet, P.R. (2006), Invitation to Oceanography fourth edition, 594pp, Jones and Bartlett Publishers, Sudbury, MA, USA
(3) Nansen, F. (1999), Farthest North; the incredible three-year voyage to the frozen latitudes of the north, 544pp, Modern Library
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