Tracing the ocean overturning circulation. Science Olympiad Resource Anand Gnanadesikan NOAA Geophysical Fluid Dynamics Lab. Welcome!- In this lesson you will learn. How we first learned about deep ocean currents.
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Science Olympiad Resource
NOAA Geophysical Fluid Dynamics Lab
Print out a copy of this slide to build up your own version of the circulation
In 1751, Captain Henry Ellis made a surprising discovery using a new scientific instrument- the thermometer.
Upon the passage, I made several trials with the bucket sea-gage, in latitude 25'-13" north; longitude 25'-12" west. I charged it and let it down to different depths from 360 feet to 5346 feet; when I discovered, by a small thermometer of Fahrenheit's, made by Mr Bird, which went down in it, that cold increased regularly, in proportion to depths, till it descended to 3900 feet: from whence the mercury in the thermometer came up at 53 degrees; and tho' I afterwards sunk it to a depth of 5346 feet, that is a mile and 66 feet, it came up no lower. The warmth of the water upon the surface, and that of the air, was at the time by the thermometer 84 degrees. I doubt not but that the water was a degree or two colder, when it enter'd the bucket, at the greatest depth, but in coming up had acquired some warmth"
Capt. Henry Ellis- Report to the Royal Society, 1751
Photo of early water sampler:NOAA
Ellis also reported that the cold water enabled him to get cold drinks at sea!
Research vessels (above) lower CTDs with water sampling rosettes (right) which take samples of water at different depths as well as measuring a high-resolution profile of conductivity and temperature.
Since it never gets that cold on the equator, it was very quickly realized that this cold water had to come from polar regions.
What does this tell you about where the water in the ocean is sinking to depth?
Draw these sinking pathways on your base map in blue.
25% of the radiocarbon in the North Pacific has decayed.
This implies a ventilation age of ~2000 yr.
We get this from the fact that the water gets older (lower radiocarbon) as you move into the Atlantic and the Pacific.
If the surface and deep are exchanging rapidly, we’d expect the radiocarbon at the surface to be closer to that at depth.
The plot at left shows the size of the difference. Notice where the smallest differences occur.
But are these small differences due to water coming up rapidly or water sinking from the surface rapidly?
We can get some sense of the answer by comparing the radiocarbon difference with the CFC12 difference. Remember that radiocarbon differences are generated in the deep ocean, while CFC differences are generated at the surface. Notice the difference between the North Atlantic and the Southern Ocean.
Both tracers low- high mixing
Radiocarbon, somewhat low, CFC high, weak upwelling
Radiocarbon very low, CFC relatively high, strong upwelling with some mixing.
So most of the deep water flowing into the Indian/Pacific comes right back to the Southern Ocean!
Minima and maxima pentrating into ocean interior. Let’s look at the minimum coming out of the south. We can follow the salinity and pCFC concentrations along a density surface that lies just above this minimum.
The colors show CFCs, the lines concentrations of salinity. You can see plumes emanating (sometimes in one tracer, sometimes in both) from the Southern Ocean. Add these to your plot in purple.
Now we are forming colder,denser waters in the North Atlantic and
Now we are forming colder,denser waters in the North Atlantic and turning the dense water back into warmer,lighter intermediate water in the Southern Ocean. How do we get this water back to the North Atlantic?
Direct current and tracer measurements have shown that there is a large flow of water through the Indonesian archipelago.
The plots above show surface speed from a computer model. The model on the left is a low resolution model, the one on the right a much finer-resolution model. You can see red circles that correspond to eddies in this model. These eddies are part of a flow of warm water from the Indian into the Atlantic.
You will see this picture in a lot of textbooks and websites. From what we’ve done here, you should now be able to tell what’s wrong with it!