Water Masses and the Thermohaline Current
Five water masses
There are five water masses, and each corresponds with the layers discussed in the Seawater Lesson. Each has different properties and they don’t mix well. They include:- Surface water (0-200 m) - aka the photic zone
- Central water (to bottom of main thermocline)
- Intermediate water (to ~1500 m) - aka the aphotic zone from this point downward.
- Deep water (between Intermediate water and 4000 m)
- Bottom water (at the sea floor)
Each layer moves in a different direction from the one above it. This keeps the water moving and not stagnate. And those "rings" of water moving about Antarctica? They are a part of that moving water within the water masses.
Image from: http://www.antarctica.gov.au/science/climate-processes-and-change
Note in the image above where the sea ice is located. When ice forms, only a small fraction of the salts in seawater are incorporated into the ice. This makes the seawater super saturated with salt - a brine. This brine sinks to the continental shelf and flows down the continental slope and rise with the downwelling current formed in the Antarctic Convergence.
Deep-ocean Circulation
Deep-ocean currents form as a response to density differences in the water. Factors creating a dense mass of water include:
- Cold sea water temperature
- Increase in salinity
- click here for a shockwave animation of brine exclusion in sea ice!
Bottom currents also called contour currents because they generally moves around features rather than over them. They typically move 1-2 m/day, but locally can move 60 cm/s.
Antarctic Bottom Water
The Antarctic Bottom Water forms in the Weddell Sea during winter. Sea ice only holds 15% of salt, the rest forms a frigid brine beneath. This brine sinks towards the continental shelf and mixes with the Circumpolar Current.
From the University of Bergen: Schematic of deep water formation in the Atlantic Ocean. Filchner overflow is an important component for the production of Antarctic Bottom Water (AABW).
Arctic Bottom Water
The Arctic Ocean still has a fair amount of ice covering it, which makes exploring the region difficult. The Arctic Bottom Water is mostly a closed system due to the continental shelf between Russia and Alaska blocking the path to the Pacific Ocean. Some does “escapes into the North Atlantic and becomes a part of the North Atlantic Deep Water.
The amount sinking in the polar regions = the amount rising in the equatorial areas.
The Global Heat Connection
Thus, thermohaline currents help moderate Earth’s temperature by bringing cold water to the equators and warm water to the poles. It also distributes between ocean basins:
- Dissolved gases
- Dissolved solids
- Nutrients
- Organisms
But what happens when the oceans heat up? Let's explore this.
The Earth heats up and ice at polar caps melt. This increases amount of fresh water in oceans and decreases density of sea water. So sea ice can't form, which means the brine doesn't form which means the thermohaline /deep ocean currents can’t form. So the thermohaline circulation belt slows or stops and Earth's climate is not moderated.
OK, so the current doesn't form. Why is this a problem?
Europe's "Little Ice Age" was from about 1350 to about 1850, although the start and end dates are still being debated. There are many hypotheses as to the cause, but one is a disruption to the thermohaline current due to a large influx of freshwater entering the North Atlantic Ocean. How much the temperatures declined depends on the region, but winters in Europe were cold enough to freeze canals and rivers that today are ice-free all year. Crops failed in many areas. A migration of people from the farmlands to the cities began, which may have made the plagues that frequently occurred during this time worse.
Could this happen again. Could the poles freeze, and start of new ice age? It is possible. Current research on the thermohaline current shows that it is slowing down. But that does not mean we're headed for an ice age.
Global warming, from a human perspective, is way better than global cooling. Check out the image below of what Earth looked like during the height of the last Ice Age. Much of North America and large parts of Europe, Russia and China were under ice. We can't live on ice! This concentrates the world's population to the areas that are ice free. Eighteen thousand years ago that wasn't a big deal. But what about today? How happy would Europe be if the Russian people had to move south?
Now let's look at global warming. All that area that was under ice is now ice free. Now let's assume that the poles are ice free all the time. That's a lot of land that is free for humans to inhabit. We'd still have the "don't move into our country" problem, however.
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