Deep water circulation
How does water get from the deep ocean back to the surface?
- "Pushing" and "Pulling" - Upwelling brings water back up to the surface. - Must be balanced.
What is the pathway of the global ocean conveyor belt?
- "Starts" in the North Atlantic, the "piston" that drives the system. - Sinks at high latitudes. - Flows south through Atlantic. - Slow upward mixing along path or at upwellings. - Returns to North Atlantic.
Circumpolar Deep Water (CDW)
- "The greatest mixmaster of the world." - Flows out into the Pacific and Indian Ocean. - Mix of AABW, AAIW, and NADW in circumpolar current.
What happens to the water and its chemistry as it travels along that path?
- As water spends more time away from the surface and is not as subjected to surface processes, it gets "older," - Decay starts to take place of certain chemicals, which can be used as tracers.
What evidence do we have to support the global ocean conveyor pathway?
- Can see the pattern in temperature and salinity. - Flow rates are too slow to measure. - Aging of water along the flow path via tracers.
what are water masses?
- Come from different areas of the ocean. - Layers that have different densities and are identifiable by their temperature and salinity (density). - Do not mix readily. - Tend to flow over and under each other for long distances.
Antarctic Bottom Water (AABW)
- Densest water in the world and forms in the Weddell sea, a shallow embayment in South Atlantic. - Cold and salty. - Downwells and flows down the slope to hug the ocean bottom and flow into other ocean basins.
Where does intermediate water form today, and why?
- Flows out into the Pacific and Indian Ocean due to the mixing with other water masses. - Antarctic convergence zone.
Antarctic Intermediate Water (AAIW)
- Forms at Antarctic convergence zone (Polar Front). - Sinks to about 1000 m. - Combination of Ekman convergence and polar cooling.
North Atlantic Deep Water (NADW)
- Forms in the Norwegian-Greenland Sea and where the salty Gulf Stream waters enter these seas. - Overflow from 1) Norwegian Sea 2) Denmark Straits 3) Labrador Sea 4) Mediterranean Sea, all mix here.- Gulf Stream water cools at high latitudes, especially over winter, which increases the density. - Flows out over sills and flows south. - Freshened by rainfall. - Overrides AABW and mixes with other waters near Antarctica.
How does the depth of the mixed layer and the shape of the thermocline vary with location (ideally at least)?
- In the tropics, the thermocline separates deep water from the mixed layer. - At high latitudes, deep water extends to the surface because the thermocline is weak or non-existent here since surface waters are just as cold as deep waters.
What is the "push" in global ocean circulation?
- Thermohaline (density) as dense water sinks at high latitudes into the deep ocean to form deep waters. - Relatively rapid. - In only a few locations.
Where does the deep water form today that fills the deep Atlantic, Pacific and Indian basins and why?
- Near Antarctica in the Weddell Sea. - In the NE Atlantic in the Norwegian-Greenland Sea. - Generally, can form where water can downwell to the deep sea since the stratification is weak..
How do we get a sense of how "old" the water is (how long it's been away from the surface)?
- Rain of organic matter falling through the water column as it decays. - "Older" water allows for more time for reactions to occur. - Certain reactions happen at specific depths.
What are the major deep water masses in the oceans?
- Surface - Intermediate - Deep - Bottom - All increase with density as depth increases.
What is the mixed layer?
- Surface layer (about 20 - 100 m deep). - Little to no change in temperature and salinity (density) with depth. - Has several interactions with the atmosphere, making it well-mixed. - Depth varies with wind intensity, season, latitude, and currents.
Where do deep waters get their characteristic temperatures and salinities?
- Temperatures come from solar insulation as it heats up the surface water, and then at high latitudes, that water cools, and sinks. - Salts are put into the ocean via continental weathering (carbonate and silicate), hydrothermal circulation, and reactions with sediments.
What role does deep water circulation play in controlling climate? - Last Glacial Maximum circulation, potential future scenarios, Cretaceous Greenhouse.
- There were no ice caps present at any of the poles, and the Earth was much warmer due to CO2. - Due to the position of the continents, NS flow was reduced, while EW flow was increased. - Dense water formed from evaporation in the subtropics to create warm, salty, deep water. - Poleward heat transport by surface and deep waters. - Reduced pole to equator temperature gradient.
What are interactions between the atmosphere and the mixed layer of the ocean?
- Wind-driven circulation. - Evaporation and precipitation. - Exchange of solar radiation and gases.
What is the "pull" in global ocean circulation?
- Wind-driven upwelling at divergences. - Particularly seen in the Southern Ocean. - Mixing along isopycnals to bring deep water back to the surface. - Must be balanced by rising deep water.
What is the thermocline?
- Zone of rapid temperature change. - Separates layers of water that are stably stratified. - Isolates deep ocean from exchange with the atmosphere in most locations. - About 200 m - 1,000 m deep. - Stability limits vertical movements and insulates the deep ocean from seasonal changes in temperature and salinity.
What are the 7 most common salts in the ocean and what form are they in?
Cl-, Na+, SO4^2-, Mg2+, Ca2+, K+, and HCO3- and they exist in the forms of ions.
salinity
Salinity The amount of salt compared to pure water.- Salts (dissolved ions (Na, Cl, Ca...) - Mostly products of chemical weathering of the continents.
What causes salinity variations?
The surface process of evaporation and precipitation (water is moved, but salts stay behind).
What causes temperature variations?
The surface process of solar radiation.
What determines the density of water?
The temperature and salinity. - Colder and saltier water tend to be more dense.It also depends on the location. - Warm water density is dominated by temperature. - Cold water density is dominated by salinity.
In-Situ Temperature (T - tau)
The temperature of the water at depth (under pressure). - This temperature tends to be higher since molecules are closer together when under pressure.
Potential Temperature
The temperature of the water if it is brought up to the sea level (atmospheric pressure) and analyzed.
What drives deep water circulation?
Thermohaline Circulation (Thermo = temp., haline = salinity, and temp + salinity = density).
What processes drive the global ocean conveyor belt?
density
