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Ocean Circulation. Atmospheric circulation is directly tied to ocean circulation in distributing the worlds energy budget. Circulation in the oceans is very different that that of the atmospheres in several ways.
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Ocean Circulation Atmospheric circulation is directly tied to ocean circulation in distributing the worlds energy budget. Circulation in the oceans is very different that that of the atmospheres in several ways. The atmosphere is heated from below and the ocean from above where 90% is absorbed in the upper 100m.
Because warmer surface water is less dense than cooler water below, this increases stability. However, because surface heating also increases evaporation, the water becomes more saline and thus more dense. The thermal inertia of the ocean prevents it from responding directly to changing insolation on a small scale the way that air is affected. It is the movement of the atmosphere that forces the movement of surface waters. The movement of one fluid (air) against the other (water) results in friction/drag (known as wind stress) on the surface that pushes it along as a wind-drift current.
The Coriolis effect turns water to the right as the wind pushes it such that currets are at 20-25° angles from the direction of the wind. This results in the large semi-circular gyres in each ocean basin. Why would water pile up in the middle of these gyres?
Real world surface currents approximate the simple model. There are some further complications to consider for deeper water currents.
The Ekman spiral leads to Ekman transport of water at a 90° angle to the wind that forces the motion.
Upwelling and Downwelling In areas of convergence, water piles up and the surface layer thickens causing downwelling. In areas of divergence, the surface water moves away and the surface layer thins causing upwelling.
Western boundary currents tend to be narrow and fast because of the vorticity effect.
The Gulf Stream is a great example of just such a current. How was it discovered?
Circulation of the Deep Ocean Salinity is the driving mechanism for circulation in the deep ocean. Salinity is the number of grams of salt dissolved in one kilogram of water. Thermohaline circulation- driven by temperature and salinity. Vertical structure of the deep ocean
Bottom-Water Formation- begins with the formation of cold and/ or salty water (that is relatively dense) at high latitudes. This is the densest water in the oceans. There are two primary ways to form dense bottom water. Sea Ice formation- when seawater freezes, the salt is excluding leaving extra salty water behind. This is obviously cold and dense and therefore sinks.
Antarctic Bottom Water (AABW)- forms in the Weddell Sea and circles Antarctica eventually seeping north into all ocean basins finally reaching 45-50° in about 250 years according to 14C data.
North Atlantic Deep Water (NADW)- this water has a more complex formation than AABW. The warm salty Gulf Stream travels north and cools off. While jus chill’n the water bumps into water from the Greenland and Norwegian Seas that has formed sea ice. This results in formation of about half of all the deep sea water. It mingles with AABW and then spreads into the deep basins around the world.
NADW mingles with AABW and then spreads into the deep basins around the world. Flow at 4000m
So, what happens if this flow stops? Predicted change in mean annual temperatures 30 years after shutdown of circulation. Has it ever happened in the past?
Yes, it has happened before Abu Hureyra Mesopotamia (Sumer) Ur
Abu Hureyra stood close to the river floodplain and in close proximity to picket forests. People lived on hackberries, wild plums, nuts, and gazelle meat. The global warming that sustained this environment was interrupted. The Laurentide Ice sheet that covered eastern North America for 100,000 years was rapidly melting. Enormous volumes of fresh water flowed into the North Atlantic. Icebergs broke off from the eastern margins of the ice cap and deposited as much as 1/2 meter of small rocks (IRDs) from the Hudson Bay region onto the floor of the Labrador Sea.
As this ice sheet melted, Lake Agassiz filled the Depression left by the retreating ice cap. At first, the melt water spilled into the Mississippi watershed and flowed into the Gulf of Mexico. 12,000 years ago, the shrinking of ice front opened up a new channel to the east. Lake Agassiz dropped rapidly as the water shot across southern Canada into what is now the St. Lawrence valley. An enormous surge of fresh water washed into the already dilute Labrador Sea.
Within a short time, the downwelling that carried salt into the deep ocean and and it's movement southward stopped altogether. The warm conveyor belt that had nourished global warming for 3000 years had abruptly shut down. Far below the surface of the Labrador Sea, Salt ceased to flow away from the northern ocean. Global warming stopped perhaps within a few years. The northern and glaciers advanced again, plunging the world into a 600 year cold snap. Then suddenly, Atlantic down welling suddenly resumed, and the conveyor belt switched on. 10,000 years of warmer conditions began and continue to this day.
This event, called the Younger Dryas, brought severe drought to the Euphrates Valley. Cold wind, reduced rainfall, and much depleted nut harvests changed Abu Hureyra very quickly. The nut-rich forests retreated almost 100 km away from the village. The people turned to wild cereal grasses and drought resistant clovers to supplement the now meager nut harvests. 1000 years earlier, these people could have dispersed into smaller groups and relied on ties with kin to see them through the lean years. It could happen again thanks to global warming!
