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Help to the Gulf Stream

Friday, 4. December 2009, 16:13:00

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There has been some worry that the Gulf Stream might be hindered in doing its good work for north western Europe as a consequence of global warming, but now it seems that another consequence of the same global warming in another corner of the world might save its life. Unexpected help may come from the Agulhas Current.

The Agulhas Current flows down the east coast of Africa from 27°S to 40°S. It is narrow, swift, and one of the strongest ocean currents in the world. Southwest of Cape Town it abruptly turns back into the Indian Ocean. In this process huge rings of water with diameters of hundreds of kilometres are cut off at intervals of 3 to 4 months. These so-called "Agulhas Rings" carry warm and saline waters from the Indian Ocean into the Atlantic. It is known for quite some time that the Agulhas region is one of the key regions for the supply of salty waters into the Atlantic. By analyzing observational data and computers models it has been shown that this process is strengthening due to climate change in the Southern Ocean.

Westerlies in the Southern Ocean are blocking the water exchange between the Indian Ocean and the Atlantic. In the past decades, a southward shift of these westerlies has widened the corridor south of Africa for the inflow of water into the Atlantic. This trend could further intensify in the future as part of climate change.

In the Atlantic this extra amount of salty water is transported northward by the prevailing currents and could finally help to stabilize the Gulf Stream system in the northern North Atlantic. There it could act against the freshening process due to enhanced precipitation and the ice melting. Further studies are however required to prove the hypothesis.

Reference:
Biastoch et al., 2009:
Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies.
Nature, 462 (7272), doi:10.1038/nature08519.


In German:


PS: Copy and paste full German link into browser to make it work!



Academics

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Global Conveyor Belt

Thursday, 14. May 2009, 19:46:51

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Generalized model of the thermohaline circulation: 'Global Conveyor Belt'. This illustration (from NASA) shows cold deep high salinity currents circulating from the north Atlantic Ocean to the southern Atlantic Ocean and east to the Indian Ocean. Deep water returns to the surface in the Indian and Pacific Oceans through the process of upwelling. The warm shallow current then returns west past the Indian Ocean, round South Africa and up to the North Atlantic where the water becomes saltier and colder and sinks starting the process all over again.

According to a 50-year-old model of ocean currents the Labrador Current is a southbound subsurface flow of cold water forming a continuous loop with the familiar northbound flow of warm water on the surface, called the Gulf Stream.


This model of North Atlantic currents as shown in the diagram here below from a press release from Duke University has been called into question by new data from Duke University and the Woods Hole Oceanographic Institution.


The following is primarily based on the press release from the Duke University, but the real thing is of course the relevant paper of 14 May 2009 in the journal Nature.

Climatologists pay attention to the Labrador Sea because it is one of the starting points of a global circulation pattern that transports cold northern water south to make the tropics a little cooler and then returns warm water at the surface, via the Gulf Stream, to moderate temperatures of northern Europe.

Oceanographers long thought the Labrador seawater moved south along what is called the Deep Western Boundary Current (DWBC), which hugs the eastern North American continental shelf all the way to near Florida and then continues further south.


But studies in the 1990s using submersible floats that followed underwater currents "showed little evidence of southbound export of Labrador sea water within the Deep Western Boundary Current (DWBC).

Lozier and Bower launched 76 special Range and Fixing of Sound floats into the current south of the Labrador Sea between 2003 and 2006. Those "RAFOS" floats could stay submerged at 700 or 1,500 m depth and still communicate their data for a range of about 1,000 km using a network of special low frequency and amplitude seismic signals.

Only 8 percent of the RAFOS floats' followed the conveyor belt of the Deep Western Boundary Current. About 75 percent of them escaped that coast-hugging deep underwater pathway and instead drifted into the open ocean by the time they rounded the southern tail of the Grand Banks.

The new float observations and simulated float trajectories provide evidence that the southward interior pathway is more important for the transport of Labrador Sea Water through the subtropics than the DWBC, contrary to previous thinking,

• http://news.duke.edu/2009/05/conveyor.html
• http://www.nature.com/nature/journal/v459/n7244/abs/nature07979.html
• http://www.eurekalert.org/pub_releases/2009-05/whoi-sfs051309.php
• http://www.eurekalert.org/pub_releases/2009-05/du-cwo051309.php
• http://www.physorg.com/news161439846.html
• http://www.scientificblogging.com/news_articles/climate_change_modeling_complication_ocean_circulation_does_not_work_expected
• http://www.geologytimes.com/research/Cold_water_ocean_circulation_doesnt_work_as_expected.asp



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Beaufort Gyre

Friday, 1. May 2009, 09:55:23

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In oceanography a ‘gyre’ is a circular pattern of currents in an ocean basin. The Gulf Stream is a part of the North Atlantic Subtropical Gyre. Here is a map of the five major ocean gyres.


This kind of world maps often irritates me. I think that everybody should own a globe to realise what our Earth really looks like - after all it is a sphere, and not flat. Some of you may have noticed that I often use polar-centered maps, to give you another look. Here is such a map of the Arctic Ocean - well I know that we usually talk about only three oceans: the Pacific Ocean, the Atlantic Ocean, and the Indian Ocean. Again this may partly be a consequence of the sort of world maps we use. It is more correct to talk about five oceans, the three I mentioned plus the Southern Ocean (around Antarctica) and finally the smallest of the five, the Arctic Ocean.

The Beaufort Gyre is an ocean and ice circulation pattern in the Beaufort Sea, north of Alaska. I have marked it with a B on the map. This gyre moves in a clockwise direction (looking from above the North Pole). This circulation results from an average high-pressure system that spawns winds over the region. Ice that forms in or drifts into the Beaufort Gyre has historically remained in the Arctic ice system for years, accumulating snow and thickening each winter. Beginning in the late 1990s, the ice began melting away while in the southern parts of the gyre, before completing the circulation.

Over the last few weeks I have started on several posts without finishing them (getting occupied with other thoughts and ideas). One of them was supposed to be about Ekman, maybe I’ll finish that one later. Anyway Vagn Ekman was a Swedish oceanographer. During the expedition of the Fram expedition (1893-1896), Fridtjof Nansen had observed that icebergs tend to drift not in the direction of the prevailing wind but at an angle of 20°-40° to the right. Bjerknes invited Ekman, still a student, to investigate the problem, which he did, and what made his name famous within oceanography. - Sorry, I am deflecting a bit.

The short story is that Beaufort gyre is a result of the Coriolis force, the prevailing winds, and what is know as Ekman transport (in short: movement of water at a certain angle to the wind).

Finally a map showing the relationship with other ocean currents in the Arctic area. The map is from the Arctic Monitoring and Assessment Programme (AMAP)



• http://nsidc.org/seaice/processes/circulation.html
• http://www.whoi.edu/beaufortgyre/index.html
• http://www.amap.no/

More about the Fram vessel and the Fram expedition:
• http://www.fram.museum.no/en/




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