North Atlantic Drift Current and Abrupt Climate Change

[Saul]

There is a belief that a complete interruption of the North Atlantic drift current could cause abrupt climate change.

The amount of heat transported by the North Atlantic drift current and the Gulf stream is 8 times smaller than the amount of heat transported by the atmosphere transferring the summer heat that is absorbed by the ocean.

That mechanism (a complete interruption of the North Atlantic drift current) is not capable of causing the Younger Dryas 1000 year abrupt cold period. It also is not capable of causing the Heinrich events.


http://www.americanscientist.org/issues/id.999,y.0,no.,content.true,page.1,css.print/issue.aspx

Recently, however, evidence has emerged that the Younger Dryas began long before the breach that allowed freshwater to flood the North Atlantic. What is more, the temperature changes induced by a shutdown in the conveyor are too small to explain what went on during the Younger Dryas. Some climatologists appeal to a large expansion in sea ice to explain the severe winter cooling. I agree that something of this sort probably happened, but it's not at all clear to me how stopping the Atlantic conveyor could cause a sufficient redistribution of heat to bring on this vast a change.

Like many other myths, this one rests on a strand of truth. The Gulf Stream carries with it considerable heat when it flows out from the Gulf of Mexico and then north along the East Coast before departing U.S. waters at Cape Hatteras and heading northeast toward Europe. All along the way, it warms the overlying atmosphere. In the seas between Norway and Newfoundland, the current has lost so much of its heat, and the water has become so salty (through evaporation), that it is dense enough to sink. The return flow occurs at the bottom of the North Atlantic, also along the eastern flank of North America. This overturning is frequently referred to as the North Atlantic thermohaline circulation, or simply the "Atlantic conveyor." It is part of the global pattern of ocean circulation, which is driven by winds and the exchange of heat and water vapor at the sea surface.

http://www.atmos.washington.edu/~david/Gulf.pdf

Is the Gulf Stream responsible for Europe’s mild winters?
By R. SEAGER, D. S. BATTISTI, J. YIN, N. GORDON, N. NAIK, A. C. CLEMENT and M. A. CANE

It is widely believed by scientists and lay people alike that the transport of warm water north in the Gulf Stream and North Atlantic Drift, and its release to the atmosphere, is a major reason why western Europe’s winters are so much milder (as much as 15–20 degC) than those of eastern North America (Fig. 1). The idea appears to have been popularized by M. F. Maury in his book The physical geography of the sea and its meteorology (1855)which went through many printings in the United States and the British Isles and was translated into three languages.

In summary, the east–west asymmetry of winter climates on the seaboards of the North Atlantic is created by north-westerly advection over eastern North America and by zonal advection into Europe. The Pacific Ocean has an analogous arrangement with meridional advection being an especially strong cooling over Asia. Since western Europe is indeed warmed by westerly advection off the Atlantic, we next assess how the surface fluxes over the Atlantic are maintained.

In conclusion,while OHT warms winters on both sides of the North Atlantic Ocean by a few degC, the much larger temperature difference across the ocean, and that between the maritime areas of north-western Europe and western North America, are explained by the interaction between the atmospheric circulation and seasonal storage and release of heat by the ocean. Stationary waves greatly strengthen the temperature contrast across the North Atlantic and are themselves heavily influenced by the net effect of geography. In contrast, transport of heat by the ocean has a minor influence on the wintertime zonal asymmetries of temperature. Even in the zonal mean, OHT has a small effect compared to those of seasonal heat storage and release by the ocean and atmospheric heat transport. In retrospect these conclusions may seem obvious, but we are unaware of any published explanation of why winters in western Europe are mild that does not invoke poleward heat transport by the ocean as an important influence that augments its maritime climate.

 

[Xnn]

Saul;

The article's you've linked to appear to be fairly correct.

I also agree that Heinrich events were not caused by changes in Atlantic ocean currents.
The ice shelf responsible for Heinrich events was located near the Hudson strait.
That's a long way from the Atlantic and the Gulf currents. It was unstable and once it started to fall apart (just like an avalanche) there was not much that was going to stop it. However, now that it is gone, it is gone.

Now, what some people have realized, is that while we are not going to see another precisely Heinrich event in the near future, it is entirely possible that we may witness another collapse of a very large ice shelf. The West Antarctic is the area of prime concern as there is a large amount of ice grounded on land that is already underwater. Clearly, this is not a stable situation. Once it starts to break up, there is very little that can stop it.

Heinrich and the Younger Dryas were primarily a Northern Hemipshere events. So, it we are about to witness a breakup of the the West Antarctic, then there is no similar analogy.

 

[Saul]

The ice purge during the Heinrich events is from multiple ice sheets. The bing purge ice sheet mechanism could not explain ice sheet purges from multiple ice sheets.

As a side note the current abrupt Greenland ice sheet purge into the Atlantic ocean has abruptly stopped. Why it started and why it abruptly stopped (Five separate and independent ice sheet flows is not known.)

But back to the melt water pulse. A total stoppage of the North Atlantic drift current could not have caused the Younger Dryas abrupt cooling. The during the Younger Dryas abrupt cooling the Northern Hemispere went from interglacial warm (insolation at the critical 65N was the highest in 100,000 years) to glacial cold with 70% of the cooling occurring within 15 years. The remainder of the Younger Dryas cooling was complete in 40 years.

During the Younger Dryas each winter the North Atlantic ocean frozen to the latitude of mid-Spain.

The Younger Dryas is a Heinrich event. These are cyclic events. The melt water pulse is a red herring and the ice sheet surges is also a red herring.

There is a misconception as the first guess as to what caused the cyclic Heinrich event was incorrect, that a Heinrich event cannot occur during an interglacial. That is not correct. The Younger Dryas occurred. What caused the Younger Dryas? What caused the 8,200 year ago cooling event.

All of the previous interglacial periods ended abruptly. The climate forcing function is the same. Its affect on the planet, however, depends on the stage of the glacial/interglacial cycle when it occurs as well as specifics that are connected with the forcing function.

 

[dorlomin]

Has the younger dryas not been noted as starting about 500 years earlier in the Southern Hemisphere. This largely from ice cores from mid lattitude glaciers?

 

[Studiot]

Thank you for posting this, it gives cause for thought and cetainly suggests much more work need to be done before we can reliably and confidantly explain some of these things.