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posted by Fnord666 on Friday March 27 2020, @11:52PM   Printer-friendly
from the buffers-work-up-to-a-point-and-then-all-hell-breaks-loose dept.

How stable is deep ocean circulation in warmer climate? Altered circulation might have cooled northern areas of North America and Europe

If circulation of deep waters in the Atlantic stops or slows due to climate change, it could cause cooling in northern North America and Europe – a scenario that has occurred during past cold glacial periods.

Now, a Rutgers coauthored study suggests that short-term disruptions of deep ocean circulation [also] occurred during warm interglacial periods in the last 450,000 years, and may happen again.

Ironically, melting of the polar ice sheet in the Arctic region in a warmer world, resulting in more fresh water entering the ocean and altering circulation, might have caused previous coolings.

[...] The study, published in the journal Science and led by scientists at the University of Bergen in Norway, follows a 2014 study on the same topic.

"These findings suggest that our climate system, which depends greatly on deep ocean circulation, is critically poised near a tipping point for abrupt disruptions," said coauthor Yair Rosenthal, a distinguished professor in the Department of Marine and Coastal Sciences and Department of Earth and Planetary Sciences at Rutgers University–New Brunswick. "Although the disruptions in circulation and possible coolings may be relatively short-lived – lasting maybe a century or more – the consequences might be large."

The warm North Atlantic Current -- the northernmost part of the Gulf Stream -- flows into the Greenland Sea. It becomes progressively colder and saltier due to heat loss to the air, eventually sinking and forming the North Atlantic Deep Water formation -- a mass of deep, cold water that flows southward. Melting of the polar ice sheet in the Arctic region would result in more fresh water entering the ocean and disrupting that circulation pattern, potentially causing cooling in northern areas of Europe and North America.

[...] The latest study covers three other warm interglacial periods within the past 450,000 years. During all of them, regardless of the degree of global warming, the scientists found similar century-long disruptions of the North Atlantic Deep Water formation. And they found that such disruptions are more easily achieved than once believed and took place in climate conditions similar to those we may soon face with global warming.

Journal Reference:
Eirik Vinje Galaasen, Ulysses S. Ninnemann, Augustin Kessler, et al. Interglacial instability of North Atlantic Deep Water ventilation. Science, 2020 DOI: 10.1126/science.aay6381


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  • (Score: 2, Interesting) by Anonymous Coward on Saturday March 28 2020, @03:23AM (1 child)

    by Anonymous Coward on Saturday March 28 2020, @03:23AM (#976560)

    Your comment is generally correct, but I think the effect on hurricanes is probably a bit more complicated.

    All other things equal, increasing sea surface temperatures will result in stronger hurricanes. This is not in dispute. But there are other factors like vertical wind shear that affect hurricane formation and intensity. Vertical wind shear is the change in wind speed and direction as you go up in the atmosphere. Strong vertical wind shear is favorable for supercell thunderstorms and tornadoes, but is unfavorable for hurricanes.

    One way we calculate vertical wind shear is to pick two different levels in the atmosphere. We start at the origin of a graph and draw two vectors outward, one for the winds at each of the levels. Then we draw a third vector connecting the heads of the first two vectors. That third vector is the shear vector. The length of that third vector is the amount of vertical wind shear. As that vector gets longer, there's more shear in the atmosphere. Winds in the upper atmosphere are usually quite a bit stronger than near the surface, so increasing the upper-level winds usually produces more vertical wind shear.

    What you're describing is that you'll have warmer water near the equator and cooler water at high latitudes. There is some coupling between sea surface temperatures and the air above the surface. That means the lower atmosphere will probably be warmer in the tropics and a bit cooler at higher latitudes. In other words, you'll have a bigger temperature difference between the equator and, say, 40 degrees north. These temperature differences in the lower atmosphere cause differences in air pressure in the upper atmosphere. So if there's a bigger temperature gradient in the lower atmosphere, there will be a stronger pressure gradient in the upper atmosphere. And increasing the pressure gradient in the upper atmosphere will also result in stronger upper-level winds. And increasing the upper-level winds also probably means more vertical wind shear.

    To summarize, sea surface temperatures will get warmer, but the vertical wind shear will probably get stronger. This article discusses the North Atlantic basin. And climate models generally predict that sea surface temperatures will increase in the tropical North Atlantic, but that vertical wind shear will also get stronger. One is favorable for stronger hurricanes, but the other is unfavorable.

    Sea surface temperatures tend to vary on a seasonal time scale, with the warmest water in September and the coldest water in February and March. However, vertical wind shear tends to vary more on weather time scales, increasing and decreasing on the other of days instead of months. On average, there will probably be more vertical wind shear, which has the effect of suppressing hurricane activity. But because there's significant variability on weather time scales, there will be times during the season when the shear abates. When that happens, the very warm sea surface temperatures may more strong hurricanes. But the rest of the time, when the shear is stronger, it may suppress hurricane activity. One hypothesis is that we'll see an overall suppression of hurricane activity in the North Atlantic, but hurricanes that do develop will tend to be stronger. We may see fewer hurricanes, but the ones that occur will be more extreme.

    This is an active area of research and we don't know for sure how global warming will actually affect hurricane activity in the North Atlantic. And it's also quite possible that some of the other tropical cyclone basins around the world won't behave in the same manner as the North Atlantic.

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  • (Score: 2) by maxwell demon on Saturday March 28 2020, @07:30AM

    by maxwell demon (1608) on Saturday March 28 2020, @07:30AM (#976583) Journal

    One way we calculate vertical wind shear is to pick two different levels in the atmosphere. We start at the origin of a graph and draw two vectors outward, one for the winds at each of the levels. Then we draw a third vector connecting the heads of the first two vectors. That third vector is the shear vector. The length of that third vector is the amount of vertical wind shear.

    Let me shorten that for you:

    Wind shear is the vector difference of wind velocities at two different heights.

    But apart from the wordiness, interesting comment.

    --
    The Tao of math: The numbers you can count are not the real numbers.