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Author Topic: Gulf Stream stall  (Read 2497 times)

macfly

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Gulf Stream stall
« on: May 07, 2017, 08:01:59 PM »
I wondered if the board members here could help me discover if there is there a number or a calculation of the sea ice loss that you are aware of that will result in the stalling of the thermo-inclines at the Nansen and Amundsen Basins?

I am aware that there is also a theory that the melt water from the Greenland Ice Sheet may also be the primary cause of the stalling of the Gulf Stream.

I would love to get the clearest understanding of the individual and combined danger of these two events.

Thanks so much in advance.

mati

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Re: Gulf Stream stall
« Reply #1 on: May 08, 2017, 05:51:44 PM »
I remember reading about this one which was a "big" pulse of fresh water:
http://www.livescience.com/31810-big-freeze-flood.html

but im not sure what effect the gradual freshening from the greenland ice sheet will have...
and so it goes

FishOutofWater

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Re: Gulf Stream stall
« Reply #2 on: May 15, 2017, 03:16:46 AM »
The global thermohaline circulation is very complicated. Different parts of the global system do not move in concert. Over decades things average out.

A huge amount of melt water is stored in the Beaufort gyre so if it suddenly dumped a third of its fresh water the overturning in the Labrador sea could be slowed down and that could impact the Gulf stream.

Over the past year the Florida current branch of the Gulf Stream has been flowing a normal to above normal rate except for several weeks following the passage of hurricane Matthew.


F.Tnioli

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Re: Gulf Stream stall
« Reply #3 on: May 16, 2017, 04:16:43 PM »
I wondered if the board members here could help me discover if there is there a number or a calculation of the sea ice loss that you are aware of that will result in the stalling of the thermo-inclines at the Nansen and Amundsen Basins?

I am aware that there is also a theory that the melt water from the Greenland Ice Sheet may also be the primary cause of the stalling of the Gulf Stream.

I would love to get the clearest understanding of the individual and combined danger of these two events.

Thanks so much in advance.
I think that even if such calculations exist, they would be extremely imprecise due to complexity of things involved and general insufficience of modern coupled models capability to predict on such a large scale and into rather far future.

What can be done and is indeed being done is analysis of things which already happened. Like this one, for example.

What i'm pretty sure about is that we won't have "stalling" as such. Instead, i think that we'll have it changed, and indeed massively changed by some time 2040 or so (and onwards). Where, how and how-much it'll be happening by then - is one darn difficult question, though.

Of course, such "change" may well spell much doom for particular regions and nations. Still, note how "global warming" was much more used term until researchers got relatively better understanding of climate, after which "climate change" began to be used much more often instead. Same thing is to happen on this subject also, i suspect.

wili

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Re: Gulf Stream stall
« Reply #4 on: August 01, 2017, 05:57:27 AM »


Loss of Arctic sea ice impacting Atlantic Ocean water circulation system


https://news.yale.edu/2017/07/31/loss-arctic-sea-ice-impacting-atlantic-ocean-water-circulation-system

“Conventional thinking has been that if ocean circulation weakens, reducing the transport of heat from low to high latitudes, then it should lead to sea ice growth. But we have found another, overlooked, mechanism by which sea ice actively affects AMOC on multi-decadal time scales,” said professor Alexey Fedorov, climate scientist at the Yale Department of Geology and Geophysics and co-author of a study detailing the findings in the journal Nature Climate Change.

Earlier this year, a different Yale-led study cautioned that the AMOC system was not as stable as previously thought. That study said the possibility of a collapsed AMOC under global warming conditions is being significantly underestimated.

“We’ve now found this new connection between sea ice and AMOC,” Liu said. “Sea ice loss is clearly important among the mechanisms that could potentially contribute to AMOC collapse.”

The researchers based their findings on a combination of comprehensive climate model simulations and novel computations of the sensitivity of ocean circulation to fluctuations in temperature and salinity at the ocean’s surface over time.

“In our experiments we saw a potential loss of 30% to 50% of AMOC’s strength due to Arctic sea ice loss. That is a significant amount, and it would accelerate the collapse of AMOC if it were to occur,” Fedorov said.

In the short-term, changes in the subpolar North Atlantic have the greatest impact on AMOC, the researchers found; but over the course of multiple decades, it was changes in the Arctic that became most important to AMOC, they said.

thnx to mlparish at robertscribbler's blog for this
"A force de chercher de bonnes raisons, on en trouve; on les dit; et après on y tient, non pas tant parce qu'elles sont bonnes que pour ne pas se démentir." Choderlos de Laclos "You struggle to come up with some valid reasons, then cling to them, not because they're good, but just to not back down."

johnm33

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Re: Gulf Stream stall
« Reply #5 on: August 01, 2017, 10:29:44 AM »
I think the basins will continue being topped up by warmer waters from the south as the cooler lighter layers go south, through CAA. The Gulf stream imho is a residual of the tidal cycle which will continue to cycle whatever water is present, whether thats from the Gulf or, seasonally, Arctic waters from Baffin/Greenland.

SteveMDFP

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Re: Gulf Stream stall
« Reply #6 on: August 01, 2017, 12:50:36 PM »
I think the basins will continue being topped up by warmer waters from the south as the cooler lighter layers go south, through CAA. The Gulf stream imho is a residual of the tidal cycle which will continue to cycle whatever water is present, whether thats from the Gulf or, seasonally, Arctic waters from Baffin/Greenland.


It's a pretty graphic, but I don't think it makes physical  sense to say that tidal forces can permanently create sea water circulation, with surface waters moving towards the poles and deep waters moving towards the equator.  It takes massive amounts of energy to force the world's oceans to circulate in observed patterns, and the tidal forces from gravitatioal effects don't seem to be adequate.

The conventional model is that ocean circulation is a heat engine effect.  Solar energy warms surface waters at the equator, and polar water cools and sinks, creating a kind of down-hill gradient for surface waters to move poleward.

Much of the tendency for polar waters to sink derives from ice formation, with brine exclusion causing cold, salty water to sink.  It makes more sense to see the Gulf Stream poleward directional flow as being driven strongly by the total volume of annual ice formation in the arctic.

When the arctic is perennially ice-free, there will be much less heat engine efficiency in moving Atlantic circulation.  This is a negative feedback mechanism that will tend to retard the final disappearance of winter ice.

Edit:
I found the above graphic at:
https://www.uwgb.edu/dutchs/EarthSC102Notes/102TheOceans.HTM

It indicates that the graphic relates to vertical (tidal) motion, not bulk horizontal motion.  A separate section describes ocean circulation.
« Last Edit: August 01, 2017, 02:28:52 PM by SteveMDFP »

gerontocrat

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Re: Gulf Stream stall
« Reply #7 on: August 01, 2017, 12:59:28 PM »
More of a question than a statement....
And on the other hand, CO2 concentrations up, heat retention up, more energy into the oceans, plus   less summer sea ice meaning more open water meaning more energy captured by the oceans. i.e. the efficiency of the heat engine declines but the amount of energy available to drive it increases ?

Anyone got a few supercomputers and hydraulic maths geniuses lying around doing nothing to spare ?

binntho

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Re: Gulf Stream stall
« Reply #8 on: August 01, 2017, 01:46:43 PM »
Seem to remember a rather good article that divided the forces driving the Gulf Stream into three more-or-less equivalent forces:

1) Subduction in the northern end due to cooling.
2) Surface winds driving the sea towards the North-East
3) Tidal effects, particularly coupled with the Mid-Atlantic ridge, where the tide lifts a certain amount of water over the ridge every time.

But it´s been many years since I read this article, so perhaps my memory is way off, and the article itself may have been totally off track.

oren

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Re: Gulf Stream stall
« Reply #9 on: August 01, 2017, 02:07:12 PM »
This probably belongs in the stupid questions thread, but with the reduction in sea ice cover during the summer comes additional refreezing in the winter, so why should the engine (cold briny water sinking) stall?

Dharma Rupa

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Re: Gulf Stream stall
« Reply #10 on: August 01, 2017, 02:44:50 PM »
All a nice theory, but the evidence so far is that the Western Boundary Currents (e.g. Gulf Stream) have shifted poleward over the years.  Just ask the lobster fishermen and the fishermen on the Grand Banks.

SteveMDFP

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Re: Gulf Stream stall
« Reply #11 on: August 01, 2017, 02:55:45 PM »
This probably belongs in the stupid questions thread, but with the reduction in sea ice cover during the summer comes additional refreezing in the winter, so why should the engine (cold briny water sinking) stall?

Quite right.  That's why I said *perenially* ice free.  I.e., little winter ice formation, perhaps a century from now.

Total annual volume of new ice formation is essentially equal to total annual volume of melting.  I think it may make intuitive sense that as the ice pack has thinned over decades, there may currently be more annual ice formation than 50 years ago, which would tend, I think, to support a stronger pole-ward flow of the Gulf Stream.  But that's just my intuition, not actual science.

Ice Shieldz

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Re: Gulf Stream stall
« Reply #12 on: August 01, 2017, 06:13:57 PM »
Yes this makes sense - especially when one considers that young ice builds quicker and therefore has a greater net effect in driving AMOC then a bunch of multi-year ice more slowly building on itself.

jdallen

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Re: Gulf Stream stall
« Reply #13 on: August 02, 2017, 12:39:36 AM »
I'm still not convinced of the "slowdown" hypothesis.

Annual flow of it into the Arctic is greater that the total winter ice volume by a factor of 20.  I have a hard time believing brine expulsion is the primary driver of that.  Even without that, even at higher temperatures, thermal contraction will provide a huge amount of energy to drive the current.
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Ice Shieldz

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Re: Gulf Stream stall
« Reply #14 on: August 02, 2017, 05:58:09 AM »
Assuming air temps are cooler than the water/ice below it, wouldn't thermal contraction also be greater in water that is ice-free or that has thinner ice - which provides less insulation to the air above it?