Decrease in Atlantic circulation?

[x-y-no]

by Gavin Schmidt and Michael Mann

In a sure-to-be widely publicized paper in the Dec. 1 Nature, Bryden et al. present results from oceanographic cruises at 25°N across the Atlantic showing a ~30% decline in the ocean overturning circulation. These cruises have been repeated every few years since 1957, and the last two cruises (in 1998 and 2004) show notable changes in the structure of the deep return circulation. In particular, the very deepest part of the return flow (at around 3000 to 5000 m) has reduced and moved up in the water column compared to previous decades. How solid is this result and what might it imply for climate?

The first question that is asked is usually how these calculations are done. Due to the predominantly "geostrophic" nature of the ocean circulation (i.e. velocity is generally horizontally perpendicular to pressure gradients because of the Coriolis effect), you can calculate changes in North-South velocities by only considering the East-West changes in temperature and salinity. So given a section across the ocean (say 25°N), oceanographers can estimate the transport across that section. The error in these numbers is a little hard to know, but Bryden et al estimate around +/- 6 Sv (1 Sv is 10^6 m^3/s, the Amazon output is around 0.1 Sv for perspective).

What did Bryden et al find? Their calculations indicate that the Gulf Stream itself has been remarkably stable over the almost 40 years, and this accords with other measurements of the Gulf Stream flow itself. Since what goes north must eventually go south (after taking into account the very small amounts of atmospheric transport and the amount of flow through the Bering Strait), all of the other changes will balance. They show that the amount of deep return flow seems to have gone down about 8 Sv (out of 25 Sv), and the amount of mid-ocean to surface transport has gone up by about the same amount. This corresponds to a roughly 30% apparent weakening in the so-called "Thermohaline Circulation" (see our previous discussion here). Since the surface flow is warmer than the deep flow, there is a consequent decrease in the northward heat flux of about 0.2 PW (or about 15%).

http://www.realclimate.org/index.php?p=225

I haven't had a chance to read this paper yet, but I thought I'd post this link and discussion here, since if this result stands up it does impact the issue of influences on the activity of the Atlantic hurricane season.

There are definitely some odd aspects to this, since the prevailing theory for our current active storm phase is predicated in part on and increased rate of the THC, whereas this data indicates a decreased rate. Furthermore, if this has been going on for any extended time, we would expect to see some cooling of the north Atlantic surface waters, and we're seeing the opposite.

Jan

[DoctorHurricane2003]

Hmm I just had a thought.

We all know that the water that comes back in the thermohaline circulation from the north is going to be cooler, even though it is a deep water current.

Perhaps what we see in "Active" THC cycles (where active means more ATLC hurricanes), the THC decreases. While the THC is on the increasing side, perhaps slight increase of upwelling of cooler waters in tropical regions is responsible for the decrease in SST anomalies/hurricanes.

Just some passing thoughts as I read this.

[P.K.]

I haven't read this but I know pervious studies have claimed a decrease in the THC which could make it interesting over here given how it keeps us warmer over here.

How is an increased THC going to reduce TC activity? (Almost midnight so I'm probably missing something obvious) As far as I can tell there would be more upwelling as a result and less sinking at the poles? Quite worrying what could happen though if the THC reduced so much it was able to flow the other way, might get a tiny bit cold here.

[DoctorHurricane2003]

That's what I was thinking, PK

[P.K.]

Yes I took a while to write that by looking through some notes so you beat me to it. I'll go through it properly tomorrow.

[brunota2003]

Here is the article on it from Yahoo!: http://news.yahoo.com/s/nm/20051130/sc_ ... t_ocean_dc
My Theory: The Warm Water pools up in the tropics, and temps at higher latitudes get around average to maybe a touch above, because of TC's, but in the tropics, the SST's get warmer than average, resulting in more, and more intense TC's, thus you have this season...

[Dr. Jonah Rainwater]

Wouldn't shutting down the Gulf Stream also shut down the Loop Current?

...Or would all that warm tropical seawater just sit in the tropics and simmer because none of the heat is being transferred north? In the absense of the Gulf Stream, hurricanes would be pretty much nature's ONLY way of maintaining the global heat balance.

[DoctorHurricane2003]

Shutting down the Gulf Stream is just about a physical impossibility unless the Earth stopped rotating. It can go weaker and grow stronger, but shutting it down is not going to happen.

[x-y-no]

I haven't read this but I know pervious studies have claimed a decrease in the THC which could make it interesting over here given how it keeps us warmer over here.

How is an increased THC going to reduce TC activity? (Almost midnight so I'm probably missing something obvious) As far as I can tell there would be more upwelling as a result and less sinking at the poles? Quite worrying what could happen though if the THC reduced so much it was able to flow the other way, might get a tiny bit cold here.

No, an increased THC would increase activity, at least according to Dr. Gray's ideas. The reason for this is that the THC crosses the equator, providing a net heat transport from the southern hemisphere to the northern. So an increased THC warms the entire north Atlantic basin.

So this is what's perplexing about this result - if the THC really has slowed to this extent, then what's causing the observed warming in the basin? There are the changes in the wind-driven gyre, but are they adequate to explain it, or is something oing on with the large-scale circulation that we don't understand?

[x-y-no]

Wouldn't shutting down the Gulf Stream also shut down the Loop Current?

...Or would all that warm tropical seawater just sit in the tropics and simmer because none of the heat is being transferred north? In the absense of the Gulf Stream, hurricanes would be pretty much nature's ONLY way of maintaining the global heat balance.

The Gulf Stream is for the most part a component of the wind-driven gyre, and has not changed appreciably in the last 40 years at least.

[terstorm1012]

http://news.bbc.co.uk/1/hi/sci/tech/4485840.stm

[Jim Hughes]

Wouldn't shutting down the Gulf Stream also shut down the Loop Current?

...Or would all that warm tropical seawater just sit in the tropics and simmer because none of the heat is being transferred north? In the absense of the Gulf Stream, hurricanes would be pretty much nature's ONLY way of maintaining the global heat balance.

Mother nature has built in system and she seems to have been doing a pretty good job so I am sure she could dig up some old recipes to handle the warmer tropical SST's. Different cloud cover patterns. A different smaller THC...etc..

The THC is complicated and one has to consider the memory or residual effect in what we are seeing now. Sure they are related to the current atmospheric feedbacks but it also related prior conditions.

We know about the salinity and water temperature part of the THC as it goes from the tropics to the polar latitudes but you have to consider what were the variables like before.

There is about a 10-14 year gyre in the NA as well as some longer ones involving the AMO and NAO and some of them are very long.

So let's just use some bogus numbers here to make my point. The warmer and more saline SST's are brought north by the gulf stream and they cool. The water cools and the salt content makes it sink.

Okay so we know that both the water temperature and the salt content play a role. Right? But we also should know from observations that both the water temperatures and it's salinity can very from time to time.

So the THC continues during one year when the tropical waters coming up are in the 80-85 range (I know they cool before hand ) and the waters way up north are 37-42 degrees.

In another year the SSt temperatures shift a few degrees. All of this plays into the whole picture. Eddies or pockets of warmer water will exist below the surface compared to their surroundings. They will eventually come up in both the long and short term cycles.


Jim

[ggaryg]

My understanding of the theory is that as global warming causes the earth to heat up, there is an increase in melting of the polar ice fields resulting in a tremendous increase of fresh water. It is the cooler fresh water which is disturbing the thermal conveyor belt. It has been postulated that previous mini ice ages, like the one in the 1600's were caused by this effect.

[x-y-no]

My understanding of the theory is that as global warming causes the earth to heat up, there is an increase in melting of the polar ice fields resulting in a tremendous increase of fresh water. It is the cooler fresh water which is disturbing the thermal conveyor belt. It has been postulated that previous mini ice ages, like the one in the 1600's were caused by this effect.

There is significant evidence of freshening of the surface waters in the North Atlantic (see for example http://www.sciencemag.org/cgi/content/abstract/308/5729/1772) and that could indeed play a role in slowing the THC. Until this new publication, however, we have had no direct evidence of such a slowing.

[no advance]

* 18:00 30 November 2005
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The ocean current that gives western Europe its relatively balmy climate is stuttering, raising fears that it might fail entirely and plunge the continent into a mini ice age.

The dramatic finding comes from a study of ocean circulation in the North Atlantic, which found a 30% reduction in the warm currents that carry water north from the Gulf Stream.

The slow-down, which has long been predicted as a possible consequence of global warming, will give renewed urgency to intergovernmental talks in Montreal, Canada, this week on a successor to the Kyoto Protocol.
NS Forum
What do you think about these dramatic findings?
Discuss this story >>

Harry Bryden at the National Oceanography Centre in Southampton, UK, whose group carried out the analysis, says he is not yet sure if the change is temporary or signals a long-term trend. "We don’t want to say the circulation will shut down," he told New Scientist. "But we are nervous about our findings. They have come as quite a surprise."
No one-off

The North Atlantic is dominated by the Gulf Stream – currents that bring warm water north from the tropics. At around 40° north – the latitude of Portugal and New York – the current divides. Some water heads southwards in a surface current known as the subtropical gyre, while the rest continues north, leading to warming winds that raise European temperatures by 5°C to 10°C.

But when Bryden’s team measured north-south heat flow last year, using a set of instruments strung across the Atlantic from the Canary Islands to the Bahamas, they found that the division of the waters appeared to have changed since previous surveys in 1957, 1981 and 1992. From the amount of water in the subtropical gyre and the flow southwards at depth, they calculate that the quantity of warm water flowing north had fallen by around 30%.

When Bryden added previously unanalysed data – collected in the same region by the US government’s National Oceanic and Atmospheric Administration – he found a similar pattern. This suggests that his 2004 measurements are not a one-off, and that most of the slow-down happened between 1992 and 1998.

The changes are too big to be explained by chance, co-author Stuart Cunningham told New Scientist from a research ship off the Canary Islands, where he is collecting more data. "We think the findings are robust."
Hot and cold

But Richard Wood, chief oceanographer at the UK Met Office’s Hadley Centre for climate research in Exeter, says the Southampton team's findings leave a lot unexplained. The changes are so big they should have cut oceanic heating of Europe by about one-fifth – enough to cool the British Isles by 1°C and Scandinavia by 2°C. "We haven’t seen it yet," he points out.

Though unseasonably cold weather last month briefly blanketed parts of the UK in snow, average European temperatures have been rising, Wood says. Measurements of surface temperatures in the North Atlantic indicate a strong warming trend during the 1990s, which seems now to have halted.

Bryden speculates that the warming may have been part of a global temperature increase brought about by man-made greenhouse warming, and that this is now being counteracted by a decrease in the northward flow of warm water.

After warming Europe, this flow comes to a halt in the waters off Greenland, sinks to the ocean floor and returns south. The water arriving from the south is already more saline and so more dense than Arctic seas, and is made more so as ice forms.
Predicted shutdown

But Bryden’s study has revealed that while one area of sinking water, on the Canadian side of Greenland, still seems to be functioning as normal, a second area on the European side has partially shut down and is sending only half as much deep water south as before. The two southward flows can be distinguished because they travel at different depths.

Nobody is clear on what has gone wrong. Suggestions for blame include the melting of sea ice or increased flow from Siberian rivers into the Arctic. Both would load fresh water into the surface ocean, making it less dense and so preventing it from sinking, which in turn would slow the flow of tropical water from the south. And either could be triggered by man-made climate change. Some climate models predict that global warming could lead to such a shutdown later this century.

The last shutdown, which prompted a temperature drop of 5°C to 10°C in western Europe, was probably at the end of the last ice age, 12,000 years ago. There may also have been a slowing of Atlantic circulation during the Little Ice Age, which lasted sporadically from 1300 to about 1850 and created temperatures low enough to freeze the River Thames in London.

Journal reference: Nature (vol 438, p 655).
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Now I am really confused. No advance

[P.K.]

Though unseasonably cold weather last month briefly blanketed parts of the UK in snow, average European temperatures have been rising, Wood says. Measurements of surface temperatures in the North Atlantic indicate a strong warming trend during the 1990s, which seems now to have halted.

lol, what does the snow last week have to do with the NAD?

[P.K.]

No, an increased THC would increase activity, at least according to Dr. Gray's ideas. The reason for this is that the THC crosses the equator, providing a net heat transport from the southern hemisphere to the northern. So an increased THC warms the entire north Atlantic basin.

So this is what's perplexing about this result - if the THC really has slowed to this extent, then what's causing the observed warming in the basin? There are the changes in the wind-driven gyre, but are they adequate to explain it, or is something oing on with the large-scale circulation that we don't understand?

If it spreads further into the North Atlantic then surely the heat loss due to Sandstrom's theorum would help to partly cancel this out? (I'm well aware it isn't that simple though)

If the changes aren't salinity driven then the other possible factors are changes to the surface wind stress or some sort of tidal forcing.

Is this really the first publication to show a slowing I've heard things about it slowing before this.

[x-y-no]

If it spreads further into the North Atlantic then surely the heat loss due to Sandstrom's theorum would help to partly cancel this out? (I'm well aware it isn't that simple though)

Yeah, I'm going to have to do a lot more thinking about this before I can claim to have a handle on what's going on.

If the changes aren't salinity driven then the other possible factors are changes to the surface wind stress or some sort of tidal forcing.

If the figures in Bryden et al. are accurate, I don't see how it could be anything but salinity-driven, given the size of the measured change. And as I noted above there has been substantial freshening of the north Atlantic surface waters recently.

But the question is why don't we see the expected cooling of the surface waters in the north Atlantic, if this slowdown is real?

Is this really the first publication to show a slowing I've heard things about it slowing before this.

I've seen publications of model results which predicted a slowdown of the THC, and also some research showing localized decrease in subsidence off the Greenland coast, but this is the first time I've seen direct evidence that the entire THC has slowed.

[P.K.]

If the figures in Bryden et al. are accurate, I don't see how it could be anything but salinity-driven, given the size of the measured change. And as I noted above there has been substantial freshening of the north Atlantic surface waters recently.

But the question is why don't we see the expected cooling of the surface waters in the north Atlantic, if this slowdown is real?

8 Sverdrups is a large difference I'll admit that. - Looking at my notes it appears there is claimed to be a consistent freshening of the surface layers and NADW since the 1960s. (Dickson et al. 2002) This could either be due to changes in the atmosphere eg. NAO changes, or an indication of a THC weakening. Most of the models that have been made seem to show a decrease in the strength with time bu there are large variations in the different models. Models have been run with increases in CO2 levels due to AGW and theses showed a substancial decrease in the tranpsort in the North Atlantic (Manabe and Stouffer 1994).

The reason I was saying it isn't necessarily salinity driven is due to what I have been taught in the last couple of weeks that it is suspected that it is more likely to be mechanically driven (Tidal forcing, or Wind forcing). My lecturer is bound to mention this next Tuesday so I'll be interested to see what he has to say.

Nothing is certain though which is why this area is good for research at the moment.

[Jim Hughes]

No, an increased THC would increase activity, at least according to Dr. Gray's ideas. The reason for this is that the THC crosses the equator, providing a net heat transport from the southern hemisphere to the northern. So an increased THC warms the entire north Atlantic basin.

So this is what's perplexing about this result - if the THC really has slowed to this extent, then what's causing the observed warming in the basin? There are the changes in the wind-driven gyre, but are they adequate to explain it, or is something oing on with the large-scale circulation that we don't understand?

If it spreads further into the North Atlantic then surely the heat loss due to Sandstrom's theorum would help to partly cancel this out? (I'm well aware it isn't that simple though)

If the changes aren't salinity driven then the other possible factors are changes to the surface wind stress or some sort of tidal forcing.

Is this really the first publication to show a slowing I've heard things about it slowing before this.

There was a paper written a while back about the great saline anomaly. I forget when it was written but it was related to the possible slow down of the THC if I remember correctly. The data they were researching was much older. Sometime in the early to mid 70's I believe. Of course we know it never shut the THC but I think it may have slowed it down.


Jim