Dr. Gray at it again...

[x-y-no]

I'm very confused. Where do you see "assumptions about the release of fresh water from the poles" in Bryden et. al.?

that would be the "what if" scenarios

Could you give me an example of a "what if" scenario in Bryden et. al.? I'm still at a complete loss as to what you're referring to.

[Aric Dunn]

simply enough !!
i was going off what windspeed had said.
about the release of fresh water from the poles
have read the previous post?

[Aric Dunn]

which has been repeatedly debunked by people from realclimate, and others. If anything, evidence is pointing to a THC slowdown.

Repeatedly debunked? By whom? Dr. Bryden? RealClimate's article is heavily reliant on Bryden et al.'s findings from 2005, which was based on modeling trends between the AMO and THC (if a significant amount of fresh water was input into the system). The model shows a decrease in salinity and a decrease in the THC. However, I argue that the data to prove that current melt from Greenland currently having any negligible impact on salinity concentrations in the Atlantic is still a great deal unknown. In other words, is the model accurately emulating the relationship between the AMO and THC with a high percentage accuracy based on what is really occurring? There may be variables unaccounted for since the premise of such modeling was also based on "what if" scenarios of an introduction of fresh water like that in North America 8-10,000 years ago. Such a significant introduction of fresh water is an unknown variable because we do not have any such source like the giant freshwater lake that existed in North America 10,000 years ago when continental glaciers were retreating. Furthermore, slow melt water from Greenland may not produce enough freshwater to impact the system in real time. Again, Bryden's paper was based on modeling of "what will happen to the THC if x amount of fresh water is introduced." Clearly, we do not have evidence to suggest this is actually occurring right now.

On the other hand, Dr. Grey's work is based on Knight et al.'s research that incorporates real time measurements that date back to the 19th century; and furthermore, documents actual increases and decreases spanning the past 100 years in the THC, including an increase since the 1970s that was preceded by a decline in the 1940s and 50s. William Grey is also using this research as a one part of his research on AMO and THC decadal trending and Atlantic hurricane activity. He acknowledges other variables when you speak with him, such as lag in cycles of poleward heat transfer, mid-to-upper tropospheric wind patterns and global surface pressures in and around the ITCZ. I would consider Knight's data more than Bryden et al.'s for now until we observe an actual response in the AMO and THC based on model initializations. I would also not be so quick to refute William Grey. There are many climatologists that do not debunk him repeatedly. The RealClimate site you speak of has continually failed to explain increases and decreases in hurricane activity that are not based on anthropogenic influences.

see read..

what so hard

[x-y-no]

simply enough !!
i was going off what windspeed had said.
about the release of fresh water from the poles
have read the previous post?

Ummm ... OK, I guess I missed that claim in windspeed's post. I guess I should be asking windspeed what he/she is talking about.

Bryden et. al. reports on the results of a pair of oceanographic cruises measuring temperature and salinity across the Atlantic basin at 25° N. They compare the measurements with those of a series of cruises every few years since 1957. There are no "what if" scenarios involved.

[Aric Dunn]

and read this...

its helps you understand
http://www.worldclimatereport.com/index ... rculation/

[x-y-no]

see read..

what so hard

That was seriously unneccesary.

I suggest you take your own advice.

[Windspeed]

x-y-o, the I apologize for not clarifying, let me just paste what I had read. So direct your attention at me, not Aric. Although that's difficult at times. Afterall, he is... Aric...

Previously, measurements from ships have been used as a
means of estimating the overturning circulation and associated heat transport. Bryden et al
analysed a new 25° N transatlantic profile and compared it with four previous profiles taken over
the past five decades (measurements taken in 1957, 1981, 1992, 1998 and 2004). Initial results
suggested that the AMOC had slowed by about 30%, mostly between 1992 and 1998, although it
was hard to determine if these observed changes were due to natural variability or longer-term
trends. Investigating this would require a longer, continuous observational record which is
currently unavailable. Computer models have suggested a weakening circulation in a warming
world over the next century, but do not suggest a rapid change of the magnitude identified in
Bryden et al.

The preliminary findings of the RAPID project (2006)

Although the upper 1km of the North Atlantic has been found to be warmer since 1990,
investigation continues into the potential impacts of this warming on the AMOC. Bryden’s
presentation at the RAPID conference in September 2006 suggests that there is evidence for a
small decrease in the overturning circulation, but perhaps not as big as that suggested in his
earlier paper. He also concluded that the inter-annual variability between the years 1980 and
2005 is not significantly different from the variability in the first year of observations undertaken in
the RAPID project (2004 – 2005). Therefore it is still unclear as to whether the observations
gathered to date reflect natural variability within the AMOC or a longer-term trend.
Other research presented at the RAPID conference on the theme ‘Is the Atlantic Overturning
Circulation slowing down?’ provided further clarification of the uncertainties and highlighted the
need for long-term continuous observation and monitoring. The results of modelling future
changes in the AMOC showed a projected slowing of the current in the 21st century with a
recovery in the 22nd century, in simulations where CO2 concentrations are held fixed after 2100.
Reported results indicate that any weakening could be substantially increased by the melting of
the Greenland ice-sheet and, especially, by the rate at which this melting occurs. Modelling this
freshwater input suggests the impact may not be as dramatic as previously thought; with a 42%
weakening of the AMOC for a high melt estimate according to one climate model. Results from a
study by Jungclaus et al. indicate that abrupt climate change caused by the melting of the
Greenland ice sheet does not appear to be a realistic possibility.

Again, sorry, I should have clarified.

[x-y-no]

Reported results indicate that any weakening could be substantially increased by the melting of
the Greenland ice-sheet and, especially, by the rate at which this melting occurs. Modelling this
freshwater input suggests the impact may not be as dramatic as previously thought; with a 42%
weakening of the AMOC for a high melt estimate according to one climate model. Results from a
study by Jungclaus et al. indicate that abrupt climate change caused by the melting of the
Greenland ice sheet does not appear to be a realistic possibility.[/i]

Sorry, I should have clarified.

OK, but you understand that's not from Bryden et. al., right?

The problem with the Bryden result as regards Dr. Gray's hypothesis is that if the THC really has slowed by 30% as indicated, then by Dr. Gray's reasoning we should now be in a slow period for Atlantic tropical activity.

[Windspeed]

Reported results indicate that any weakening could be substantially increased by the melting of
the Greenland ice-sheet and, especially, by the rate at which this melting occurs. Modelling this
freshwater input suggests the impact may not be as dramatic as previously thought; with a 42%
weakening of the AMOC for a high melt estimate according to one climate model. Results from a
study by Jungclaus et al. indicate that abrupt climate change caused by the melting of the
Greenland ice sheet does not appear to be a realistic possibility.[/i]

Sorry, I should have clarified.

OK, but you understand that's not from Bryden et. al., right?

The problem with the Bryden result as regards Dr. Gray's hypothesis is that if the THC really has slowed by 30% as indicated, then by Dr. Gray's reasoning we should now be in a slow period for Atlantic tropical activity.

It was, however, from Byden's own presentation. That's what alarmed me. The idea here is that the original hypothesis which came out several years ago alluded to a shut down of the THC as being a precursor to a cooler Europe and possibly even new continental glaciation event. The explanation was fresh water melt from the poles (Greenland in patricular) was a source and discussed the introduction of the giant fresh water lake from 10,000 and modeled its effects on the THC in the past. But salinity in the Tropical Atlantic has increased since the 1970s according to Knight. And Grey was basing his research on Knight's work. Now if I may offer some of my own observations:

It seems that if the THC was increasing, then poleward heat transfer through currents would increase, decreasing mean OHC in the tropical Atlantic. It also seems to me that if THC was decreasing, salinity would increase in the tropical Atlantic but decrease at higher latitudes, all the way up into the poles, if you consider that the Caribbean and Gulf of Mexico has the highest concentration of salinity in the Atlantic basin, and it also is the source of the Gulf Stream Current. The problem here is that the THC is not necessarily the same current that originates in the Gulf of Mexico and Caribbean. In fact, here is an image:



The warmer and initial salinity based drive is also near near to but several tens of meters below the surface, and moves through the main MDR of the Atlantic, where as the cooler "bottom" current run along the ocean floor, directly underneath. The main upper "warm" current of the THC goes through the eastern Atlantic and middle, and is only joined by the Gulf Stream well north of 25ºN. Ironically at the same time this report came out in 2005, some of our highest temperatures ever recorded were observed in the MDR. The 26º isotherm was near 100 meters in depth at places in the month of July then drastically cooled in August and September. Maybe it was a coincidence that Dennis and Emily formed right over it so early in the season.

[terstorm1012]

Re: the Salinity Increasing in the Atlantic, this reminds me of something I read in the State of the Climate Report 2006 from the AMS, seems the two years of drought the Amazon basin experienced had a major impact on that.

[x-y-no]

It was, however, from Byden's own presentation. That's what alarmed me. The idea here is that the original hypothesis which came out several years ago alluded to a shut down of the THC as being a precursor to a cooler Europe and the explanation was fresh water ocean melt from the poles (Greenland in patricular) was responsible. But salinity in the Tropical Atlantic has increased since the 1970s according to Knight. And Grey was basing his research on Knight's work. Now if I may offer some of my own observations:

A couple of things about this:

I've long been skeptical of the notion that Greenland and polar ice melt could shut down the THC. Mostly, my opinion on that derives from discussions with my dad (who incidentally was the one who initially proposed the draining of Lake Agassiz as the cause of the Younger Dryas episode.)

Second, an increase in salinity in the tropical Atlantic is exactly what one would expect if the THC slowed.

Third, Dr. Gray's hypothesis long predates Knight's work. He's been discussing this hypothesis for decades, at least since the mid-80's.

It seems to me that if the THC was increasing, then poleward heat transfer through currents would increase, decreasing mean OHC in the tropical Atlantic.

Actually, that may be true overall but in the Northern Hemisphere tropics you get a net warming because of the cross-equatorial flow. That's the reason Dr. Gray's hypothesis makes intuitive sense.

It also seems to me that if THC was decreasing, salinity would increase in the tropical Atlantic but decrease at higher latitudes, all the way up into the poles, if you consider that the Caribbean and Gulf of Mexico has the highest concentration of salinity in the Atlantic basin, it also is the source of the Gulf Stream Current. The problem here is that the THC is not necessarily the same current that originates in the Gulf of Mexico and Caribbean.

Yes. And indeed it does appear that the North Atlantic is freshening. See:

Dickson et al. Rapid freshening of the deep North Atlantic Ocean over the past four decades, Nature, 416, 832-836. 2002

In fact, here is an image:



The main upper "warm" current of the THC goes through the eastern Atlantic and middle, and is only joined by the Gulf Stream well north of 25ºN. The warmer and initial salinity based drive is also near near to but several tens of meters below the surface, and moves through the main MDR of the Atlantic.

Well, yes, it's definitely a misnomer to refer to the THC as the "Gulf Stream."

BTW, as long as you posted this, I have a real issue with this representation of the THC. A substantial part (possibly a majority) of the upwelling occurs not in the North Pacific and Indian basins as depicted here, but in the Southern Ocean in the vicinity of the Drake Passage.

Ironically at the same time this report came out in 2005, some of our highest temperatures ever recorded were observed in the MDR. The 26º isotherm was near 100 meters in depth at places in the month of July then drastically cooled in August and September. Maybe it was a coincidence that Dennis and Emily formed right over it so early in the season.

My opinion (only maginally backed up by data) is that the extreme warmth in 2005 in the MDR was largely due to reduced trade winds that year, resulting in less evaporative cooling.

[TCmet]

The problem with the Bryden result as regards Dr. Gray's hypothesis is that if the THC really has slowed by 30% as indicated, then by Dr. Gray's reasoning we should now be in a slow period for Atlantic tropical activity.

This was the point I was trying to get at with my original post. Assuming Bryden et al is valid and the THC is slowing down, Gray's theory would expect decreasing Atlantic TC activity. So given the sharp increase in Atlantic TC activity since 1995... either the THC really isn't slowing down (Bryden et al is wrong), or the link between THC and TC activity isn't as strong as Gray claims.

I'd actually be inclined to believe there's no silver bullet in this case, and that it's probably a mix of many things occuring, some of which are too subtle for us to detect (even if comprehensive ocean observations did exist, past a few midlatitude transects). To me, one of those things that's somewhere in the mix is recent anthropogenic increases of oceanic heat content in the MDR. It's not the entire answer (and surely doesn't explain the complex multidecadal oscillation of atlantic activity all by itself), but it's important.

[TCmet]

also, thanks for the warm welcome!

[Windspeed]

My opinion (only maginally backed up by data) is that the extreme warmth in 2005 in the MDR was largely due to reduced trade winds that year, resulting in less evaporative cooling.

I agree with this and I wish to look into it more. Do you know if the Azores High was weaker or displaced earlier in the year in 2005? Usually, in late summer, surface pressures in the eastern Atlantic decrease. This allows OHC to gain latitude in the MDR. Where as in the early part of summer, surface pressures are much higher and drive lower tropospheric winds much stronger along and just north of the ITCZ. This brings the SAL west over the MDR and with a very low moisture content, allows radiational cooling to bleed off much of the OHC what would be there otherwise. In 2005, I am sure this was the overall influence.

[philnyc]

Here's an assessment of Dr. Gray's theories on the THC and global warming by a group of climate scientists trying to keep the record straight:

http://www.realclimate.org/index.php/archives/2006/04/gray-on-agw/

Some of you may have read this already. For those who haven't, it should prove quite interesting.

[x-y-no]

I admit I haven't looked into the issue of the 2005 warm anomaly in any depth.

[x-y-no]

Here's an assessment of Dr. Gray's theories on the THC and global warming by a group of climate scientists trying to keep the record straight:

http://www.realclimate.org/index.php/archives/2006/04/gray-on-agw/

Some of you may have read this already. For those who haven't, it should prove quite interesting.

One of the very few RealClimate threads I ever participated in. I'm almost exclusively a lurker there for some reason.

[philnyc]

My opinion (only maginally backed up by data) is that the extreme warmth in 2005 in the MDR was largely due to reduced trade winds that year, resulting in less evaporative cooling.

I agree with this and I wish to look into it more. Do you know if the Azores High was weaker or displaced earlier in the year in 2005? Usually, in late summer, surface pressures in the eastern Atlantic decrease. This allows OHC to gain latitude in the MDR. Where as in the early part of summer, surface pressures are much higher and drive lower tropospheric winds much stronger along and just north of the ITCZ. This brings the SAL west over the MDR and with a very low moisture content, allows radiational cooling to bleed off much of the OHC what would be there otherwise. In 2005, I am sure this was the overall influence.

Nice conjecture, windspeed. Here's the climo for the Bermuda Azores High 1968-1996 for June 1st to July 21st:



And here's 2005 for the same period:



The high was not weaker, but there was some displacement to the northeast and the bottom edge and western side were weaker than normal. Actually, I'll go get the wind speeds to see if that's more clear...

[Dean4Storms]

I have no problem with what Dr. Gray is saying, prove him wrong. Hurricane history does not lie neither does observations, but you can get a model to say anything you want.

[wxmann_91]

My opinion (only maginally backed up by data) is that the extreme warmth in 2005 in the MDR was largely due to reduced trade winds that year, resulting in less evaporative cooling.

I agree with this and I wish to look into it more. Do you know if the Azores High was weaker or displaced earlier in the year in 2005? Usually, in late summer, surface pressures in the eastern Atlantic decrease. This allows OHC to gain latitude in the MDR. Where as in the early part of summer, surface pressures are much higher and drive lower tropospheric winds much stronger along and just north of the ITCZ. This brings the SAL west over the MDR and with a very low moisture content, allows radiational cooling to bleed off much of the OHC what would be there otherwise. In 2005, I am sure this was the overall influence.

The surface pressures over much of the Atlantic basin were well below normal from June-July 2005. I strongly believe the weaker Azores High led to less upwelling, more atmospheric instability (weakened trade wind inversion), and an ITCZ that was further north than usual, thus the abnormally active early season.