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Decrease in Atlantic circulation?

Filed under: — group @ 30 November 2005 - (Français)

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 106 m3/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%).

It should be stressed that should this be a sustained feature (and not affected by the +/- 6 Sv uncertainty estimated in the paper), this would be extremely significant. Modelling experiments suggest that this kind of decrease should be associated with a decrease in ocean temperatures in the North Atlantic of up to 2°C or so, and maybe 0.5° over Europe. Since these changes have not been observed (both the North Atlantic and Europe have warmed significantly over this time period), it might be premature to assert that the circulation definitely has changed. Alternatively, the models may not entirely be capturing the fairly complex oceanic processes involved. Continuous monitoring of this section has already been funded through the UK RAPID program and should provide much better data in the future, and a potential solution to this and other remaining puzzles.

It will take some time to integrate the findings of this study with other evidence of changes in North Atlantic ocean circulation, including the changes seen in salinity, changes in the so-called Atlantic Multidecadal Oscillation (AMO) (see e.g. Knight et al, 2005 and references therein) and other indicators of Atlantic climate change (e.g. Dickson et al, 2002). Right now, there isn’t an obvious synthesis of what these disparate studies are telling us.

While this is quite a serious issue, there are a few amusing points. Firstly, this study does present some awkward reading for some who hold that natural cyclical changes in the thermohaline circulation (rather than, say, anthropogenic influences), are responsible for the anomalous increase in Atlantic Hurricane activity in recent decades. Hurricane prognosticator William Gray (whose public statements we have commented on previously), has, in his recent senate testimony, confidently asserted that a putative increase in the intensity of the Atlantic Thermohaline circulation over recent decades was entirely responsible for this increase:

The Atlantic has large multi-decadal variations in major (category 3-4-5) hurricane activity. These variations are observed to result from multi-decadal variations in the North Atlantic Thermohaline Circulation (THC) – Fig. 4. When the THC is strong, it causes the North Atlantic to have warm or positive Sea Surface Temperature Anomalies (SSTA) and when the THC is weak, cold SSTAs prevail. Figure 5 shows these North Atlantic SSTAs over the last century with a projection for the next 15 years.

By Gray’s very clearly articulated reasoning, there should have been a downturn, not the observed upturn in major Atlantic hurricane activity over the past several decades (in the absence of other—including anthropogenic–influences on tropical Atlantic climate) if Bryden et al.’s results are correct. It will be interesting to see if Gray, and others, will change their line of argument in the face of this new study. Today, the last day of the official 2005 Atlantic Hurricane season, might be a fitting opportunity for them to do so.

Secondly, since the Gulf Stream itself is remarkably stable in these analyses, headline writers may have to adjust the standard ‘Gulf Stream may reverse’ titles that they normally come up with when dealing with this topic (though we note that The Independent succumbed anyway) . See here for a previous discussion.

And finally, for those of you who frequent some of the more contrarian websites, tried to pull a bit of a fast one in predicting ‘imminent’ headlines on this issue on Tuesday (Nov 29th). This was before the official release of the paper, but after the embargoed copies of this paper had been sent out, which as journalists they would have seen, but their readers had not. You didn’t need to be Nostradamus to predict the headlines here! They were trying to suggest that ‘alarmists’ will automatically blame the ocean circulation for the current wintry weather in Europe. Since the Bryden paper is talking about a multi-decadal trend, this week’s weather is obviously not relevant…

Harry L. Bryden, Hannah R. Longworth and Stuart A. Cunningham, Slowing of the Atlantic meridional overturning circulation at 25° N, Nature, 438, 655-657. 2005
Dickson et al. Rapid freshening of the deep North Atlantic Ocean over the past four decades, Nature, 416, 832-836. 2002
Knight, J.R., Allan, R.J., Folland, C.K., Vellinga, M., Mann, M.E., A signature of persistent natural thermohaline circulation cycles in observed climate, Geophysical Research Letters, 32, L20708, doi:10.1029/2005GL024233, 2005.

60 Responses to “Decrease in Atlantic circulation?”

  1. 51
    Mauri pelto says:

    Gavin- Thank you. I am sure about the output in cubic kilometers from the glaciers and I am sure I am wrong as you point out with the Sverdrup calculation, haste made waste in this case. The point I wanted to make and is better made with the correction, is that the contribution of individual glacier in terms of the current flows that have been discussed in this topic are small. .0012sv for Jakobshavns Isbrae the glacier with the greatest volume of flow in Greenland. And that is the consistent contribution the recent change represents only a fraction of this. Helheim and all of the other glaciers an even smaller flow contribution.

  2. 52
    Hank Roberts says:

    Another pointer to a journalist’s weblog (that commends this particular Realclimate thread) quoted from Benny Peiser today:
    It’s an odd day when I find myself agreeing with Benny Peiser on a climate
    change question, but his post Thursday to his CCNet list, in which he
    blasted media of coverage of the Nature paper on possible changes in the
    thermohaline circulation, seems on point.
    –John Fleck, 3 December 2005 <--- refers to this Realclimate thread End Quote

  3. 53
    Lynn Vincentnathan says:

    RE #29 on alarmist warnings. They aren’t alarming anyone I know. And even if they did, what would people do, rush to the store to buy energy efficient appliance? Watch out for those colliding shopping carts!

    No, unfortunately environmental problems, no matter how serious & deadly they may turn out to be, just don’t stir people to action much. Too much competition from terrorism, TV, Monday night football, and that Saturday night date.

    Even if we totally exaggerate & double the worse case scenario & accompany it with really scary percussive music, I can’t see it winning over many new converts to an energy/resource efficient/conservative lifestyle. I can’t even appeal to people’s economic self-interests with “save $$ while saving the earth.”. People would rather burn their money like heaps of fall leaves. It’s like everyone’s already dead, drugged, or tuned totally out. So I guess the worse case scenario really could (possibly, if not likely) happen – since people aren’t doing much to avert it.

    So, what is it re this post: Is it now more likely that GW has contributed to Katrina, than it was before this new info about the ocean conveyor? Maybe 2″ instead of 1″?

  4. 54

    James Hansen has been arguing, for instance here,

    that Greenland might be much more unstable than the IPCC has assumed. He suggests that in a warming world, surface melting leads immediately to basal lubrication and acceleration of iceflows. This correlation documented in Greenland by Zwally et al.

    Hansen suggests this is similar to the process at the root of very rapid sea level rises during past deglaciations (eg Heinrich events). He argues that deglaciation of Greenland might be a process that takes O(100yrs), rather than O(1000yrs). Thus, given the delays in the system: both the ocean responding to CO(2), and the delays in humanity changing it’s behavior, there is a risk of guaranteeing a future deglaciation of Greenland before drastic changes are observed (with the attendant O(7m) rise in sea level).

    It sounds like he’s now concerned that the point of no return could be crossed in a decade:

    I’m curious what RealClimate contributors think of his line of thought?

  5. 55
    smyyga says:

    This is just out, they seem to confirm Rignot’s findings on Kangerdlugssuaq- and Helheim- glaciers:

  6. 56
    Steve Bloom says:

    I assume someone must be working on something similar to for Greenland. This, combined with a physical model for the ice dynamics (does one exist yet?) tested against the observed behavior of the ice and the usual assumptions about the future of the climate in the Greenland region over the coming decades, comprise the pieces of the puzzle needed to come up with an estimate of how fast things are really proceeding in Greenland. Does anyone know the status of this stuff?

  7. 57
    Tim Jones says:

    How do Vostok, Dome C and other Antarctic and Greenland ice core records of historic levels of atmospheric CO2 compare with changes in THC and the AMO?

  8. 58
    Hank Roberts says:

    Are you asking for a comparison of the ice cores with some other data set that tracks the ocean currents and atmospheric circulation?

    Maybe seafloor cores for currents and lakebed sediment cores for air?

    I’m just a bystander/reader here, trying to make sense of your question — how do you find a basis for comparison?

  9. 59
    smyyga says:

    Rignot’s abstract from the AGU fall meeting:

    Leakage of the Greenland Ice Sheet through accelerated ice flow
    AU: * Rignot, E
    AF: Jet Propulsion Laboratory/Caltech, MS 300-319 4800 Oak Grove Drive, Pasadena, ca 91109-8099 United States
    AB: A map of coastal velocities of the Greenland ice sheet was produced from Radarsat-1 acquired during the background mission of 2000 and combined with radio echo sounding data to estimate the ice discharge from the ice sheet. On individual glaciers, ice discharge was compared with snow input from the interior and melt above the flux gate to determine the glacier mass balance. Time series of velocities on several glaciers at different latitudes reveal seasonal fluctuations of only 7-8 percent so that winter velocities are only 2 percent less than the yearly mean. The results show the northern Greenland glaciers to be close to balance yet losing mass. No change in ice flow is detected on Petermann, 79north and Zachariae Isstrom in 2000-2004. East Greenland glaciers are in balance and flowing steadily north of Kangerdlussuaq, but Kangerdlussuaq, Helheim and all the southeastern glaciers are thinning dramatically. All these glaciers accelerated, Kangerdlussuaq in 2000, Helheim prior to 2004, and southeast Greenland glaciers accelerated 10 to 50 percent in 2000-2004. Glacier acceleration is generally brutal, probably once the glacier reached a threshold, and sustained. In the northwest, most glaciers are largely out of balance. Jakobshavn accelerated significantly in 2002, and glaciers in its immediate vicinity accelerated more than 50 percent in 2000-2004. Less is known about southwest Greenland glaciers due to a lack of ice thickness data but the glaciers have accelerated there as well and are likely to be strongly out of balance despite thickening of the interior. Overall, I estimate the mass balance of the Greenland ice sheet to be about -80 +/-10 cubic km of ice per year in 2000 and -110 +/-15 cubic km of ice per year in 2004, i.e. more negative than based on partial altimetry surveys of the outlet glaciers. As climate continues to warm, more glaciers will accelerate, and the mass balance will become increasingly negative, regardless of the evolution of the ice sheet interior.

  10. 60
    N Guda says:

    I am not a climatologist, but a biologist with a decent understanding of statistics. Biological systems frequently have very large uncertainties associated with them, much like the uncertainties reported in Bryden. It seems to me that the data reported in this paper is interesting because of the continued trend despite the uncertainties.

    Is there a reason why the error (±6 Sv) might be in the same direction for all of the surveys, or at least for the two most recent surveys (1998 & 2004)? If so, an error of that size is an important issue. If there is no reason to assume that the errors would be in the same direction, then I would not fault them for their interpretation.

    This is a wonderful site. Thanks to all who have contributed to these discussions.