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, JunkScience.com 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.