CO2-Amplifier-in-the-Physical-Climate-System-(62703) by sdaferv


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									Opening Remarks for Ocean Circulation Session Cambridge 10 March 2008
J. R. Toggweiler Geophysical Fluid Dynamics Laboratory / NOAA
Princeton, NJ 08542 USA

I've been intrigued ever since I got out of graduate school by the CO2 changes in the atmosphere that took place along with the ice ages. Wally Broecker showed us early on that these CO2 changes were a response to something that happened in the ocean and it was always fairly clear that whatever that something turned out to be, it was going to be pretty major. I work at a lab that builds climate models, and the models that we build should be able to say, from first principles, how the ocean would be different in a warmer or colder climate. But every time I look at the ocean that our models produce for the Last Glacial Maximum, I fail to see anything that could accommodate or explain the CO2 changes. So, this has made me somewhat skeptical about what our models have been saying about the ocean in our warmer future. The generic forecast starts with an expectation that the Earth's hydrological cycle will be stronger in a warmer climate. This means that there will be a larger input of fresh water into the polar oceans from precipitation and rivers. The enhanced fresh water input and the warming of the surface from higher CO2 should make the surface waters in the polar regions less dense and less able to sink. This leads to a weaker overturning circulation in the models. The first generation of coupled models said that some weakening of the overturning should be apparent by now. Parts of the polar oceans have indeed become fresher, as expected, but no one has noticed that the overturning has weakened.


One of the consequences of a weakened overturning is that the warming of the ocean is confined to the ocean's near surface layers. This means that the surface layers warm more for a given CO2 increase, which means that the atmosphere warms a bit more too. If the circulation managed to stay the same or get stronger, the surface ocean and atmosphere would not warm as fast because the heat taken up at the surface would be distributed more evenly through the interior. My reading of the situation during the ice ages is that the ocean during the cold glacial state had a weak circulation. This was especially true for the deep and bottom waters with an origin around Antarctica. The circulation then got stronger as the Earth warmed. Do you see the problem? Our models are saying that the overturning circulation, if anything, will weaken as the Earth warms in the future. But the overturning got a lot stronger when the Earth warmed in the past. Despite the stronger hydrological cycle that came with the warming climate in the past, the overturning managed to get a lot stronger. So, how did this happen? The answer seems to be that the winds got stronger, or at least, the wind effect on the ocean got stronger. The winds drive the gyre circulations in the upper ocean, which mix the salty waters in the subtropics with the fresher water near the poles. This stirring limits the salinity differences that we see in the ocean. The winds also stir the deep ocean by drawing water from the interior up to the surface. So, if the wind effect got stronger as the Earth warmed there would be more mixing and stirring of the interior. This extra mixing and stirring explains how so much CO2 was able to escape from the deep ocean up to the atmosphere. A few years ago, Joellen Russell came to work with me as a new postdoc and she brought with her something that meteorologists had started to notice about the midlatitude westerly winds that blow around the world between Australia, Africa, and Antarctica. The westerlies in this part of the world have been getting stronger and the location of the strongest winds has shifted poleward over the last 40 years or so. This is a very interesting part of the world oceanographically because of the band of ocean in this part of the world that is not blocked by land. There is a big current in this 2

band of ocean, called the Antarctic Circumpolar Current or ACC, which is driven around and around the world by the mid-latitude westerlies. The ACC extends to the bottom and has an impact on the circulation everywhere where else, even, I dare say, in the Arctic Ocean. The ACC is limited by the continents to a more southerly position than the westerly winds. Thus, the strongest westerlies do not overlie the core of the ACC. However, because they have been shifting poleward, the strongest westerlies are now better aligned with the ACC than they were before and are in a better position to produce a more vigorous circulation throughout the ocean. So Joellen and I said to ourselves, aha, if this is a general tendency, i.e. if the westerlies shift poleward in a warming climate and shift equatorward in a cooling climate, then the circulation in the cold glacial ocean was extremely weak because the band of westerlies was too far to the north of the ACC to have much of an impact on the circulation. If the westerlies then got stronger and shifted poleward as the Earth warmed they would have come into better alignment with the ACC. This would have produced a more vigorous circulation and more mixing in the ocean in time to release a large amount of CO 2 up to the atmosphere. The extra CO2 in the atmosphere would have warmed the Earth further, leading to an even larger wind effect, more mixing and stirring, more CO 2 in the atmosphere, etc. This tendency of the westerly winds to get stronger and to shift poleward in a warming climate is now showing up in our climate models. The tendency is not as pronounced as it is in the real world but the models are clearly getting better. The predicted weakening of the overturning is also not as pronounced. Some models are not calling for any weakening. A lack of weakening is a far cry, however, from the very obvious strengthening that took place in the past. Thus, Joellen and I would argue that the models are not there yet. The ocean’s circulation is going to get significantly stronger because the wind effect is going to get significantly bigger. It should be clear in a few years if this speculation is correct. If we start to see warmer and more poorly oxygenated water in the overflow regions south of Iceland or 3

along the Antarctic continental slope then the freshening and warming would appear to be winning out. On the other hand, if we continue to see cold well-oxygenated water in these locations then the freshening and warming at the surface is not winning out. If we start to see saltier water at or near the surface around Antarctica then we will know that stronger winds are negating or even overwhelming the freshening and warming tendencies as they did at the end of the last ice age. If this is the case, the interior of the ocean will warm faster and the atmosphere and surface ocean will warm less fast than predicted by the early climate models. In summary, it is no longer a mystery why there was more CO2 bottled up in the deep ocean during the last ice age and it is no longer a mystery how it came out. To paraphrase Bill Clinton's famous campaign quip from 1992 − it's the circulation, stupid. And one does not get a major change in the circulation without a major change in the winds. The wind effect is stronger now than it was 40 years ago and it could get a lot stronger. I think we will have a better mixed and better ventilated ocean at the end of this century than we have now despite all the warming and freshening that will occur. You may be bothered by the fact that I do not distinguish between the winddriven circulation in the upper ocean and the less-obviously wind-driven circulation in the interior. This is intentional. My sense is that the wind forcing dominates at the surface and in the interior. Jean Lynch-Stieglitz showed about ten years ago that the transport of the Gulf Stream was lower at the LGM than it is now. I think Jean’s result is telling us that both components of the Gulf Stream were weaker at the LGM, the gyre component and the overturning component. Both were weaker for the same reason – because the wind effect was weaker.


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