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Natural Climate Variability on Decade-to-Century Time Scales
layer can be colder in capped oceans than in overturning oceans. Simplistically, relatively fresh surface water can be cooled to freezing without overturning. So on very long time scales, the saltier ocean has the greater influence, and that probably comes down to the one that has the evaporative Mediterranean.
For deep-water formation in the Atlantic, the depth to which the overturn extends—and probably the surface temperature as a result—depends on the history of the overturn. Thus, if you want to understand the surface temperature or the temperature of the upper 500 meters, you have to be able to follow the history of the overturn.
I have one question I would like to pose to both Dr. Lazier and Dr. Dixon: Is there a possibility of capping off the North Atlantic completely in the current regime? You see the anomaly coming through. Could that anomaly be large enough or sustainable enough that North Atlantic conditions would be significantly changed?
It is clear to the oceanographic community that salinity is really important; it controls ventilation and overturning. Sea ice, evaporation, precipitation—all of which are things that we do not measure very well—are therefore important as well. If you have a global observing system without salinity measurements, you will be looking only at the result of what is going on; you will not be getting at the mechanism.
Obviously, salinity is not the whole answer; you need to know what the atmosphere is doing, and the forcing and all the interactions. But without salinity, you are stuck because the salinity controls overturn. The TOGA people are finding that to be true in the tropical Pacific. If you are thinking about where to monitor in a global observing system, you need to follow the history of the overturns, identify where overturns generally appear, and monitor them all the way through the depth of overturn. In the North Atlantic, you have to care about Labrador Sea Water down to where it shows up, and about overflow water—in short, the whole water column in the northern North Atlantic. In the North Pacific, I agree with Claes Rooth that you do not need to worry about deep water, but you might want to measure to 1500 meters. In each of the ocean basins some sort of argument can be made for where to monitor and, if you are looking at the water-mass structure, how deep to go.
We know very little about time scales, since we have an incredible dearth of time series. We are fortunate to have had the ocean weather ships in the past, as well as efforts such as John Lazier's to continue making measurements in the Labrador Sea. In the Pacific, there are equivalent data sets only in the California Current and around Japan.
Dr. Lazier's paper has demonstrated nicely the effect of two consecutive salinity anomalies. In both the convection was capped, with big property changes down below. A trend toward decreasing salinity and temperature can also be seen.
I had several questions in relation to the data shown. I was interested in the little blips of higher salinities during the capping. What is the source of the high salinity at depth during capping?
Second, I wondered whether there are enough data to tell whether there are 10- to 15-year cycles, or whether the overall decrease is part of a longer cycle. The earlier data are rather sporadic, so it may not be appropriate to use them to look at trends. And how are the two cycles related to the cycles that Syd Levitus showed?
And my last question is, how cold would you have had to make the water during those capping events in order to break through, and what kind of heat flux would it require?
LAZIER: Capping the low-salinity water in the late 1970s was coincident with mild weather, and the same was true in the early 1980s. The succeeding severe weather resulted in mixing. Incidentally, two recent records indicate that the very-low-salinity Labrador Sea water is showing up in the Irminger Sea.
DICKSON: I think the real question was how you cap convection to the point that you interrupt North Atlantic Deep-Water production in some way. To start with, I'd say that we should be using the word "entrainment" rather than "convection" there. It's what's coming over the sill and doubling itself you worry about, not the deep-water convection where it got started.
Now if we compare the Denmark Strait overflow with the Mediterranean and gets up to a faster speed. It therefore entrains a large volume of unrelated, much less dense water—it multiplies itself by three—and comes down the slope with lots of turbulence. It ends up coming across at mid-depth, while the colder, more homogeneous Denmark Strait water outflow, the latter starts off denser, comes up a steeper slope, goes all the way to the bottom.
For the Greenland Sea, it would be easier to effect model changes by altering what is entrained rather than the density of the top 400 meters of the Icelandic Sea. And that could indeed change the production of deep water.
SWIFT: I wonder whether Walter Munk would comment on the scales of convection.
MUNK: Yes, I have a slide [see Munk, discussion after Lazier paper, in the color well] from a Greenland Sea experiment of