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Natural Climate Variability on Decade-to-Century Time Scales
Commentary on the Paper of Mysak
DOUGLAS G. MARTINSON
Lamont-Doherty Earth Observatory
I want to start by commending Dr. Mysak for attempting to combine a wide variety of observations and processes into a single feedback model. Having said that, I want to play a "friendly devil's advocate." First, as you point out, there is a decrease in the Siberian river runoff into the Arctic that coincides with an increase in the Mackenzie River runoff. What is the net effect of that sort of competition?
That leads to the next point, the influence of runoff on ice growth. Are you suggesting that the runoff influences the ice growth through its impact on stability? If so, is the Beaufort Sea less stable than the region receiving the Siberian river inflow? Or are you suggesting some other mechanism? Alternatively, might the increased runoff be indicative of some other atmospheric anomaly (e.g., winds, temperature, cloud cover) that more directly affects the ice cover? Also, do you know whether the increased areal distribution of ice actually represents more fresh-water? Spreading of the ice in the Greenland Sea by increased winds would also lead to thinner ice, in which case the net volume of fresh-water has not necessarily changed.
Now for some general points. You suggest that local convection changes the SST, which in turn influences the cyclogenesis. As Walter Munk asked previously, can the small spatial scales of convection significantly influence SST? In other words, how exactly do you envisage the convection's influencing SST to the extent that it could affect cyclogenesis? Might not the lateral influences be more important? Also, if the cyclogenesis in the Greenland Sea is increased, it is still difficult for me to imagine that it drives increased precipitation so far to the west.
Finally, a comment regarding the shutdown of convection in response to an increase in advected sea ice (fresh-water). I just want to point out that in the Weddell Sea today, a large output of glacial ice from the Filchner, Ronne, and Larson ice shelves is streaming ice down the eastern side of the Antarctic peninsula, and that is the region where we have the largest source of Antarctic bottom water. If you track this ice, it reenters the Weddell in the vicinity of the Greenwich meridian where the open ocean convection was observed in the mid-1970s. Therefore, the relationship between advected ice and a convection shutdown must not be as straightforward as it might otherwise seem.
Those are a few interesting lead-in points, and I appreciate the model you have given us to talk about.
GROISMAN: As I remember, 70 to 80 percent of the fresh-water input into the Arctic from Eurasia comes from rivers, some of which are highly variable in volume. You would have to take that variability into account in that region.
MYSAK: The tremendous mixing due to tides and even convection on the wide continental shelf in the Arctic Ocean that borders Eurasia seems to minimize Eurasian runoff water's impact on sea ice. In the Beaufort Sea region, where the shelf is very narrow, there seems to be an immediate effect.
MOREL: One must be careful not to interpret an association of anomalies, even if it occurs more than once, as a causal relationship, particularly when it involves precipitation. The models tend to yield different anomalies.
TRENBERTH: If sea ice melts, it creates fresh-water that substantially affects salinity. But I believe that observational evidence suggests that you get more ice created because it's cold than because the water is fresher.
MYSAK: For a given air temperature you'll get more ice if the water's fresher. Also, some data I looked at didn't show a very good correlation between cold-air temperature anomalies and sea-ice formation, but then it was limited to spring and summer.
LEHMAN: I'd like to comment on topics from the last several presentations. First, Bob Dickson said that he didn't feel the net overflow in the Greenland/Icelandic/Norwegian Sea area was being affected—in other words, that its strength was invariant on, say, decadal time scales. It seems to me that the topographic barrier acts to filter out variability there on short time scales. I should note, however, that I disagree with him about the short-time-scale stability of the thermohaline circulation.
Now let's consider some consensus views on the relative transports for the two different limbs of the North Atlantic Deep Water: overflow water and the Labrador Sea water (ignoring recirculation). You might have about 6 sverdrups for the former, and 7 to 8 for the latter, which is not insignificant. Together they constitute a potent climate forcing agent.
If the modelers can get the deep-water formation north of the