If this is the mechanism for the variation at OWS Charlie it should also be the mechanism for the variations at OWS Bravo, Rockall Channel, and the other places where the anomaly signal has been observed. At 50°N 51.5°W, OWS Bravo is near the center of the cyclonic gyre that dominates the circulation of the Labrador Sea. There are no horizontal gradients that could be moved to lower the salinity of the top 800 m of the water column without drastically altering the dynamics of the sea, and no such alteration has been observed. This is also true in the Rockall Channel, so the interpretation of Dickson et al. (1988), an advecting anomaly, fits more comfortably with the observations.
To look further at the relationship between OWS Charlie and the Rockall Channel I have prepared Figure 3, which shows the sea-surface temperature variations at these two points. The data for OWS Charlie is from Dr. Levitus's Figure 5, and the surface temperature anomaly (for winter) in the Rockall Channel is from Ellett and Edelsten (1983) and Ellett (pers. comm.). The amplitudes of the variations at OWS Charlie are larger than at Rockall; this is in part because they are based on annual averages, while those at Rockall are 5-year running means. The interesting feature is that in each record there is a variation of about 10 years' period that occurs at Rockall roughly 5 years later than at OWS Charlie, whereas Dickson et al. (1988) demonstrated that the Great Salinity Anomaly moved from OWS Charlie to Rockall Channel in 9 months. If we assume that the temperature variations at OWS Charlie are connected to the Great Salinity Anomaly, we are then faced with the problem of explaining why the variations due to salinity move to Rockall Channel five times faster than those due to temperature.
GOODRICH: Was there a demonstrable air-temperature signal associated with the presumably colder, fresher water of the Great Salinity Anomaly?
LEVITUS: I'm not sure. But there certainly was for this decadal-scale oscillation and for the decreasing trend.
GROISMAN: If I had only two stations on land, I wouldn't dare claim that I was seeing interdecadal variability in surface temperature. If your two ocean stations are near strong temperature fronts, how can you be sure from those observations that long-term changes aren't the result of a 200-km shift east or west in such a front?
LEVITUS: We're mostly trying to document the variability, and we have confirmation from other data sets for that. There may be some contribution from front changes in the upper ocean, but I really don't think that they can explain the horizontal shifts in the deep ocean that we're seeing at OWS "C".
MOREL: I have two questions. First, can we associate these demonstrated changes in the ocean that are on decadal or longer time scales with actual changes in climate, such as atmospheric circulation? Ocean variability would not qualify as climate research otherwise. And second, could you suggest a practical sampling strategy for a global climate—not just ocean-—observing system?
LEVITUS: I think that what's most important in connection with both your questions is bringing together the huge amount of data that already exists. We at NOAA are digitizing data from the U.S., Europe, and Russia. We are seeing a decadal-scale oscillation in the COADS, subsurface, and SAT data, but it will take several years before we can evaluate all this historical data and design a better global observing system.
One of the important things we've already learned is that we need long-term commitments if we are to look at long-time-scale problems. As we saw at the Panulirus station, you can have 10 years with no changes and suddenly see very large changes. The assurance of continuing financial support is as essential as good design for an observing system.
MYSAK: In response to Pierre Morel's first question, I wanted to point out that Rosanne D'Arrigo's reconstructions of tree rings