Commentary on the Paper of Levitus et al.

JOHN R.N. LAZIER

Bedford Institute of Oceanography

Dr. Levitus and his co-authors suggest that the low-frequency temperature variations in the upper layers at Ocean Weather Ship (OWS) Charlie may be related to the lower-than-normal salinity that appeared in the subpolar gyre between 1968 and 1978. To explore this possibility I wish to examine some features of this phenomenon, which has come to be known as the Great Salinity Anomaly (Dickson et al., 1988).

In the latter half of the 1960s the salinity in the Iceland Sea became anomalously fresh (Dickson et al., 1975). Subsequently, water of lower-than-normal salinity was observed at various stations around the subpolar gyre. Taken together, these observations led Dickson et al. (1988) to propose that all these signals were due to a single pulse of low-salinity water moving around the subpolar gyre. They demonstrated that the low-salinity water moved out of the Iceland Sea into the North Atlantic Ocean via the East Greenland Current, and thence to the Labrador Sea (1972), the Labrador Current (1971-1972), OWS Charlie (1974), the Faeroe-Shetland Channel (1976), the Barents Sea (1978), and the Greenland Sea (1981-1982). The average rate of advance over the 14-year circuit was approximately 0.03 m s-1. The total salt deficit was estimated to be 72 × 1012 kg along the Labrador shelf, but by the time the anomaly moved north through the Faeroe-Shetland Channel into the Barents and Greenland Seas it was reduced to about 47 × 1012 kg.

The decrease in salinity caused by the passage of the anomaly is illustrated in Figure 1. Here the time series of salinity averaged over the upper 800 m at OWS Charlie is presented, along with eight similar values I calculated for September of the years 1966 to 1973 at OWS Bravo. The latter have been displaced along the time axis; the OWS Bravo values were actually obtained three years earlier than the year indicated.

FIGURE 1

The 0-800 m average salinity at OWS Charlie (solid line), adapted from Dickson et al. (1988), and at OWS Bravo (circles) for September of the years 1966 to 1976. The data for OWS Bravo have been plotted 3 years late to match the variation at OWS Charlie.

The salinity variation through the years is clearly similar in magnitude at the two weather ships, but the minimum occurs at OWS Bravo about three years before it occurs at OWS Charlie, which supports the conclusion of Dickson et al. (1988) that the low-salinity water could have moved from OWS Bravo to Charlie. Farther east, the salinity variation through these years is similar to that at the weather ships. This is illustrated by the five-year running mean of the surface salinity in Rockall Channel shown in Figure 2. According to Dickson et al. (1988), the decrease in the Rockall Channel of about 0.08 between the late 1960s and 1976 occurs about 9 months after the decrease at OWS Charlie, which fits well with their advection hypothesis.

It is unfortunate that Levitus et al. were unable to present the salinity data for OWS Charlie to compare with the temperature data, since the Great Salinity Anomaly is noted for a change in salinity rather than a change in temperature. The temperature records in their Figures 4, 5, and 6 do exhibit minima in the mid-1970s at the same time that the salinity is minimum in my Figure 2; however, the temperature data also indicate minima of similar magnitude in the early 1960s and mid-1980s that do not appear to be connected to the Great Salinity Anomaly or any other large-scale salinity minimum.

Another possibility raised by Levitus is that the low salinity signal at OWS Charlie could be due to long-period horizontal displacements of the water masses, which would bring water of lower salinity into the region of OWS Charlie.

FIGURE 2

Five-year running mean of the winter sea surface salinity anomaly in Rockall Channel, adapted from Ellett and Edelsten (1983) and Ellett (pers. Comm.).



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