FIGURE 4

Drake Passage transport for white-white forcing with different levels of fresh-water flux variability. Atmospheric models and observation/analyses suggest that values of 1 to 2 mm/day are realistic in the higher-latitude regions.

FIGURE 5

Principal components for Atlantic salinity section EOFs: white-white forcing.

FIGURE 6

As in Figure 5, but for the Pacific section. The trajectory of the variability (the attractor) is shown in the phase space of the first two principal components.

The sensitivity of the Antarctic Circumpolar Current (ACC) transport to the magnitude of the forcing in the WW run was studied by repeating the experiment for values of s between 0.5 and 2.0 mm/day (Figure 4). The large-scale mode is excited rather uniformly by values above 1.0 mm/ day, but not by values of 0.5 mm/day or less (which resemble the control run). Note the similar shape of the responses in the range I to 2 mm, as well as a magnitude that is essentially independent of s. As the magnitude of the forcing is reduced, the mode takes longer to become excited. Apparently a threshold value between 0.5 to 1.0 mm is required to trigger the mode, and this fact suggests a highly nonlinear generation mechanism.

Meridional Structure

Salinity and temperature data from the WW run6 were saved along key meridional sections in both the Pacific and the Atlantic and subjected separately to EOF analysis. The principal components (PCs) for modes I and 2 for both oceans are shown in Figure 5 in standard format and in Figure 6 in two-dimensional phase space. It is clear that the roughly 300-year oscillation described above extends into the high latitudes of both major oceans. In the Atlantic, the PCs are in quadrature, and this means that the salinity anomalies propagate. The sense of the motion revealed by the EOFs (Figure 7) is that more (or less) saline water moves northward from Antarctica in the near-surface waters to the central North Atlantic where it sinks and returns at depth to the Southern Ocean. While this is happening, an

6  

Given the similarity in the responses of the OGCM to different atmospheric forcings, we concentrate in the rest of the paper on the results of the WW run.



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