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Mean zonal flow, mean potential vorticity and its evolution, and trajectories in exp2_tΒΆ

First, I realized that I never plotted the Eulerian-mean zonal flow for this type of experiment with strong forcing and dissipation. It is now shown in Fig. 1 together with the trajectories of the six parcels from day 2000 to 2100. The potential vorticity (PV) for two different days are shown in Fig. 2a and b and the 100-day averaged PV for two different periods is shown in Fig. 3a and b. These figures show that there is an area to the west of the forcing (in the middle of the domain) where the PV is nearly homogenized which might explain why some of the parcels have such wild trajectories, they are pretty much free to do what they want. The two parcels that are still within a strong meridional gradient of PV have, on the other hand, a rather closed trajectory. This regime of homogenized PV is interesting because it should be related to the flow below the western boundary currents, such as the Gulf Stream and the Kuroshio. I am not sure, however, our recent view –changes of PV and meridional transport of water mass are due solely to dissipation– is adequate for this regime.

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Figure 1: Mean zonal flow between day 2000 and 2100 in exp2_t. In red are plotted the trajectories of six parcels from day 2000 to 2100.

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Figure 2: Snapshot PV and their differences 100 day apart in exp2_t. The red lines are the same as in Fig. 1.

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Figure 3: 100-day averaged PV and their differences in exp2_t. The red lines are the same as in Fig. 1.

The difference in time between two snapshots of PV at 100 day interval is shown in Fig. 2c and d, and the difference between successive 100-day averaged PV for two different periods is shown in Fig. 3c and d. In both cases, the difference decreases in magnitude (consistent with Fig. 1 of this note) without much changing its spatial structure. I am not sure what is the relationship between these differences and the shape of the trajectories.