Hi,
Sorry, i did not read carefully your first post. This is strange, and I have tested it just to try, and I do have similar results wether I use DURATION = 698400 or 6984.
The only difference with you is that I stopped the long simulation after 3600 s and I look at the result on the local submesh. But as you can see from figures below, I do have the same surface current pattern at t = 3600 s.
By checking the 3D result file I obtained by running your case, it looks like what you depict on your 698400 s picture does correspond to the current at the bottom (level 1). Can it be that you were actually looking at the bottom current instead of at the surface?
Good luck!
PL