I don't know for sure how that's defined (I ctrl-f'd and it's not explained in the paper), but this says the "North" is to the right of the image, and from context it sounds like it's the north pole of the accretion disk, i.e., the direction of the rotation axis with the right-hand rule.
> The approaching side of the large-scale jet in M87 is oriented west–northwest (position angle $\mathrm{PA}\approx 288^\circ ;$ in Paper VI this is called ${\mathrm{PA}}_{\mathrm{FJ}}$), or to the right and slightly up in the image.
> The approaching side of the large-scale jet in M87 is oriented west–northwest (position angle $\mathrm{PA}\approx 288^\circ ;$ in Paper VI this is called ${\mathrm{PA}}_{\mathrm{FJ}}$), or to the right and slightly up in the image.