Relative vorticity (vertical component) for a horizontal flow is zeta = dv/dx - du/dy; a purely zonal (east-west) current with a north-south shear in its speed produces vorticity from the -du/dy term alone.
The current is easterly (u-component only, v=0), decreasing toward the north at a rate du/dy = -10 m/s per 100 km = -1x10^-4 s^-1. Since...
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