I am reading an Oceanography Textbook which explains that in the ocean, planetary vorticity tends to be much larger than relative vorticity therefore, the potential vorticity can be simplified to be f/H.
"This requires that the flow in an ocean of constant depth be zonal."
"...in general, currents tend to be east-west rather than north south. Wind makes small changes in relative vorticity, leading to a small meridional component to the flow."
If planetary vorticity is far larger than relative vorticity, how exactly do changes in f (planetary vorticity) cause ocean flow to be more zonal?