Why is the zonal component of flow in the ocean much larger than the meridional component

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?

• I try to reason it out, but other than hand-waving it to being different rotation speed -> different fluid movement rates... or thinking along the lines of meteorology, where the geostrophic wind equation of steady stead reduces to $u=-1/(f\rho) \cdot dP/dy$ (and $v=1/(f\rho) \cdot dP/dy$), and because the large-scale temperature is N/S, the greater pressure variation as a whole is N/S, leading to a heavily zonal wind overall. But not sure either of those cut it for your question... – JeopardyTempest May 24 '17 at 0:12
• Perhaps you should post of preceding text, as often in textbook derivations they'll be quietly alluding back to some information in a recent discussion without explicitly explaining it. Or perhaps this meteorologist just isn't enough help in the water where things like salinity show their strange head! – JeopardyTempest May 24 '17 at 0:12
• Most of the answer is wind curl. – arkaia May 24 '17 at 15:17