Ocean & Atmospheric circulation redux

1) Unequal planetary heating creates pressure gradients which make winds blow on ocean surface in trade-wind & westerly patterns due to balance of pressure gradient and Coriolis effect.

Ocean & Atmospheric circulation redux

2) Winds drive Ekman transport in surface waters (~top 100m) creating “lenses” or “hills” of water between the westerlies and trades (centers of basins).

Ocean & Atmospheric circulation redux

3) Gravity drives water downhill (pressure gradient) and Coriolis effect balances it to create geostrophic flow in circles around the center. Note that this flow is like a fly-wheel compared to the winds in the atmosphere...it responds to changes (such as storms, etc.) very slowly and will keep going for long time even after wind stress stops.

4) Presence of continents causes modifications, particularly closed gyre patterns. All gyres have strong western boundary currents due to differences in planetary vorticity (see next).

Ocean & Atmospheric circulation redux

Ocean & Atmospheric circulation redux

Western Boundary Currents result from a change in vorticity necessary to balance the difference in latitude of the flowing ocean. This change in “planetary” vorticity comes from the fact that the Coriolis effect is zero at the equator and maximum at the poles.

The basic equations* for the northern hemisphere are: Western side of ocean: wind stress () + planetary vorticity () = friction () Eastern side of ocean: wind stress () = planetary vorticity () + friction ()

So, the western side must flow faster and narrower to get more friction to build up the extra vorticity needed to go poleward.

*from Knauss, J.A., “Introduction to Physical Oceanography, Second Edition”, 1997, Prentice-Hall Inc.