Lecture 4: Forces on a rotating planet

What determines the magnitude of the coriolis force?

U is the velocity

f is the coriolis parameter

The faster water moves, the larger the coriolis force

The higher the latitude, the larger the coriolis force

It is zero at the equator, and maximum at the poles

When is the coriolis force important?

Using he rosby number, Ro

Small Ro<1 system is affected by coriolis force

Large Ro>1 system with negligible influence from the coriolis force

Important when travelling relatively slowly over large distances, at which point rotation of our frame of reference noticeable affects the fluids motion

Parcel of water set in motion in the northern hemisphere and subjected to only the coriolis force:

continue to deflect to the right and eventually make a circle

Friction

Wind stress Momentum transferred from atmosphere into the ocean, creates surface currents

Viscosity (internal friction) Momentum transferred between parts of fluid that are flowing at different speeds

Frictional drag at ocean boundaries Momentum extracted from overlying flow as water molecules collide with the seafloor

The equations of motion

The equations of motion - used by physical oceanographers to describe the dynamics of the ocean - are simply Newton’s second law applied to a fluid: