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For an inviscid and barotropic ocean of constant depth (D), a water parcel with initial vorticity 2Ω is displaced from the equator to the north pole. Latitudinal variation of the parcel vorticity (ζ) is well represented by the curve

Vorticity zeta against latitude from 0 to 90 N, all curves starting at 2 Omega: P rises to 4 Omega, Q stays at 2 Omega, R falls linearly to 0 and S falls to 0 along a curve that bends below R and flattens near the pole

  • A.S
  • B.Q
  • C.P
  • D.R

Answer: A

The curve is S.

For an inviscid barotropic ocean of constant depth, potential vorticity conservation reduces to conservation of absolute vorticity, \(\zeta + f\), where \(f = 2\Omega\sin\phi\). At the equator f = 0, so \(\zeta + f = 2\Omega\) throughout, and

\[ \zeta = 2\Omega(1 - \sin\phi) \]

This falls from \(2\Omega\) at the equator to 0 at the pole. It falls fastest near the equator, where \(\sin\phi\) grows quickly, and flattens near the pole, so it bends below the straight line: curve S.

R falls linearly with latitude, which would need \(\sin\phi\) to grow uniformly. Q keeps the relative vorticity constant, ignoring the change in f, and P adds f instead of subtracting it.

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