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Which property of Rossby waves explains downstream development?

  • A.The phase velocity is always westward relative to the mean flow in which the wave is embedded
  • B.The waves are non-dispersive
  • C.The waves are trapped within a narrow latitude band and cannot spread their energy meridionally at all
  • D.The group velocity can exceed the phase velocity

Answer: D

Because energy travels at the group velocity, which can be substantially faster and in the opposite sense to the phase, a disturbance in one place seeds development well downstream within a day or two. Westward phase propagation is real but is not what carries the energy. Rossby waves are strongly dispersive, which is precisely why the two velocities differ.

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