Fig. 1
Schematic illustration of the Rossby wave propagation mechanism on a β-plane for the case where the zero-order geostrophic balance is independent of latitude [eqs. (6a–6c)]. Long vertical arrows represent the zero-order geostrophic meridional velocity v 0, whereas short vertical arrows represent their first-order ageostrophic correction . The first-order ageostrophic zonal component , represented by the short horizontal arrows, is in phase with the zero-order height (pressure) anomaly h 0. The interaction between the zero-order geostrophic field and the first-order ageostrophic correction explains the westward propagation of the zero-order field (illustrated by being shifted by a quarter of a wavelength, after a quarter of a period of . The residual Coriolis force acting on shifts v 0 westward, but its contribution to the divergence field, acts to shift h 0 eastward. is in (out of) phase with v 0 at low (high) latitudes. This keeps the Coriolis force constant on the β-plane so it can be balanced by the constant PGF. Its contribution to the divergence field , opposes and overwhelms , therefore allows h 0 to propagate westward coherently with the zero-order momentum field.