It is hard to exaggerate the importance of that account by the Bergen School of the evolution of a frontal disturbance from a trivial kink in the polar front to an occluded front. Nevertheless, we now know about Rossby waves, jet streams and the complex nature of fronts (Figure 13.4) and realise their significance in the growth and decline of midlatitude cyclones (Note 12.L). Surface and upper-level winds are now thought of as separate but interacting closely, and we regard the growth of a kink in the polar front as due to the waviness of the upper westerlies. This occasionally causes an eastward jet stream to lunge towards the equator and back, thereby making it move clockwise (in the southern hemisphere), as shown in Figure 12.15. Such cyclonic rotation implies an upper-level trough of low pressure, since a reciprocal association between rotation and pressure is implied in Buys-Ballot's Rule (Section 12.2). At the same time, cold air at the surface is advected towards the equator behind a cold front, and there is warm advection towards the pole ahead of the wave. (This is explained in Note 13.B.) Detailed consideration then shows that the effect of these cold and warm flows is to amplify the wave, which deepens the low pressure at the surface and shifts its centre, until eventually it is below the upper-level trough. Then the trough's convergence prevents uplift from the surface low, so that the latter is filled by surface convergence and consequently disappears, restoring the status quo.
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