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3. Possible Mechanism
I. Downward propagation in the stratosphere
A positive zonal-mean zonal wind anomaly (hereafter [u]', where the accent denotes the anomaly) in the stratosphere can propagate downward as follows. Planetary waves from the troposphere are refracted anomalously equatorward by anomalously large vertical [u] gradients, resulting in a positive anomaly of convergence of meridional momentum flux (hereafter Fm' ) at high latitudes just below the original [u]'. The Fm' causes an acceleration of [u], so that the original [u]' is followed by a similar anomaly at the level below. This process is then repeated for lower and lower levels, resulting in a downward propagation of the [u] anomalies. Also the Fm anomalies are expected to propagate downward.
II. Downward propagation from the lower stratosphere to the lower troposphere
The upcm variations can then propagate further down to the lower troposphere in the following 4 steps (see Fig. 3):
Figure 3: Illustration of the hypothesized mechanism for the downward propagation of zonal wind variations from the lower stratosphere to the lower troposphere.