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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 F_m^' ) at high latitudes just below the original [u]'. The F_m' 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 F_m anomalies are expected to propagate downward.

II. Downward propagation from the lower stratosphere to the lower troposphere

The u_pcm variations can then propagate further down to the lower troposphere in the following 4 steps (see Fig. 3):

1. F_m' in layer 1 drives, through the positive [u]' and associated meridional Coriolis force (F_c,u), a polar cap mean meridional wind (v_pcm), which associated zonal Coriolis force (F_c) counteracts the original F_m'. The v_pcm implies an equatorward v at 60Ú N in the stratosphere (v₁) and upper troposphere (v₂).
2. v₁ and v₂ at 60Ú N induce a lower tropospheric poleward flow (v₃), which slightly lags the upper equatorward flow. The difference between v₁+v₂ (denoted as v₁₊₂) and v₃ determines the tendency of the polar cap mean surface pressure ('p_0,pcm/'t).
3. The anomalous meridional surface pressure gradient around 60Ú N implies an anomalous [u] around 60Ú N and thus an anomalous u_pcm near the surface.
4. The Coriolis force F_c due to v₃ maintains the anomalous zonal flow against friction.

Figure 3: Illustration of the hypothesized mechanism for the downward propagation of zonal wind variations from the lower stratosphere to the lower troposphere.