Background zonal asymmetry allows MJO to either enhance or suppress the SSWs
Motivation:
Seeing that the MJO-related signal can only propagate to the Arctic in some specific phase (See Fig.1), we anticipated that the zonal asymmetry of either the background state or the forcing itself plays a role and pursued it in Kang and Tziperman 2018 [4].
Findings:
Using idealized circumglobal MJO forcing with k=1 structure, we studied the role played by the background zonal asymmetry. MJO-forced waves can only penetrate the jet in appearance of the background stationary wave pattern. In perfect zonally symmetric experiment (0% below), the transmission rate of the MJO-forced waves decreases to almost zero, consistent with the lackness of MJO-related signal in the Arctic stratosphere (Fig. 5). The transmission rate in the MJO forced experiments (MJO[N]) seem to follow a universal rule as the background changing experiments.
Fig.4: Each solid dot is one background changing experiment (with increasing stationary wave amplitudes in the background flow from 0% to 100%); each empty circle (MJO[N]) denotes one realistic background dry core model forced by MJO forcings with maximum heating rate of N K/day. Shown is the transmission rate of MJO-excited waves through the mid-latitude jet as a function of the maximum U wind convergence at the jet exit region. Copied from [4].Fig.5: Temperature fluctuation amplitude at the MJO forcing frequency, plotted as a function of pressure and latitude. Without background zonal asymmetry (2dMJO5), the MJO-excited waves cease to propagate before the mid-latitude jet. Copied from [4].
As the transmission of MJO-forced waves being prohibited by the zonally symmetric background flow, the Arctic stratosphere responds in an opposite manner compared to the zonally asymmetric background case: the temperature goes down, the PNJ gets stronger, and the SSW becomes less frequent.
Fig.6: Similar as above, showing the response of temperature, U wind, and SSW time series, but for the response to 5K/day circumglobal MJO-like forcing in a dry core experiment with zonally symmetric background. Copied from [4]The reason for the suppression of SSWs and the cooling of the Arctic stratosphere is revealed in the U wind response above. The MJO forcing at the Equator works as a wave source, exciting Rossby waves propagating outside the Equator, and in the mean while, transporting momentum to the Equator. As a result, the mid-latitude jet decelerates, the temperature gradient (baroclinicity) falls due to the thermal wind constraint, weakening both the stationary waves and the transient waves which feed on the baroclinicity there.
Wanying Kang 