MJO-SSW teleconnection

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.

Transmission-jet-exit-convergence.png
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].
T_Amp_MJO5_2D3D.png
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.

Climatology_response_MJO_2D.png
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.

Reference:

Kang, W. and E. Tziperman2018The Role of Zonal Asymmetry in the Enhancement and Suppression of Sudden Stratospheric Warming Variability by the Madden–Julian Oscillation. J. Climate, 312399–2415, https://doi.org/10.1175/JCLI-D-17-0489.1

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