More Frequent Sudden Stratospheric Warming Events due to Enhanced MJO Forcing Expected in a Warmer Climate

Sudden stratospheric warming (SSW) events influence the Arctic Oscillation and midlatitude extreme weather. Observations show SSW events to be correlated with certain phases of the Madden-Julian oscillation (MJO), but the effect of the MJO on SSW frequency is unknown, and the teleconnection mechanism, its planetary wave propagation path, and time scale are still not completely understood. The Arctic stratosphere response to increased MJO forcing expected in a warmer climate using two models is studied: the comprehensive Whole Atmosphere Community Climate Model and an idealized dry dynamical core with and without MJO-like forcing. It is shown that the frequency of SSW events increases significantly in response to stronger MJO forcing, also affecting the averaged polar cap temperature. Two teleconnection mechanisms are identified: a direct propagation of MJO-forced transient waves to the Arctic stratosphere and a nonlinear enhancement of stationary waves by the MJO-forced transient waves. The MJO-forced waves propagate poleward in the lower stratosphere and upper troposphere and then upward. The cleaner results of the idealized model allow identifying the propagating signal and suggest a horizontal propagation time scale of 10-20 days, followed by additional time for upward propagation within the Arctic stratosphere, although there are significant uncertainties involved. Given that the MJO is predicted to be stronger in a warmer climate, these results suggest that SSW events may become more frequent, with possible implications on tropospheric high-latitude weather. However, the effect of an actual warming scenario on SSW frequency involves additional effects besides a strengthening of the MJO, requiring further investigation.