Doubling of SSW occurrence due to stronger MJO as expected in a warmer climate.
Motivation:
As the Sudden Stratospheric Warming (SSW) was observed to be led by specific phase of Madden-Julian Oscillation (MJO) [1] , and the MJO was predicted to get stronger under a global warming scenario [2], we started thinking of the behind mechanism of the MJO-SSW teleconnection and the changes in a future warmer climate.
Findings:
We first validated the MJO-SSW teleconnection by showing that the Arctic stratospheric temperature fluctuate with the MJO phase significantly.
Fig.1: Composite of polar cap (65–90N and 10 mb) temperature (K), calculated as an average over all occurrences of a given MJO phase and shown as a function of that MJO phase (horizontal axis) and days since each phase (vertical axis), following Garfinkel et al. (2012a). Results are shown for (a) forced idealized model and (b) WACCM with enhanced entrain- ment and therefore stronger MJO. Copied from [3].The message deliverer is the large-scale waves excited by the MJO forcing. Shown below is a hovmoller plot of the meridional component of wave activity flux, where the northward propagation of the flux is clear in both WACCM and Idealized dry core model. The signal seems to be halted and noisy around 30N, possibly due to the interaction with the jet there, which turned out to be a key process determining the SSW response (will discuss later).
Fig.2: Hovmoller plot of the meridional component of wave activity flux as a function of latitude and lag time after MJO phase 4 for (left) Idealized dry dynamic core experiment forced by k=1 MJO-like forcing, and (right) WACCM simulation. Copied from [3]
In response to a stronger MJO forcing as expected in a warmer future climate [2], the Arctic stratosphere is warmed by over 4K, the polar night jet is weakened by over 3m/s and the SSW frequency almost doubles, in both a Held-Suarez type idealized model and the Whole Atmospheric Community Climate Model (WACCM).
Fig.3: Changes in climatology (a,b) zonal mean temperature and (c,d) zonal wind in response to MJO forcing in (a),(c) the idealized model and (b),(d) SC-WACCM. Stippled areas are 95% significant with the Student’s t test. Copied from [3]Tab.1: SSW occurrence during (top group) the 50 simulated years in a perpetual DJF idealized model, and (bottom group) 30 simulated years in full WACCM. SSWs are identified by the 10 mb wind reversal (or almost reversal <5m/s) at 60N, 65N and 70N. Copied from [3].
Wanying Kang 