Intraseasonal modulation of Sudden Stratospheric Warmings by ENSO

Using the complete ERA-Interim reanalysis and three state-of-the-art models, this study explores how El Niño-Southern Oscillation (ENSO) can influence the frequency and seasonal cycle of sudden stratospheric warmings (SSWs) by modulating the background upward wave propagation. Reanalysis data shows that in the last four decades, winters with SSWs were significantly more common than inactive winters, particularly under La Niña (LN) conditions but not during El Niño (EN), regardless of the ENSO/SSW definitions considered herein. In agreement with previous studies, our models tend to simulate a linear ENSO-SSW relationship, with more SSWs for EN, that show a peak of occurrence around January — as occurs in reanalysis —, and less SSWs for LN when compared to neutral conditions. Independently of ENSO, the main tropospheric precursor of SSWs appears to be an anomalous wave-like pattern over Eurasia, but it is dominated by wavenumber 1 (WN1) for EN and shows an enhanced wavenumber 2 (WN2) for LN. The differences in this Eurasian wave pattern, which is largely internally generated (ENSO-unforced), emerge from the distinct configuration of the background, stationary wave pattern induced by ENSO, favouring a relative stronger WN1 (WN2) component during EN (LN). Our results suggest that the ENSO-forced signal relies on preconditioning the seasonal-mean polar vortex, becoming weaker and displaced (stronger and more stable) for EN (LN), while ENSO-unforced wave activity represents the ultimate trigger of SSWs. This supports the view that ENSO and SSWs are different sources of variability of the winter atmospheric circulation and may reconcile previous findings in this context.