On the Influence of ENSO on Sudden Stratospheric Warmings. Issue 8 (20th April 2023) - Palmeiro, F. M.
Abstract: Using the extended ERA5 reanalysis and three state‐of‐the‐art models, this study explores how El Niño‐Southern Oscillation (ENSO) can influence the total frequency, seasonal cycle and preconditioning of sudden stratospheric warmings (SSWs). Reanalysis data shows that in the last seven decades, winters with SSWs were more common than winters without, regardless El Niño (EN) or La Niña (LN) occurrence or the ENSO/SSW definitions. In agreement with previous studies, our models tend to simulate a linear ENSO‐SSW relationship, with more SSWs for EN, around mid‐winter (January–February) as in reanalysis, and less 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, emerge from the distinct configuration of the background, stationary wave pattern induced by ENSO in the North Pacific, favoring a stronger WN1 (WN2) component during EN (LN). Our results suggest that the ENSO‐forced signal relies on modulating the seasonal‐mean polar vortex strength, becoming weaker and more 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 distinct sources of variability of the winter atmospheric circulation operating at different time‐scales and may reconcile previous findings in this context. Plain Language Summary: Sudden stratospheric warmings (SSWs) are extreme events of the stratospheric polar vortex that consist in its deceleration and breakdown for several days, with implications on surface weather for several weeks. On the other hand, the atmospheric response to El Niño‐Southern Oscillation (ENSO) has been shown to weaken the stratospheric polar vortex during El Niño and strengthen it during La Niña winters, and hence, potential modulation of the occurrence of SSWs by ENSO is expected. Reanalysis data shows that in the last seven decades, winters with SSWs were more frequent than winters without regardless of ENSO, and instead of having an impact on the SSW total frequency, El Niño seems to favor events in mid‐winter (January–February). The main mechanism driving SSWs is thought to be anomalous upward wave activity from the troposphere into the stratosphere, reaching and perturbing the polar vortex. Independently of the ENSO phase, enhanced wave activity before SSWs is here found over Eurasia, thus pointing to this region as the main precursor of SSWs. While ENSO modulates the polar vortex at monthly/seasonal time‐scales, SSWs do so at daily/weekly time‐scales, representing distinct sources of variability of the polar vortex. Key Points: El Niño‐Southern Oscillation (ENSO) and sudden stratospheric warmings (SSWs) modulate the stratospheric polar vortex at different time‐scales The main SSW precursor independently of the ENSO phase is associated with wave activity over Eurasia The anomalous Eurasian wave pattern can be dominated by different wave numbers depending on the ENSO phase