The salience of nonlinearities in the boreal winter response to ENSO: Arctic stratosphere and Europe

Abstract

The Arctic stratospheric response to El Niño (EN) and La Niña (LN) is evaluated in a 41 member ensemble of the period 1980 to 2009 in the Goddard Earth Observing System Chemistry-Climate Model. We consider whether the responses to EN and LN are equal in magnitude and opposite in sign, whether the responses to moderate and extreme events are proportionate, and if the response depends on whether sea surface temperature anomalies (SSTs) peak in the Eastern Pacific (EP) or Central Pacific (CP). There is no indication of any nonlinearities between EN and LN, though in ~ 15% of the ensemble members the stratospheric sudden warming (SSW) frequencies for EN and LN are similar, suggesting that a similar SSW frequency for EN and LN, as has occurred over the past ~ 60 years, can occur by chance. The response to extreme EN events is not proportionate to the amplitude of the underlying SST anomalies in spring. EP EN events preferentially increase zonal wavenumber 1 and decrease zonal wavenumber 2 as compared to CP EN events, however the zonal-mean Arctic stratospheric and subpolar surface response is generally little different between EP EN and CP EN once one accounts for the relative weakness of CP events. These differences between EP and CP events and between moderate and extreme EN events only emerge if at least 25 events are composited, however, due to the small signal-to-noise ratio, and hence these differences may be of little practical benefit.

Weakening of the Teleconnection From El Niño–Southern Oscillation to the Arctic Stratosphere Over the Past Few Decades: What Can Be Learned From Subseasonal Forecast Models?

“…This apparent decadal variability is unforced and rather occurred by chance. Model simulations run over a limited period can also simulate a relative weakening of the ENSO-vortex effect that is reminiscent of that observed, despite showing a robust connection over a longer period (Weinberger et al, 2019) 5. The tendency of EN to be associated with more SSW and LN with fewer SSW events in the first half of this period only but not the second (Butler & Polvani, 2011;Domeisen et al, 2019).…”

“…In addition, the North Pacific ridge associated with LN has shifted closer to North America since 1979 (Kumar et al, ; Yang et al, ) where it leads to an enhanced wave‐2 signal (Yang et al, ). This apparent decadal variability is unforced and rather occurred by chance. Model simulations run over a limited period can also simulate a relative weakening of the ENSO- vortex effect that is reminiscent of that observed, despite showing a robust connection over a longer period (Weinberger et al, ) The tendency of EN to be associated with more SSW and LN with fewer SSW events in the first half of this period only but not the second (Butler & Polvani, ; Domeisen et al, ). Over the entire period there is little difference in the frequency of SSW events between LN and EN winters (Butler & Polvani, ; Garfinkel et al, ), though there is sensitivity to how LN events and SSW events are classified (Polvani et al, ; Song & Son, ). It is possible that there may never have been a strong connection in the first place and the satellite network since 1979 better constrains reanalysis products.…”

Multi-model assessment of the late-winter extra-tropical response to El Niño and La Niña

“…While Brönnimann (2007) described this canonical pattern as "close to symmetric" for El Niño and La Niña, recent studies revisiting the topic and targeting linearities/non-linearities deliver contradictory results, with some reporting a symmetric signal (e.g. Deser et al 2017;Ayarzagüena et al 2018;Weinberger et al 2019) and others claiming asymmetry (e.g. Trascasa-Castro et al 2019;Hardiman et al 2019; Jiménez-Esteve and Domeisen 2019).…”

Nonlinearity in the Tropospheric Pathway of ENSO to the North Atlantic

“…Using an atmospheric model ensemble following observed SSTs, Weinberger et al (2019) find that the ENSO stratospheric Several factors might be able to explain the differences among the above studies: the background SSTs (e.g., Xie et al, 2018), the role of the extratropical SST anomalies, the location and pattern of the ENSO SST forcing, or the location and intensity of the climatological planetary-scale stationary waves in the model. Further work will be necessary to assess the model dependence in relation to the ENSO stratospheric connection.…”