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.
Highlights
El Niño Southern Oscillation (ENSO), i.e. the warm (El Niño hereafter EN) and cold (La Niña hereafter LN) phases of the equatorial Pacific coupled atmosphere-ocean phenomenon, impacts the global atmospheric circulation in both the troposphere (Horel and Wallace 1981; Ropelewski and Halpert 1987; Halpert and Ropelewski 1992; Trenberth et al 1998) and stratosphere
In the rest of this paper we refer to the atmospheric response as ‘linear’ when the responses to EN and LN are equal in magnitude and opposite in sign, the response to moderate vs extreme events is proportional and there is no significant difference between the responses to Eastern Pacific (EP) EN and Central Pacific (CP) EN events
How many Central Pacific El Niño (CP EN) events and Eastern Pacific El Niño (EP EN) events must be composited before the wave-1 heat flux in the lower stratosphere in winter becomes significantly different? To answer this question we introduce a bootstrapping methodology that will be used in Sects. 5 and 6
Summary
El Niño Southern Oscillation (ENSO), i.e. the warm (El Niño hereafter EN) and cold (La Niña hereafter LN) phases of the equatorial Pacific coupled atmosphere-ocean phenomenon, impacts the global atmospheric circulation in both the troposphere (Horel and Wallace 1981; Ropelewski and Halpert 1987; Halpert and Ropelewski 1992; Trenberth et al 1998) and stratosphere (van Loon and Labitzke 1987; Hamilton 1993; Domeisen et al 2019). Episodes of prolonged upward wave flux can lead to sudden stratospheric warming (SSW) events (Polvani and Waugh 2004; Sjoberg and Birner 2012), and on average, modulate surface climate (Cagnazzo et al 2009; Ineson and Scaife 2009; Bell et al 2009) and increase weather predictability (Sigmond et al 2013) in the Euro-Atlantic region for weeks. While in the seasonal mean strong LN events appear to lead to a stronger and colder than normal Arctic polar vortex (Garfinkel and Hartmann 2007; Iza et al 2016), there is no reduction in the frequency of SSW events during LN winters (Butler and Polvani 2011; Polvani et al 2017; Domeisen et al 2019). Two explanations have been offered as to why LN should not lead to reduced SSW frequency: first, the North Pacific ridge associated with LN does not reach the subpolar Northwest Pacific where it could most efficiently destructively interfere with the climatological stationary waves
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