Analysis of the Nonlinearity of El Niño–Southern Oscillation Teleconnections*

Abstract

El Niño-Southern Oscillation (ENSO) has significant variations and nonlinearities in its pattern and strength. ENSO events vary in their position along the equator, with some located in the central Pacific (CP) and others in the east Pacific (EP). To study how these variations are reflected in global ENSO teleconnections, both observations and idealized atmospheric general circulation model (AGCM) simulations are analyzed. Clear nonlinearities exist in observed teleconnections of sea level pressure (SLP) and precipitation. However, it is difficult to distinguish if these are caused by the different signs, strengths, or spatial patterns of events (strong El Niño events mostly being EP events and strong La Niña events mostly being CP events) or by combinations of these. Therefore, sensitivity experiments are performed with an AGCM forced with idealized EP and CP ENSO sea surface temperature (SST) patterns with varying signs and strengths. The response is generally stronger for warm events than for cold events and the teleconnection patterns vary with changing SST anomaly patterns. EP events show stronger nonlinearities than CP events. The nonlinear responses to ENSO events can be explained as a combination of nonlinear responses to a linear ENSO (fixed pattern but varying signs and strengths) and a linear response to a nonlinear ENSO (varying patterns). Any observed event is a combination of these aspects. While in most tropical regions these add up, leading to stronger nonlinear responses than expected from the single components, in some regions they cancel each other, resulting in little overall nonlinearity. This leads to strong regional differences in ENSO teleconnections.

The salience of nonlinearities in the boreal winter response to ENSO: North Pacific and North America

“…By forcing the model with historical SSTs, we allow for a more natural comparison to the observed response to ENSO as compared to simulations with annually repeating identical SST anomalies [as analyzed by Garfinkel et al (2013)] or idealized SST patterns (Frauen et al 2014;Hegyi et al 2014). However we acknowledge that the imposing of SSTs violates energetic constraints and does not allow for the generation of self-consistent SST anomalies and teleconnections.…”

“…While stronger EN events should be expected to induce a stronger tropical/subtropical Rossby wave source (e.g. Frauen et al 2014), identical sea surface temperature anomalies located in different regions will not, in general, lead to the same tropical rainfall response due to the existence of a minimum threshold for deep convection (Chiodi and Harrison 2013;Frauen et al 2014;Johnson and Kosaka 2016). This asymmetric tropical rainfall response, could in principle drive nonlinearities in the extratropical troposphere Hoerling et al , 2001.…”

“…Hoerling et al , 2001Hoerling and Kumar 2002;Garfinkel et al 2013;Frauen et al 2014;Johnson and Kosaka 2016), though with the caveat that the nonlinearities tend to be weak in comparison with internal variability and are only identifiable upon averaging over many events (Hoerling and Kumar 2002;Garfinkel et al 2013). This motivates the following two questions: (a) In what regions will nonlinearities first begin to become identifiable above the noise as sample sizes increase?…”

Asymmetry of the winter extra-tropical teleconnections in the Northern Hemisphere associated with two types of ENSO

“…For example, asymmetry roles of the CP (EP) El Niño and La Niña in Australia and the southeast China rainfall variability were reported in the work of Cai et al (2010) and Karori et al (2013), respectively. Frauen et al (2014) documented the asymmetric responses of the sea level pressure (SLP) and tropical rainfall anomalies to the two types of ENSO through the atmospheric general circulation model.…”

Importance of Late Fall ENSO Teleconnection in the Euro-Atlantic Sector

“…related to the Niño-3.4 index, so we do not address the asymmetry between cold and warm phases, nor do we distinguish between central Pacific and eastern Pacific ENSO, although these aspects might be important (Pozo-Vázquez et al 2001;Garfinkel et al 2013;Frauen et al 2014;Feng et al 2017).…”