Nonlinearity in the Tropospheric Pathway of ENSO to the North Atlantic

Abstract

<p><strong>Abstract.</strong> El Niño Southern Oscillation (ENSO) can exert a remote impact on North Atlantic and European (NAE) winter climate. This teleconnection is driven by the superposition and interaction of different influences, which are generally grouped into two main pathways, namely the tropospheric and stratospheric pathways. In this study, we focus on the tropospheric pathway through the North Pacific and across the North American continent. Due to the possible non-stationary behavior and the limited time period covered by reanalysis data sets, the potential nonlinearity of this pathway remains unclear. In order to address this question, we use a simplified physics atmospheric model forced with seasonally varying prescribed sea surface temperatures (SST) following the evolution of different ENSO phases with linearly varying strength at a fixed location. To isolate the tropospheric pathway the zonal mean stratospheric winds are nudged towards the model climatology. The model experiments indicate that the tropospheric pathway of ENSO to the North Atlantic exhibits significant nonlinearity with respect to the tropical SST forcing, both in the location and amplitude of the impacts. For example, strong El Niño leads to a significantly stronger impact over the North Atlantic Oscillation (NAO) than a La Niña forcing of the same amplitude. For La Niña forcings, there is a saturation in the response, with no further increase in the NAO impact even when doubling the SSTforcing, while this is not the case for El Niño. These findings may have important consequences for long-range predictions of the North Atlantic and Europe.</p>

Resampling of ENSO teleconnections: accounting for cold-season evolution reduces uncertainty in the North Atlantic

“…The fall-to-winter evolution has implications for the underlying teleconnection mechanism and associated impacts. Recently, King et al (2018a) highlighted these intra-seasonal differences and suggested that the November-December ENSO teleconnection to the North Atlantic involves a tropospheric pathway, unlike the January-February teleconnection which involves both tropospheric and stratospheric pathways (also see Ayarzagüena et al, 2018;Domeisen et al, 2019;Hardiman et al, 2019;Jiménez-Esteve and Domeisen, 2018). A statistically significant relationship between ENSO and western European temperatures in November-December was also presented by King et al (2018a).…”

“…Note that these analyses are performed using observed SLP anomalies identified with Nino3.4 events as in other parts of this study. We do not investigate nonlinearity related to other factors such as ENSO SST types (except briefly below) or amplitudes, other atmospheric variables and regions, as this is a challenging question in its own right and is addressed by other studies (e.g., Garfinkel et al, 2013;Jiménez-Esteve and Domeisen, 2020;Trascasa-Castro et al, 2019;Weinberger et al, 2019). Consistent with our results, these studies also generally agree that elucidation of nonlinearities using the limited sample of events in the reanalysis record is difficult, and therefore this research requires larger samples of model data.…”

Stratospheric Nudging And Predictable Surface Impacts (SNAPSI): a protocol for investigating the role of stratospheric polar vortex disturbances in subseasonal to seasonal forecasts

“…Imposing stratospheric anomalies through a nudging procedure has been shown to significantly impact the near surface flow (e.g., Douville, 2009), even if only the zonally symmetric component is imposed (e.g., Simpson et al, 2011;Hitchcock and Simpson, 2014;Zhang et al, 2018;Jiménez-Esteve and Domeisen, 2020). By comparing the difference between the nudged and control ensembles, the processes that drive this downward coupling can be diagnosed in each model for a variety of events of interest.…”

“…The protocol presented here is primarily based on a zonally symmetric nudging technique that has been used successfully to identify stratospheric influences on the tropospheric circulation in both hemispheres (Simpson et al, 2011;Hitchcock and Simpson, 2014;Zhang et al, 2018;Jiménez-Esteve and Domeisen, 2020). In essence, by comparing an ensemble hindcast in which the stratosphere is constrained to the observed evolution to a second hindcast in which the stratospheric circulation is constrained to climatology, the tropospheric impacts of the stratospheric anomalies can be isolated.…”

Modulation of the El Niño teleconnection to the North Atlantic by the tropical North Atlantic during boreal spring and summer

“…The complexity present in the ENSO impact over Europe results in part from several ENSO teleconnections that propagate through different geographical regions, as well as different atmospheric levels (Rodríguez-Fonseca et al, 2016). For example, teleconnections can propagate through the extratropical troposphere (Bulić and Kucharski, 2012;Jiménez-Esteve and Domeisen, 2018;Mezzina et al, 2020), the extratropical stratosphere (Butler et al, 2014;Ayarzagüena et al, 2018;Domeisen et al, 2019), or the Caribbean region (Wulff et al, 2017;Hardiman et al, 2019;Rieke et al, 2021). Furthermore, the ENSO teleconnection can be modulated by different regions and processes along its way, such as the tropical North Atlantic (Sung et al, 2013;Casselman et al, 2021) and the Indian Ocean (Zhong et al, 2005;Fletcher and Cassou, 2015;Joshi et al, 2021).…”