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

Abstract

El Niño-Southern Oscillation (ENSO) is known to affect the Northern Hemisphere tropospheric circulation in late-winter (January-March), but whether El Niño and La Niña lead to symmetric impacts and with the same underlying dynamics remains unclear, particularly in the North Atlantic. Three state-of-the-art atmospheric models forced by symmetric anomalous sea surface temperature (SST) patterns, mimicking strong ENSO events, are used to robustly diagnose symmetries and asymmetries in the extra-tropical ENSO response. Asymmetries arise in the sea-level pressure (SLP) response over the North Pacific and North Atlantic, as the response to La Niña tends to be weaker and shifted westward with respect to that of El Niño. The difference in amplitude can be traced back to the distinct energy available for the two ENSO phases associated with the non-linear diabatic heating response to the total SST field. The longitudinal shift is embedded into the large-scale Rossby wave train triggered from the tropical Pacific, as its anomalies in the upper troposphere show a similar westward displacement in La Niña compared to El Niño. To fully explain this shift, the response in tropical convection and the related anomalous upper-level divergence have to be considered together with the climatological vorticity gradient of the subtropical jet, i.e. diagnosing the tropical Rossby wave source. In the North Atlantic, the ENSO-forced SLP signal is a well-known dipole between middle and high latitudes, different from the North Atlantic Oscillation, whose asymmetry is not indicative of distinct mechanisms driving the teleconnection for El Niño and La Niña.

El Niño teleconnection to the Euro-Mediterranean late-winter: the role of extratropical Pacific modulation

“…These features are well captured by the three models, with a slightly weaker signal in EC-EARTH (Fig. 5a, d, g; see Mezzina et al 2021). As a first approximation, PDO conditions do not appear to change the structure of El Niño teleconnection but may affect its amplitude, suggesting that the same fundamental dynamics is at play.…”

“…13a, c, e) together with the WAF modulation under The El Niño teleconnection is characterized by the well-known wave-train arching northeastward over the North Pacific-American region, whose vertical tilt with height projects on the canonical dipolar pattern over the North Atlantic-European region (Fig. 5; Mezzina et al 2020Mezzina et al , 2021. The changes in K s , induced by PDO − (Fig.…”

“…In this idealized framework, the two modes are considered as independent, and the focus is put on the potential modulating role of extratropical Pacific SSTs. Note that the atmospheric response to ENSO as such, in both its positive and negative phase, has been comprehensively reported in Mezzina et al (2021).…”

“…1, a suitable candidate for the tropospheric pathway of the El Niño-NAE teleconnection involves the propagation of planetary Rossby waves crossing the North Pacific-American region (e.g. García-Serrano et al 2011;Mezzina et al 2020), analyzed in depth using the MEDSCOPE multi-model by Mezzina et al (2021). As shown in Sect.…”

On the Dependency of Atlantic Hurricane and European Windstorm Hazards

“…Over Europe, several authors have explored how ENSO impacts the North Atlantic storm track. ENSO produces an extratropical response through Rossby wave propagation (Branstator, 2014; Goss et al., 2018; Held et al., 1989; Mezzina et al., 2020; Soulard et al., 2019; Stan et al., 2017), influencing atmospheric hazards globally (Steptoe et al., 2018). European temperature and precipitation anomalies associated with ENSO are spatially similar to those associated with the North Atlantic Oscillation (NAO) (Pozo‐Vázquez et al., 2001, 2005), attributed to a predominantly positive phase relationship between the two indices during November to December and a negative phase relationship during January to March (Huang et al., 1998; Moron & Gouirand, 2003).…”