Central Pacific El Niño, the “subtropical bridge,” and Eurasian climate

Abstract

[1] This study contributes to the discussion on possible effects of El Niño on North Atlantic/European regional climates. We use NCEP/NCAR reanalysis data to show how the two different types of El Niños (the central Pacific, or CP, and the east Pacific, or EP) result in remarkably different European winter temperature anomalies, specifically weak warming during EP and significant cooling during CP El Niños, the latter being associated with a negative phase of the winter North Atlantic Oscillation (NAO). Our results diverge from former suggestions addressing the weakened stratospheric polar vortex as the dominant factor contributing to the El Niño/NAO teleconnection. We propose a tropospheric bridge as the mechanism primarily responsible for the establishment of a negative NAO phase and of associated cold European winters. This mechanism includes the subtropical jet (STJ) waveguide being activated only during CP El Niños, when anomalous convective heating occurs near the edge of the Pacific warm pool. Under these conditions the STJ is enhanced by planetary wave flux divergence in the subtropical upper troposphere, providing favorable conditions for the propagation of a wave number 5 disturbance around the subtropical Northern Hemisphere. This wave contributes to weakening of the Azores High and, hence, to the negative NAO phase. As global warming scenarios project an increase in the frequency of CP El Niño events, the distinctive nature of this mechanism implies that the probability of cold European winters may increase as well in future decades.

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

“…Jia et al (2009) suggested that when the tropical forcing moves to the west of the tropical central Pacific, the atmospheric responses tend to change from the PNA-like to AO-like pattern. This difference is already discussed by previous studies (Graf and Zanchettin 2012;Jia et al 2009;Li et al 2006). In this study however, we mainly put our focus on the asymmetric impacts between the CP (EP) El Niño and CP (EP) La Niña.…”

“…This result is in accordance with the previous findings that the atmospheric responses tend to change from the PNA-like to AO-like pattern with the westward movement of the tropical diabatic forcing (Jia et al 2009). This difference is also revealed by Graf and Zanchettin (2012). They suggested that CP El Niño usually yields an AO-like atmospheric response, while EP El Niño induces a PNA-like teleconnection.…”

Stratospheric response to Arctic sea ice retreat and associated planetary wave propagation changes

“…Graf and Zanchettin (2012) find an NAO signal induced by central Pacific El Nino events. Their proposed mechanism does not include the polar vortex, but a subtropical bridge: sea surface temperature anomalies of these El Nino events occur in warmer Pacific regions, leading to excitation of stronger Rossby waves, maintaining a zonal subtropical waveguide.…”

Satellite Observations of El Niño Impacts on Eurasian Spring Vegetation Greenness during the Period 1982–2015

“…For example, we found that spring NDVI over west and central parts of Eurasia has closer linkage to CP El Niño in Section 3. Graf and Zanchettin [114] analyzed the relationship between CP El Niño and European climate and proposed a tropospheric bridge as the mechanism primarily responsible for the establishment of CP El Niño/negative NAO teleconnection and of associated cold European winters. When the anomalous convective heating occurs near the edge of the warm pool, the subtropical jet (STJ) waveguide can be activated only during CP El Niño events and further enhanced by planetary wave flux divergence in the subtropical upper troposphere, favoring the propagation of a wave number 5 disturbance around the subtropical Northern Hemisphere.…”