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.
Highlights
The teleconnection of El Niño-Southern Oscillation (ENSO) to the North Atlantic-European (NAE) sector is a longexplored topic that, is still controversial in several aspects
Our results suggest that asymmetries are present in the NAE region associated with strong El Niño- and La Niñalike sea surface temperature (SST) patterns in terms of amplitude and zonal shift, but the structure of the sea-level pressure (SLP) pattern is similar and driven by the same dynamics: the dipolar pattern, consistent with the canonical view of Brönnimann (2007), is associated with the tropospheric Rossby wave train and its westward tilt with height (Fig. 5)
Even in the presence of a symmetric forcing, asymmetries arise in the SLP response over both the North Pacific (Aleutian Low) and NAE sector (North Atlantic dipole)
Summary
The teleconnection of El Niño-Southern Oscillation (ENSO) to the North Atlantic-European (NAE) sector is a longexplored topic that, is still controversial in several aspects. A first cornerstone on the topic—and starting point of this study—was set in a review by Brönnimann (2007), who concluded that a robust ENSO signal exists over the NAE region in late winter (January to March, JFM): a dipole in sea-level pressure (SLP) with centers over the mid-latitude and high-latitude North Atlantic (see “Appendix 1”). He referred to this signal as “canonical”, though acknowledging the existence of other, “non-canonical” views. One of our objectives is to show that the canonical NAE signal associate with El Niño and La Niña can be mostly explained in terms of the same tropospheric dynamics
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