Specificity of Extreme El Niño atmospheric teleconnections in present and future climate

– Margot Beniche, LEGOS –

Résumé :

El Niño-Southern Oscillation (ENSO), characterized by warm or cold sea surface temperature (SST) anomalies in the tropical Pacific, is a dominant driver of global climate variability. Through atmospheric circulation patterns known as teleconnections, these SST anomalies influence distant regions, including North America during winter. Typically, ENSO affects North America via the « Pacific-North American » (PNA) teleconnection pattern. However, another less common pattern – shifted eastward and known as the « Tropical Northern Hemisphere » (TNH) teleconnection – can also occur. There is currently no consensus on which ENSO type drives this diversity in North American ENSO teleconnection patterns. Furthermore, recent studies suggest that the ENSO teleconnection pattern will change under a warmer climate. This thesis examines the hypothesis that the TNH teleconnection arises specifically during extreme El Niño events, and investigates how this teleconnection might evolve under future warmer climates.

This thesis challenges the traditional view that ENSO diversity – whether El Niño warmest anomalies occur in the central or eastern Pacific – is the main driver of ENSO teleconnection diversity. Instead, I demonstrate using large atmospheric (AMIP6) ensembles that only extreme El Niño events, such as those in 1982-83, 1997-98, and 2015-16, are associated with an eastward shift of the typical PNA teleconnection toward a Tropical Northern Hemisphere (TNH) teleconnection. These events result in reproducible impacts, including warmer winters in the northeastern United States (69% probability) and increased rainfall in California (79%) and Florida (97%). Only 26 out of  the 42 examined coupled climate models from the CMIP6 database, database used in IPCC reports, are capable of simulating extreme El Niño events. These models generally reproduce a reasonable teleconnection pattern for extreme El Niño, allowing to examine its evolution under warmer climates. Under such conditions, the extreme El Niño TNH teleconnection weakens and shifts eastward, leading to amplified impacts over the Atlantic sector, and a reduction of droughts over Northeast Brazil.

This research opens interesting perspectives, such as the highly predictable impacts of extreme El Niño events on North America, or a probable strong contribution of extreme El Niño events to the overall ENSO teleconnection changes in warmer climates.

Jury :

• Mme Andréa Taschetto, referee, CCRC/UNSW, Australia

• M. Fabio D’Andrea, referee, ENS/CNRS/IPSL, Paris

• Mme Juliette Mignot, examiner, IRD LOCEAN/IPSL, Paris

• M. Matthieu Lengaigne, thesis Advisor, IRD MARBEC, Sète

• M. Jérôme Vialard, thesis co-Director, IRD LOCEAN/IPSL, Paris

• M. Nicholas Hall, thesis co-Director, Université Toulouse III, Toulouse