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Vous êtes ici : Accueil / Actualités / Séminaires / Seminaires Septembre 2017-Aout 2018 / Jeudi 16 novembre - M. Khodri - Volcanic forcing and Pacific decadal variability

Jeudi 16 novembre - M. Khodri - Volcanic forcing and Pacific decadal variability

Par SEMSOU Dernière modification 06/11/2017 14:19
Quand ? Le 16/11/2017,
de 11:00 à 12:00
Où ? Salle Coriolis
Participants Myriam Khodri, Chercheur, LOCEAN/IPSL
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Séminaire exceptionnel

 

Dr. Myriam Khodri

Chercheure, Laboratoire d'Océanographie et du Climat : Expérimentation et Approches Numériques (LOCEAN) / IPSL

 

Présentation en français.

 

Titre : The influence of volcanic forcing on Pacific Ocean inter-annual to decadal variability over the last centuries

 

Résumé :

The understanding of ENSO intrinsic dynamics has improved over recent decades, in particular with the identification of useful predictors. The understanding of ENSO response to external forcing is however primitive compared to our understanding of its internal dynamics. There is, for instance, neither a clear consensus on how anthropogenic forcing influences ENSO nor how it is influenced by volcanism. Some explosive tropical volcanic eruptions are sufficiently strong to inject aerosols into the stratosphere. Those aerosols backscatter incoming solar radiation, and reduce the global surface temperature by a few tenths of a degree Celsius for a couple of years. While this systematic, global effect of large tropical eruptions, is relatively well understood, their impact on regional seasonal phenomena such as the Asian monsoon or on natural modes of climate variability such as ENSO has not yet settled to a consensus. Observational in situ SST data sets dating back to 1882 indicate a large positive ENSO-like pattern following four out of five big eruptions during the historical period. While this is a small sample, and a warm event was already underway during two of these eruptions, longer ENSO-proxy records also suggest a more probable equatorial Pacific warming within 2 years after large tropical eruptions. Modelling studies have however reported contrasting responses to volcanism. There is hence a need to reconcile in situ observations and paleo-proxies, which suggest an El Niño during the 2 years that follow the eruption, and modelling studies that have reached no consensus on either the sign or the mechanism of the ENSO response to volcanism. 

In this presentation I’ll show that relying on SST anomalies relative to the tropical average (relative SST) allows us to reconcile models and observations. This metric reveals that an El Niño event tends to follow large tropical eruptions in observed SST data sets and in the Coupled Model Inter-comparison Project phase 5 (CMIP5) historical experiments. Targeted climate model simulations further emphasize that Pinatubo-like eruptions tend to shorten La Niñas, lengthen El Niños and induce anomalous warming when occurring during neutral states. Volcanically induced cooling in tropical Africa weakens the West African monsoon, and the resulting atmospheric Kelvin wave drives equatorial westerly wind anomalies over the western Pacific. This wind anomaly is further amplified by air–sea interactions in the Pacific, favouring an El Niño-like response. The general framework outlined above is useful to identify the core dynamics involved in post-eruption inter annual to decadal climate variability over the last millennium, and its applicability to adaptation to current variability and future change.

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