FENG
mardi 1 mars 2022
“Study of the Climate Variability and the Role of Volcanism in the North Atlantic-Mediterranean Sector during the Last Millennium”
Devant le Jury composé de:
M. Francis Codron LOCEAN SU (Paris) Président
Mme. Wenmin Man IAP (Chine) Rapportrice
M. Hugues Goosse UCL (Belgique) Rapporteur
M. Matthew Toohey USAS (Canada) Examinateur
Mme. Myriam Khodri LOCEAN SU (Paris) Directrice
M. Laurent Li LMD SU (Paris) Co-directeur
Abstract:
Volcanic effects from large eruptions on climate manifest for
a few years both at earth’s surface as a cooling and in the
stratosphere as a warming. Such impacts lie in principle among
the potential predictable features of climate after a volcanic
eruption took place. Previous works based on models and
observations also suggest an increased probability for a
positive phase of the North Atlantic Oscillation (NAO) during
the first winter following stratospheric tropical eruptions.
Large gaps remain however in our understanding of the climate’s
response to volcanic eruptions, related to the paucity of
observed volcanic events during instrumental era and the degree
to which such response depends on the characteristics of the
eruption (i.e., hemispheric loading, strength, season) or
initial conditions of the climate system when the eruption
occurs. Tackling all these factors is crucial to improve our
understanding of the underlying physical mechanisms and
eventually assess the potential risks associated with future
large volcanic eruptions.
This PhD work aims at exploring the above mentioned issues
with the IPSL-CM6A-LR model as part of framed PMIP4 and VolMIP
standardized CMIP6 coupled model experiments designed to
systematically tackle specific uncertainty factors. The first
part of the thesis is devoted to characterising the simulated
NAO signal in winters following stratospheric volcanic eruptions
using three long transient simulations of the last millennium
(500-1849 CE). The uncertainties related to the season, strength
and the latitude of the eruptions were also explored. The
results reveal that the model simulates increased warming in the
stratosphere, a stronger polar vortex and a surface pattern
similar to the positive phase of the NAO in the first winter
following a summer or a winter tropical eruption, with polar
night jet responses amplitudes that are linearly related to the
eruption strength. No linear relationship with the eruption
magnitude is identified for extra-tropical Northern Hemisphere
events while a tendency towards a positive NAO phase is only
significant during the same winter as the eruption occurrence.
To further assess the physical mechanisms and the robust
responses related to sampling climate background conditions, the
second part of the study uses a suite of 25-members ensemble
simulations for the well observed Mt Pinatubo tropical eruption
(Philippines, June 1991). Ensemble members all start from
predefined initial conditions sampling the full ENSO cycle and
use the same volcanic forcing dataset following the VolMIP
protocol. Our experimental protocol also comprises sensitivity
simulations designed to separate the influence of
volcanically-induced surface cooling and stratospheric heating
on key observed features during the first two winters following
Pinatubo. Results indicate that our model experiments and
observations both have a positive NAO-like pattern in the first
winter after the eruptions. The model also simulates realistic
levels of warming in the stratosphere while, both mean sea-level
pressure and temperature anomalies display similar patterns as
in observations in the first winter. Sensitivity experiments
indicate that the surface positive NAO signature in our model
experiments is primarily attributable to stratospheric heating
in the lower tropical stratosphere which generates stronger
subtropical zonal winds through the thermal wind balance and
accelerates the polar vortex.