Public thesis defense of Jerome Sauer: "Extreme Arctic sea ice lows investigated with rare event sampling"

Summary 

Various studies identified possible drivers of extremes of Arctic sea ice reduction, such as observed in the summers of 2007 and 2012, including sea ice-ocean preconditioning, large-scale atmospheric circulation variability and synoptic-scale cyclones. 

However, a robust quantitative statistical analysis and a better understanding of the predictability of extreme sea ice lows are hindered by the small number of events that can be sampled in observations and numerical simulations. Recent studies tackled the problem of sampling climate extremes by using rare event algorithms, i.e., computational techniques developed in statistical physics to reduce the computational cost required to sample rare events in numerical simulations.

In this thesis, we apply a rare event algorithm to ensemble simulations with the coupled climate models Planet Simulator (PlaSim) and European community Earth-System-Model version 3 (EC-Earth3) to study extremes of summer pan-Arctic sea ice area reduction under present-day climate conditions. 

With both models, we simulate sea ice area anomalies larger in magnitude than observed in 2012, and we compute statistically significant composite maps of dynamical quantities during events with return times of more than 100 years. We exploit the improved statistics of low sea ice states to study their drivers on synoptic to seasonal time scales, including sea ice area, sea ice volume and surface energy budget analyses to disentangle the roles of dynamic vs. thermodynamic forcing on the sea ice.

The methodological, technical and scientific achievements of this thesis open several perspectives. These concern in particular attribution and impact studies, as well as the improvement of seasonal predictions of summer sea ice conditions using data-based statistical models, machine learning-based forecasts and climate model ensemble-based initialized forecasts. The developed strategies can be adjusted to study anomalous sea ice conditions in specific regions of the Arctic and in Antarctica.

Jury members

Prof. François Massonnet (UCLouvain) (Supervisor)
Prof. Francesco Ragone (University of Leicester, U.K.) (Supervisor)
Prof. Quizhen Yin (UCLouvain) (Chairperson)
Prof. Hugues Goosse (UCLouvain) (Secretary)
Dr. Jeroen Wouters (University of Reading, U.K.)  
Dr. Steffen Tietsche (ECMWF, Bonn)