Variability and predictability of blocking regimes from idealised to general circulation models

Public Thesis Defense of Anupama Kuttikkat Xavier: "Variability and predictability of blocking regimes from idealised to general circulation models"

Summary

Atmospheric blocking patterns, defined by persistent high-pressure systems that disrupt the midlatitude circulation, are pivotal elements of weather variability and key drivers of extreme events such as prolonged cold spells, heatwaves, droughts, and persistent precipitation. Despite their importance, blocking events remain difficult to predict due to their nonlinear dynamics, sensitivity to initial conditions, and strong interactions with the background flow. This thesis investigates the variability and predictability of blocking using a hierarchy of reduced-order and comprehensive general circulation models, with a particular focus on how blocking stability and predictability depend on geographical context.

Using a reduced order quasi geostrophic land atmosphere model, we examine how blocking arises from idealised dynamical interactions. The model reveals distinct zonal, transition, and blocked regimes, with blocking stability strongly modulated by topographic positioning: upstream blocks are less stable and less predictable than downstream ones.

To assess whether such asymmetries persist in realistic conditions, blocking events in CMIP6 MIROC6 simulations are classified into Eastern and Western North Pacific regimes. An analogue-based error growth analysis shows that Eastern blocks exhibit markedly lower predictability than Western blocks, a result robust across sensitivity tests. This contrast arises from differences in basin geometry, background flow structure, and transient eddy activity.

Information transfer analysis using the Liang - Kleeman index shows that Eastern and Western blocks follow different energetic pathways, shaping their contrasting dynamics. Sensitivity experiments also reveal that model structure and spectral resolution influence simulated low-frequency variability, highlighting the importance of model formulation.

Together, these results demonstrate that blocking variability and predictability are shaped jointly by dynamical stability and geographical setting, and that model hierarchies offer complementary insights into these processes.

Jury Members

Prof. Michel Crucifix (UCLouvain) (Supervisor)
Dr. Stéphane Vannitsem (RMIB) (Supervisor)
Prof. Qiuzhen Yin (UCLouvain) (Chairperson)
Prof. Hugues Goosse (UCLouvain) (Secretary)
Prof. Lesley De Cruz (RMIB and Vrije Universiteit Brussel)
Dr. Gwendal Rivière (LMD, Paris)