THE MID-WINTER SUPPRESSION OF STORM TRACKS
(with L. Novak and T. Schneider)

Storm track activity has a local minimum in winter over the northern Pacific, maximum activity occurring in fall and spring. This phenomenology appears quite striking since pole-to-equator temperature gradients and upper-tropospheric jet speeds are stronger in winter. Explanations for this phenomenon appear incomplete in the literature. Some involve zonal asymmetries, like the role of the Tibetan Plateau, upstream seeding or jet diffluence. Others build on zonallly symmetric theories of baroclinic growth and barotropic decay, like the role of latent heat release or the role of jet structure. In order to investigate zonally symmetric theories and unravel mechanisms, we use a hierarchy of aquaplanet idealized GCMs in a zonally symmetric setting.

Publication

L. Novak, T. Schneider, and F. Ait-Chaalal, 2020
"Midwinter suppression of storm tracks in an idealized zonally symmetric setting ",
Journal of the Atmospheric Sciences, Vol. 77, 297-313
[download pdf][Official link]

TURBULENT CLOSURES AND DIRECT STATISTICAL SIMULATIONS OF GEOPHYSICAL FLOWS<
(with J.B. Marston and T. Schneider)

We are developing approaches to computing geophysical flows statistics directly, rather than through aggregation of direct numerical simulations, as is currently done, for example, in climate models. The research draws on concepts from atmospheric dynamics and theoretical physics. The long-term goal is the development of a new generation of atmosphere models that provide climate statistics directly and that are more efficient than current models in the simulation of deep-time climate and in providing insight into climatic phenomena.

Our approach is to explore how turbulent statistical closures capture atmospheric flows. Besides providing new methods to compute climate, this method might help to build physical laws for climate. For example, a comprehensive theory that would predict the dependence of eddy fluxes with mean fields is currently missing for planetary flows and constitutes an outstanding theoretical challenge.

Publication

MOMENTUM FLUX IN LARGE-SCALE ATMOSPHERIC FLOWS
(with T. Schneider)

Large-scale baroclinic Rossby waves are essential in setting the general circulation of Earth's atmosphere. They are generated at mid-latitudes through baroclinic instability, propagate meridionaly and dissipate before they reach their critical lines in the flank of the jet streams. Propagating Rossby waves transports (angular) momentum toward where they are generated. Consequently, generation and dissipation of large-scale eddies at different latitudes causes meridional momentum flux, with convergence at mid-latitudes and divergence in subtropics and to a lesser extent in polar regions. Yet it is unclear why the eddy momentum fluxes are concentrated in the upper troposphere. With that question, one of the most basic features of the general circulation of atmospheres remains unexplained.

We address the question of what controls the structure of eddy momentum fluxes with a hierarchy of dry idealized GCMs. We use simplified GCMs in which, for example, nonlinear eddy-eddy interactions (but not eddy–mean flow interactions) are suppressed, to determine which atmospheric turbulence characteristics are responsible for the structure of eddy momentum fluxes.

MIXING OF PASSIVE AND ACTIVE TRACERS IN GEOPHYSICAL FLOWS

This research is motivated by the need to better understand the effect of resolution on the ozone layer chemistry in Chemistry-Climate models.The interest and relevance of this research are also fundamental, since the mixing of chemically active tracers in chaotic and turbulent flows is far from being satisfactorily understood.