Session H02: Minisymposia: Fluids Next: Environmental Turbulent Flows Under the Effect of Climate Change

Climate Variability and Climate Change: A Unified Framework

The “death of stationarity” poses a substantial challenge to climate predictability and to the climate sciences in general. This challenge is addressed herein by formulating the problems of change in the climate’s intrinsic variability within the framework of the theory of nonautonomous and random dynamical systems (NDS and RDS) with time-dependent forcing. A key role in this theory is played by the pullback attractors (PBAs) that replace the strange attractors of the more familiar theory of autonomous dynamical systems, in which there is no explicit time dependence of either forcing or coefficients.

The concepts and methods of the NDS and RDS approach will be introduced and will be illustrated using a stochastically perturbed version of the Lorenz (1963) convection model. This illustration will be followed by applications to models of the wind-driven ocean circulation. One finds that two local PBAs, a quiescent and a chaotic one, coexist within the wind-driven ocean model's decadally modulated global PBA.

Implications for the climate sciences in general and for atmospheric, oceanic and coupled ocean-atmosphere dynamics in particular will be discussed from the perspective of the Anthropocene.

References

Ghil, M. and V. Lucarini, 2020: The physics of climate variability and climate change, Rev. Mod. Phys., 92(3), 035002, doi:10.1103/RevModPhys.92.035002.

Pierini, S., and M. Ghil, 2021: Climate tipping points induced by parameter drift: an excitable system study, Scientific Reports, 11, 11126, doi:10.1038/s41598-021-90138-1.

Vannitsem, S., J. Demaeyer, and M. Ghil, 2021: Extratropical low-frequency variability with ENSO forcing: A reduced-order coupled model study, Journal of Advances in Modeling Earth Systems, 13, e2021MS002530, doi:10.1029/2021MS002530.   

The response of the general circulation of the atmosphere to increased CO2

How Earth’s surface climate will change in the future depends on the response of the general circulation of the atmosphere to increased CO2. The emergent response of the general circulation of the atmosphere to increased CO2 in state-of-the-art climate models involves a weakening and expansion of the Hadley cell and a strengthening and poleward shift of the extratropical jet stream and storm track. In this talk I will summarize theory and numerical experiments that support a causal connection between future changes in the general circulation of the atmosphere and moist physics. The theory focuses on the Clausius-Clapeyron scaling of specific humidity with warming and its impact on surface winds and turbulent surface heat exchange. The numerical simulations show that when the response of moist physics is disabled the response of the general circulation of the atmosphere to increased CO2 is insignificant.   

Energy transfers across scales facilitated by the interaction of wind-driven internal waves and ocean fronts

In the ocean, wind-generated kinetic energy (KE) manifests itself primarily in balanced currents and near-inertial waves. The dynamics of these flows is strongly constrained by the Earth’s rotation, causing the KE in balanced currents to follow an inverse cascade but also preventing wave-wave interactions from fluxing energy in the near-inertial band to lower frequencies and higher vertical wavenumbers. How wind-generated KE is transferred to small-scale turbulence and dissipated is thus a non-trivial problem. In this talk I will present an overview of theoretical calculations and numerical simulations that demonstrate how some surprising modifications to internal wave physics by the lateral density gradients present at ocean fronts allow for strong interactions between balanced currents and near-inertial waves that can ultimately result in energy loss for both types of motion.   

Small and large scale driving icebergs and glaciers melting

Rising global temperatures have led to an increase in the discharge of ice from the Antarctic and Greenland ice sheets. For the Greenland Ice Sheet, this mass flux to the ocean includes: surface melt, leaving the ice sheets as runoff and subglacial discharge; subsurface melt; and calving of icebergs. The freshwater discharged from Greenland is transformed by fjord processes before being released into the large-scale ocean. Hence, knowledge of the fjords’ dynamics is fundamental to understand how the input of modified freshwater into the ocean impacts ocean dynamics and climate.

Major gaps in understanding include the interaction of the buoyancy-driven circulation (forced by the glacier) and shelf-driven circulation, and the dynamics in the near-ice zone. The former occurs on scales of hundreds of meters to kilometers, the latter on scales of millimeters to meters. The links between small and large scales processes that influence the climate system, via the melting of icebergs and glaciers and the freshwater discharge in the ocean, must be understood before appropriate forcing conditions can be supplied to ice sheet and ocean/climate models.   

New ways for dynamical prediction of extreme heat waves: rare event simulations and stochastic process-based machine learning.

In the climate system, extreme events or transitions between climate attractors are of primarily importance for understanding the impact of climate change. Recent extreme heat waves with huge impact are striking examples. However, they cannot be studied with conventional approaches, because they are too rare and realistic models are too complex. 

We will discuss several new algorithms and theoretical approaches, based on large deviation theory, rare event simulations, and machine learning for stochastic processes, which we have specifically designed for the prediction of the committor function (the probability of the extreme event to occur). We will discuss results for the study of midlatitude extreme heat waves and demonstrate the performance of these tools.

Using the best available climate models, our approach shed new light on the fluid mechanics processes which lead to extreme heat waves. We will describe quasi-stationary patterns of turbulent Rossby waves that lead to global teleconnection pattern in connection with heat waves and analyze their dynamics.  

We stress the relevance of these patterns for recently observed extreme heat waves with huge impact and the prediction potential of our approach.