Historic large-scale atmospheric flow changes in models and observations

The large-scale atmospheric flow plays a central role in shaping regional weather and climate on Earth. On daily to multi-decadal timescales, tropical and extra-tropical flows are responsible for transporting heat, momentum, and moisture across latitudes and longitudes, thereby affecting the distribution of precipitation, temperature, and winds. It is thus critical to assess the impacts of anthropogenic emissions on the large-scale flow. Yet, while observed thermodynamic changes have been attributed to human emissions with high confidence, there is currently a large uncertainty in recent circulation changes. One main reason for this uncertainty is the presence of discrepancies between observations and climate models, evident in various large-scale flow variables, including extra-tropical storm tracks and jet streams, and the tropical Hadley and Walker circulations. These discrepancies preclude accurately assessing both how the large-scale atmospheric flow has changed in recent decades and the impacts of human emissions on the circulation. Moreover, they prevent informing our confidence in climate projections. Here, I will provide an overview of historic large-scale flow changes in models and observations and assess the ability of models to capture observed trends in recent decades. I will then discuss several attempts to unravel the sources of model-observation differences in recent decades, and conclude with potential paths our community could take to diminish such discrepancies.