Moisture and Persistence of Annular Mode: Insights from Idealized Simulation of GFDL Spectral Model

Annular mode, one of the leading variability in midlatitude atmosphere, plays a crucial role in determining the predictability of midlatitude weather. According to its characteristics, it can be categorized into barotropic or baroclinic annular mode, each representing different parts of the Lorenz energy cycle (Ma, Hassanzadeh, and Kuang 2019). Over the past decade, extensive research has been conducted to understand the factors governing the time scales (i.e., persistence and frequency) of the annular mode, with particular emphasis on its barotropic component.

Previous research has identified a systematic bias in the persistence of annular modes across a wide spectrum of model complexity, largely attributed to positive biases in eddy-mean flow feedbacks. Several factors have been recognized as key contributors to these positive biases, with latent heat feedback being one of the primary. While the impact of global warming on latent heat feedback remains uncertain, it is crucial to understand how changes in eddy-mean flow feedback respond to variations in latent heat feedback across model hierarchy.

Lutsko and Hell (2021), adding moisture as active tracer, explored how latent heat feedback affects annular mode variability within using a two-layer quasi-geostrophic model. They find that the jet streams not only strengthen and move poleward with higher latent heat release efficiency but also show increased persistence in the annular mode due to the increase in eddy-mean flow feedback parameter and nearly constant barotropic friction.

Expanding on their work, this research employed the similar experiments of latent heat with the GFDL Dynamical core (Dycore) given that it represents the next-level of model hierarchy. Different from Lutsko and Hell (2021), the poleward shift of the subtropical jet is accompanied by a decrease in eddy-mean flow feedback, while the barotropic friction decreases due to the increase of vertical wind shear. The cancelation between the former (eddy-mean flow feedback) and the later (barotropic friction) yields a net increase in annular mode persistence. Further evidence is provided through the framework of Robinson (2000) theory.

Reference:

1. Ma, D., P. Hassanzadeh, and Z. Kuang, 2017: Quantifying the Eddy–Jet Feedback Strength of the Annular Mode in an Idealized GCM and Reanalysis Data. J. Atmos. Sci., 74, 393–407, https://doi.org/10.1175/JAS-D-16-0157.1.
2. Lutsko, N. J., and M. C. Hell, 2021: Moisture and the Persistence of Annular Modes. J. Atmos. Sci., 78, 3951–3964, https://doi.org/10.1175/JAS-D-21-0055.1.
3. Robinson, W. A., 2000: A Baroclinic Mechanism for the Eddy Feedback on the Zonal Index. J. Atmos. Sci., 57, 415–422, https://doi.org/10.1175/1520-0469(2000)057<0415:ABMFTE>2.0.CO;2.