Impact of Atmospheric Cloud Radiative Effects on Annular Mode Persistence in Idealized Simulations

• Published in: Geophysical research letters 2024-08, Vol.51 (15), p.n/a
• Main Authors: Vishny, David N., Wall, Casey J., Lutsko, Nicholas J.
• Format: Article
• Language: English
• Subjects: Antarctic Oscillation; Atmosphere; Atmospheric cooling; Atmospheric models; Atmospheric waves; Clouds; Concentration gradient; Damping; Diabatic heating; Heating; Heating and cooling; Jet stream; Jet streams (meteorology); Latitude; Nonlinear response; Nonlinearity; Rivers; Southern Hemisphere; Streams; Temperature effects; Temperature gradients; Variability; Wave generation; Wind; Wind effects; Zonal winds
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2024GL109420

The mechanisms by which clouds impact the variability of the mid‐latitude atmosphere are poorly understood. We use an idealized, dry atmospheric model to investigate the relationship between Atmospheric Cloud Radiative Effects (ACRE) and annular mode persistence. We force the model with time‐varying diabatic heating that mimics the observed ACRE response to the Southern Annular Mode (SAM). Realistic ACRE forcing reduces annular mode persistence by 5 days (−16%), which we attribute to a weakening of low‐frequency eddy forcing via modified low‐level temperature gradients, though this effect is partly compensated by reduced frictional damping due to zonal wind anomalies becoming more top‐heavy. The persistence changes are nonlinear with respect to the amplitude of ACRE forcing, reflecting nonlinearities in the response of the eddy forcing. These results highlight the ACRE's impact on low‐frequency eddy forcing as the dominant cause of changes in annular mode persistence. Plain Language Summary In this study, we conduct novel modeling experiments aimed at understanding how clouds affect the unforced variability of jet streams—concentrated bands of eastward wind in the mid‐latitude atmosphere. We focus on direct heating and cooling of the atmosphere by clouds, called Atmospheric Cloud Radiative Effects (ACRE), and how these impact the annular mode—the most significant type of jet stream variability, which consists of north‐south shifts of the jet's position. Using a simplified atmospheric model, we investigate how the persistence and structure of the annular mode are impacted by heatings and coolings that mimic the observed ACRE response to the Southern Hemisphere Annular Mode (SAM). We find that the ACRE reduces the persistence of the annular mode (i.e., deviations of the jet stream from its mean position tend to persist for less time), which we attribute to a weakening of the feedback mechanism by which atmospheric waves strengthen the jet where it is already strongest. We identify some compensating effects, but, overall, our results highlight that ACRE impacts jet streams via low‐altitude temperature gradients and atmospheric wave generation. Key Points The response of Atmospheric Cloud Radiative Effects (ACRE) to the Southern Annular Mode (SAM) reduces SAM's persistence The decrease in SAM's persistence is attributed to weakened low‐frequency eddy forcing via disruption of low‐level temperature gradients The change in persistence is nonlinear with respect t