The Impact of GCM Dynamical Cores on Idealized Sudden Stratospheric Warmings and Their QBO Interactions

Abstract

The paper demonstrates that sudden stratospheric warmings (SSWs) can be simulated in an ensemble of dry dynamical cores that miss the typical SSW forcing mechanisms like moist processes, land–sea contrasts, or topography. These idealized general circulation model (GCM) simulations are driven by a simple Held–Suarez–Williamson (HSW) temperature relaxation and low-level Rayleigh friction. In particular, the four dynamical cores of NCAR’s Community Atmosphere Model, version 5 (CAM5), are used, which are the semi-Lagrangian (SLD) and Eulerian (EUL) spectral-transform models and the finite-volume (FV) and the spectral element (SE) models. Three research themes are discussed. First, it is shown that SSW events in such idealized simulations have very realistic flow characteristics that are analyzed via the SLD model. A single vortex-split event is highlighted that is driven by wavenumber-1 and -2 wave–mean flow interactions. Second, the SLD simulations are compared to the EUL, FV, and SE dynamical cores, which sheds light on the impact of the numerical schemes on the circulation. Only SLD produces major SSWs, while others only exhibit minor stratospheric warmings. These differences are caused by SLD’s more vigorous wave–mean flow interactions in addition to a warm pole bias, which leads to relatively weak polar jets in SLD. Third, it is shown that tropical quasi-biennial oscillation (QBO)–like oscillations and SSWs can coexist in such idealized HSW simulations. They are present in the SLD dynamical core that is used to analyze the QBO–SSW interactions via a transformed Eulerian-mean (TEM) analysis. The TEM results provide support for the Holton–Tan effect.

Analysis of Planetary Scale Waves Using Idealized Sudden Stratospheric Warming Simulations in Different Dynamical Cores

“…Additionally, the amplitude of the negative temperature gradient and its variation were smaller in the FV model, as seen in Figure 3. This is consistent with the results of Yao and Jablonowski (2016), who also observed a small variance in the FV model, although they focused on dynamical cores' performance without considering the impact of global topographic forcing. These differences in the performance of each dynamical core in simulating SSWs emphasize the importance of considering the characteristics and limitations of each core when interpreting model results and understanding the processes involved in SSWs.…”

“…This difference in performance among the different dynamical cores could be attributed to their unique dynamic or numerical characteristics. Yao and Jablonowski (2016) also found that major SSWs were recorded only in the SLD model in idealized test simulations, but the role of large‐scale forcing was not clear since their test setup was not coupled with global terrain fields. Furthermore, they did not analyze the potential mechanisms behind the strong dependence of the stratospheric circulation on the dynamical cores.…”

“…Gerber and Polvani (2009;GP09) conducted a study on stratosphere-troposphere coupling in a simple perpetual Northern Hemisphere winter setup with idealized topography, and found that stratospheric variability changed due to the interaction between topographic forcing and polar vortex strength. Yao and Jablonowski (2016) and Gupta et al (2020) have demonstrated the sensitivity of the dynamical core from idealized SSW simulations. Although these studies revealed an underestimation of SSW from the finite-volume (FV) core, their research focused on the diverse results of the simulations depending on different dynamical cores.…”

Numerical impacts on tracer transport: A proposed intercomparison test of Atmospheric General Circulation Models

“…There is substantial spread in the response of the QBO period to global warming, which is chiefly attributed to uncertainty in the parametrization of gravity waves (e.g., Richter et al ., 2020). Based on the behaviour of the dycores in our study and other experiments (Yao and Jablonowski 2015; 2016), we suspect that part of this uncertainty also stems from the numerical formulation of the dynamical cores.…”

“…We focus on the circulation and transport through the stratosphere for two reasons. First, differences in the numerical representation of stratospheric dynamics between dynamical cores has motivated several past studies (e.g., Yao and Jablonowski, 2015; 2016). Subtle differences in momentum transport by waves and the mean flow, and the ability of schemes to conserve angular momentum, can lead to substantial differences in the circulation of the tropical stratosphere related to the Quasi‐Bienniel Oscillation (QBO).…”