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1. Solsticial Hadley Cell ascending edge theory from supercriticality

   https://doi.org/10.1175/JAS-D-20-0341.1  

   Hill, Spencer A. ; Bordoni, Simona ; Mitchell, Jonathan L. ( April 2021 , Journal of the Atmospheric Sciences)

   null (Ed.)

   Abstract How far the Hadley circulation’s ascending branch extends into the summer hemisphere is a fundamental but incompletely understood characteristic of Earth’s climate. Here, we present a predictive, analytical theory for this ascending edge latitude based on the extent of supercritical forcing. Supercriticality sets the minimum extent of a large-scale circulation based on the angular momentum and absolute vorticity distributions of the hypothetical state were the circulation absent. We explicitly simulate this latitude-by-latitude radiative-convective equilibrium (RCE) state. Its depth-averaged temperature profile is suitably captured by a simple analytical approximation that increases linearly with sin φ , where φ is latitude, from the winter to the summer pole. This, in turn, yields a one-third power-law scaling of the supercritical forcing extent with the thermal Rossby number. In moist and dry idealized GCM simulations under solsticial forcing performed with a wide range of planetary rotation rates, the ascending edge latitudes largely behave according to this scaling. 

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2. The Circulation Response to Volcanic Eruptions: The Key Roles of Stratospheric Warming and Eddy Interactions

   https://doi.org/10.1175/JCLI-D-18-0099.1  

   DallaSanta, Kevin ; Gerber, Edwin P. ; Toohey, Matthew ( February 2019 , Journal of Climate)

   Proxy data and observations suggest that large tropical volcanic eruptions induce a poleward shift of the North Atlantic jet stream in boreal winter. However, there is far from universal agreement in models on this effect and its mechanism, and the possibilities of a corresponding jet shift in the Southern Hemisphere or the summer season have received little attention. Using a hierarchy of simplified atmospheric models, this study examines the impact of stratospheric aerosol on the extratropical circulation over the annual cycle. In particular, the models allow the separation of the dominant shortwave (surface cooling) and longwave (stratospheric warming) impacts of volcanic aerosol. It is found that stratospheric warming shifts the jet poleward in both the summer and winter hemispheres. The experiments cannot definitively rule out the role of surface cooling, but they provide no evidence that it shifts the jet poleward. Further study with simplified models demonstrates that the response to stratospheric warming is remarkably generic and does not depend critically on the boundary conditions (e.g., the planetary wave forcing) or the atmospheric physics (e.g., the treatment of radiative transfer and moist processes). It does, however, fundamentally involve both zonal-mean and eddy circulation feedbacks. The time scales, seasonality, and structure of the response provide further insight into the mechanism, as well as its connection to modes of intrinsic natural variability. These findings have implications for the interpretation of comprehensive model studies and for postvolcanic prediction.

    

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3. Dynamical Regimes of Polar Vortices on Terrestrial Planets with a Seasonal Cycle

   https://doi.org/10.3847/PSJ/ac54b6  

   Guendelman, Ilai ; Waugh, Darryn W. ; Kaspi, Yohai ( April 2022 , The Planetary Science Journal)

   Abstract Polar vortices are common planetary-scale flows that encircle the pole in the middle or high latitudes and are observed in most of the solar system’s planetary atmospheres. The polar vortices on Earth, Mars, and Titan are dynamically related to the mean meridional circulation and exhibit a significant seasonal cycle. However, the polar vortex’s characteristics vary between the three planets. To understand the mechanisms that influence the polar vortex’s dynamics and dependence on planetary parameters, we use an idealized general circulation model with a seasonal cycle in which we vary the obliquity, rotation rate, and orbital period. We find that there are distinct regimes for the polar vortex seasonal cycle across the parameter space. Some regimes have similarities to the observed polar vortices, including a weakening of the polar vortex during midwinter at slow rotation rates, similar to Titan’s polar vortex. Other regimes found within the parameter space have no counterpart in the solar system. In addition, we show that for a significant fraction of the parameter space, the vortex’s potential vorticity latitudinal structure is annular, similar to the observed structure of the polar vortices on Mars and Titan. We also find a suppression of storm activity during midwinter that resembles the suppression observed on Mars and Earth, which occurs in simulations where the jet velocity is particularly strong. This wide variety of polar vortex dynamical regimes that shares similarities with observed polar vortices, suggests that among exoplanets there can be a wide variability of polar vortices. 

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4. Quasi‐Biennial Oscillation of Short‐Period Planetary Waves and Polar Night Jet in Winter Antarctica Observed in SABER and MERRA‐2 and Mechanism Study With a Quasi‐Geostrophic Model

   https://doi.org/10.1029/2019GL084759  

   Lu, Xian ; Wu, Haonan ; Chu, Xinzhao ; Oberheide, Jens ; Mlynczak, Martin G. ; Russell, III, James M. ( November 2019 , Geophysical Research Letters)

   Analyzing Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations from 2003 to 2018, the interannual variability of 2–5d eastward propagating planetary waves is found to correlate positively with zonal‐mean zonal winds averaged over 67.5°±10°S but negatively with the quasi‐biennial oscillation (QBO) index in austral winter. The composite‐mean wave amplitudes are ~20% larger in QBOe than in QBOw. On statistical average, the poleward flank strengthening and the equatorward flank weakening of polar night jet (PNJ) during QBOe form a dipole‐cell pattern. In contrast, only a single negative cell is seen in the Northern Hemisphere zonal‐mean zonal winds (January) previously explained by the Holton‐Tan theory. Such difference implies an interhemispheric asymmetry and other processes needed to explain the additional positive cell in Antarctica. Mechanistic modeling illustrates that the stronger PNJ generates eastward propagating planetary waves with larger growth rates (stronger waves) in QBOe than QBOw, explaining the QBO‐like signal in the Antarctic planetary waves.

    

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5. Interhemispheric Coupling Mechanisms in the Middle Atmosphere of WACCM6

   https://doi.org/10.1175/JAS-D-19-0253.1  

   Smith, A. K. ; Pedatella, N. M. ; Mullen, Z. K. ( March 2020 , Journal of the Atmospheric Sciences)

   Simulations with the Community Earth System Model, version 2, using the Whole Atmosphere Community Climate Model version 6 [CESM2(WACCM6)] configuration, show evidence of dynamical coupling from the high latitudes of the winter middle atmosphere to the tropics and the middle and high latitudes of the summer hemisphere. Analysis of monthly and daily output covering 195 simulation years indicates that the response in the summer middle and high latitudes has a weak overall magnitude of a few kelvins or less in temperature but has a repeatable pattern whose structure and phase agree with observational studies. Lag correlation indicates that perturbations in wave activity in the winter stratosphere, as quantified by Eliassen–Palm (EP) flux divergence, are accompanied by perturbations in the transformed Eulerian-mean meridional wind extending into the summer hemisphere. There is not an appreciable correlation with momentum forcing in the summer hemisphere by either resolved waves or parameterized gravity waves. The rapid circulation response and the lack of a wave response in the summer hemisphere suggest that the interhemispheric coupling that is simulated in WACCM6 in both the stratosphere and the mesosphere owes its existence to a circulation that develops to restore balance to the zonally averaged state of the atmosphere. This is an alternative explanation for the coupling from the winter stratosphere to the summer mesosphere; previous studies have assumed a necessary role for wave activity in the summer hemisphere.

    

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