This work is a direct continuation of McKinney et al., who attempted to create a planet with Earth-like temperatures and physical properties but with precipitation and circulation patterns that were Titan-like. McKinney et al. attempted to do so by changing only three basic planetary parameters: the ratio of dry land to ocean on the surface, the rotation period, and the volatility of the condensable. Each of these parameters is varied from an Earth-like value to a Titan-like one to analyze the climate transition between these two planetary archetypes. In this work, we expand on McKinney et al. by including a seasonal cycle and increasing the number of diagnostic criteria for determining Titan-like dynamics. The simulations use Earth-like obliquity and an Earth-like solar constant. We find that the presence of a dry land strip extending to at least 55°N/S is most effective at creating Titan-like climatic conditions on an otherwise Earth-like planet, such as high-latitude summer precipitation maxima and a low-humidity equator. In contrast, slow rotation and high atmospheric vapor abundance have minimal climatic impacts despite being characteristic features of Titan. Our experiments show that it is not difficult to produce distinctly Titan-like features in an Earth-like GCM with minimal changes to its fundamental parameters. This suggests that Earth-like planets could have a large range of global climate states throughout their history just through changes in topography. Similarly, Titan may have experienced more Earth-like climate states in periods where its tropics were wetter.
Dynamical Regimes of Polar Vortices on Terrestrial Planets with a Seasonal Cycle
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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- Award ID(s):
- 1902409
- PAR ID:
- 10413781
- Date Published:
- Journal Name:
- The Planetary Science Journal
- Volume:
- 3
- Issue:
- 4
- ISSN:
- 2632-3338
- Page Range / eLocation ID:
- 94
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Zonal jets are common in planetary atmospheres. Their character, structure, and seasonal variability depend on the planetary parameters. During solstice on Earth and Mars, there is a strong westerly jet in the winter hemisphere and weak, low-level westerlies in the ascending regions of the Hadley cell in the summer hemisphere. This summer jet has been less explored in a broad planetary context, both due to the dominance of the winter jet and since the balances controlling it are more complex, and understanding them requires exploring a broader parameter regime. To better understand the jet characteristics on terrestrial planets and the transition between winter- and summer-dominated jet regimes, we explore the jet’s dependence on rotation rate and obliquity. Across a significant portion of the parameter space, the dominant jet is in the winter hemisphere, and the summer jet is weaker and restricted to the boundary layer. However, we show that for slow rotation rates and high obliquities, the strongest jet is in the summer rather than the winter hemisphere. Analysis of the summer jet’s momentum balance reveals that the balance is not simply cyclostrophic and that both boundary layer drag and vertical advection are essential. At high obliquities and slow rotation rates, the cross-equatorial winter cell is wide and strong. The returning poleward flow in the summer hemisphere is balanced by low-level westerlies through an Ekman balance and momentum is advected upward close to the ascending branch, resulting in a midtroposphere summer jet.more » « less
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Abstract Saturn’s largest moon, Titan, has an Earth-like volatile cycle, but with methane playing the role of water and surface liquid reservoirs geographically isolated at high latitudes. We recreate Titan’s characteristic dry hydroclimate at the equator of an Earth-like climate model without seasons and with water as the condensable by varying a small set of planetary parameters. We use three observationally motivated criteria for Titan-like conditions at the equator: 1) the peak in surface specific humidity is not at the equator, despite it having the warmest annual-mean temperatures; 2) the vertical profile of specific humidity in the equatorial column is nearly constant through the lower troposphere; and 3) the relative humidity near the surface at the equator is significantly lower than saturation (lower than 60%). We find that simply reducing the available water at the equator does not fully reproduce Titan-like conditions. We additionally vary the rotation period and volatility of water to mimic Titan’s slower rotation and more abundant methane vapor. Longer rotation periods coupled with a dry equatorial surface meet fewer of the Titan-like criteria than equivalent experiments with shorter rotation periods. Experiments with higher volatility of water meet more criteria than those with lower volatility, with some of those with the highest volatility meeting all three, demonstrating that an Earth-like planet can display Titan-like climatology by changing only a few physical parameters.more » « less
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Vortex crystals are quasiregular arrays of like-signed vortices in solid-body rotation embedded within a uniform background of weaker vorticity. Vortex crystals are observed at the poles of Jupiter and in laboratory experiments with magnetized electron plasmas in axisymmetric geometries. We show that vortex crystals form from the free evolution of randomly excited two-dimensional turbulence on an idealized polar cap. Once formed, the crystals are long lived and survive until the end of the simulations (300 crystal-rotation periods). We identify a fundamental length scale, L γ = ( U / γ ) 1 / 3 , characterizing the size of the crystal in terms of the mean-square velocity U of the fluid and the polar parameter γ = f p / a p 2 , with f p the Coriolis parameter at the pole and a p the polar radius of the planet.more » « less
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The midwinter suppression of eddy activity in the North Pacific storm track is a phenomenon that has resisted reproduction in idealized models that are initialized independently of the observed atmosphere. Attempts at explaining it have often focused on local mechanisms that depend on zonal asymmetries, such as effects of topography on the mean flow and eddies. Here an idealized aquaplanet GCM is used to demonstrate that a midwinter suppression can also occur in the activity of a statistically zonally symmetric storm track. For a midwinter suppression to occur, it is necessary that parameters, such as the thermal inertia of the upper ocean and the strength of tropical ocean energy transport, are chosen suitably to produce a pronounced seasonal cycle of the subtropical jet characteristics. If the subtropical jet is sufficiently strong and located close to the midlatitude storm track during midwinter, it dominates the upper-level flow and guides eddies equatorward, away from the low-level area of eddy generation. This inhibits the baroclinic interaction between upper and lower levels within the storm track and weakens eddy activity. However, as the subtropical jet continues to move poleward during late winter in the idealized GCM (and unlike what is observed), eddy activity picks up again, showing that the properties of the subtropical jet that give rise to the midwinter suppression are subtle. The idealized GCM simulations provide a framework within which possible mechanisms giving rise to a midwinter suppression of storm tracks can be investigated systematically.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3847/PSJ/ac54b6
