Isca is a framework for the idealized modelling of the global circulation of planetary atmospheres at varying levels of complexity and realism. The framework is an outgrowth of models from the Geophysical Fluid Dynamics Laboratory in Princeton, USA, designed for Earth's atmosphere, but it may readily be extended into other planetary regimes. Various forcing and radiation options are available, from dry, time invariant, Newtonian thermal relaxation to moist dynamics with radiative transfer. Options are available in the dry thermal relaxation scheme to account for the effects of obliquity and eccentricity (and so seasonality), different atmospheric optical depths and a surface mixed layer. An idealized grey radiation scheme, a two-band scheme, and a multiband scheme are also available, all with simple moist effects and astronomically based solar forcing. At the complex end of the spectrum the framework provides a direct connection to comprehensive atmospheric general circulation models. For Earth modelling, options include an aquaplanet and configurable continental outlines and topography. Continents may be defined by changing albedo, heat capacity, and evaporative parameters and/or by using a simple bucket hydrology model. Oceanic Q fluxes may be added to reproduce specified sea surface temperatures, with arbitrary continental distributions. Planetary atmospheres may be configured by changing planetary size and mass, solar forcing, atmospheric mass, radiation, and other parameters. Examples are given of various Earth configurations as well as a giant planet simulation, a slowly rotating terrestrial planet simulation, and tidally locked and other orbitally resonant exoplanet simulations. The underlying model is written in Fortran and may largely be configured with Python scripts. Python scripts are also used to run the model on different architectures, to archive the output, and for diagnostics, graphics, and post-processing. All of these features are publicly available in a Git-based repository.

Polar vortices on Mars provide case‐studies to aid understanding of geophysical vortex dynamics and may help to resolve long‐standing issues regarding polar vortices on Earth. Due to the recent development of the first publicly available Martian reanalysis dataset (MACDA), for the first time we are able to characterise thoroughly the structure and evolution of the Martian polar vortices, and hence perform a systematic comparison with the polar vortices on Earth. The winter atmospheric circulations of the two planets are compared, with a specific focus on the structure and evolution of the polar vortices. The Martian residual meridional overturning circulation is found to be very similar to the stratospheric residual circulation on Earth during winter. While on Earth this residual circulation is very different from the Eulerian circulation, on Mars it is found to be very similar. Unlike on Earth, it is found that the Martian polar vortices are annular, and that the Northern Hemisphere vortex is far stronger than its southern counterpart. While winter hemisphere differences in vortex strength are also reported on Earth, the contrast is not as large. Distinctions between the two planets are also apparent in terms of the climatological vertical structure of the vortices, in that the Martian polar vortices are observed to decrease in size at higher altitudes, whereas on Earth the opposite is observed. Finally, it is found that the Martian vortices are less variable through the winter than on Earth, especially in terms of the vortex geometry. During one particular major regional dust storm on Mars (Martian year 26), an equatorward displacement of the vortex is observed, sharing some qualitative characteristics of sudden stratospheric warmings on Earth.

We examine the midlatitude jet stream responses to projected Antarctic and Arctic sea‐ice loss and global ocean warming in coordinated multi‐model experiments from the Polar Amplification Model Intercomparison Project. Antarctic and Arctic sea‐ice loss cause an equatorward shift of the winter jet stream in the southern and northern hemisphere, respectively, on average across the models. Models with stronger eddy feedback simulate farther equatorward jet shifts in response to both Antarctic and Arctic sea‐ice loss. The models simulate too weak eddy feedback compared to the real world, particularly in the northern hemisphere, resulting in an underestimation of the boreal jet response to Arctic sea‐ice loss. More precise estimates of the jet shifts are obtained by using the observed eddy feedback as a constraint and suggest that the equatorward jet shifts in response to Antarctic and Arctic sea‐ice loss exceed in magnitude the simulated poleward shifts due to ocean warming.

A longstanding mystery about Jupiter has been the straightness and steadiness of its weather-layer jets, quite unlike terrestrial strong jets with their characteristic unsteadiness and long-wavelength meandering. The problem is addressed in two steps. The first is to take seriously the classic Dowling–Ingersoll (DI) 1½-layer scenario and its supporting observational evidence, pointing toward deep, massive, zonally symmetric zonal jets in the underlying dry-convective layer. The second is to improve the realism of the model stochastic forcing used to represent the effects of Jupiter’s moist convection, as far as possible within the 1½-layer dynamics of the DI scenario. The real moist convection should be strongest in the belts where the interface to the deep flow is highest and coldest and should generate cyclones as well as anticyclones, with the anticyclones systematically stronger. The new model forcing reflects these insights. Also, it acts quasi frictionally on large scales to produce statistically steady turbulent weather-layer regimes without any need for explicit large-scale dissipation, and with weather-layer jets that are approximately straight thanks to the influence of the deep jets, allowing shear stability despite nonmonotonic potential vorticity gradients when the Rossby deformation length scale is not too large. Moderately strong forcing produces chaotic vortex dynamics and realistic belt–zone contrasts in the model’s convective activity, through an eddy-induced sharpening and strengthening of the weather-layer jets relative to the deep jets, tilting the interface between them. Weak forcing, for which the only jet-sharpening mechanism is the passive, Kelvin shearing of vortices (as in the zonostrophic instability mechanism), produces unrealistic belt–zone contrasts.

Among the great diversity of atmospheric circulation patterns observed throughout the solar system, polar vortices stand out as a nearly ubiquitous planetary‐scale phenomenon. In recent years, there have been significant advances in the observation of planetary polar vortices, culminating in the fascinating discovery of Jupiter's polar vortex clusters during the Juno mission. Alongside these observational advances has been a major effort to understand polar vortex dynamics using theory, idealized and comprehensive numerical models, and laboratory experiments. Here, we review our current knowledge of planetary polar vortices, highlighting both the diversity of their structures, as well as fundamental dynamical similarities. We propose a new convention of vortex classification, which adequately captures all those observed in our solar system, and demonstrates the key role of polar vortices in the global circulation, transport, and climate of all planets. We discuss where knowledge gaps remain, and the observational, experimental, and theoretical advances needed to address them. In particular, as the diversity of both solar system and exoplanetary data increases exponentially, there is now a unique opportunity to unify our understanding of polar vortices under a single dynamical framework.

The atmospheric response to SST anomalies is notoriously difficult to simulate and may be sensitive to model details and biases, particularly in midlatitudes. Studies have suggested that the response is particularly sensitive to a model’s background wind field and its variability. The dependence on such factors has meant that it is difficult to know what responses, if any, are robust, and whether the system itself is sensitive or whether models themselves are failing. Our goal in this work is to better understand the geographical and seasonal dependence of the atmospheric response to SST anomalies, with particular attention to the role of the background state. We examine the response of an idealized atmospheric model to SST anomalies using two slightly different configurations of continents and topography. These configurations give rise to different background wind fields and variability within the same season and therefore give a measure of how robust a response is to small changes in the background state. We find that many of the midlatitude SST anomalies considered do not produce responses that are common across our model configurations, confirming that this problem is very sensitive to the background state. Local responses in the tropics, however, are much more robust. Some of the basic-state dependence seen in midlatitudes appears to be related to the structure of both the model’s modes of internal variability and the stationary wave field. In addition, midlatitude responses involving a significant amount of vertical temperature advection produce larger-scale responses, consistent with recent studies of atmospheric responses near strong western boundary currents.