Abstract. Isca is a framework for the idealized modelling of the global circulation of planetary atmospheres at varying levels of complexity and realism.

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.

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) 1 1 2 -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 1 1 2 -layer dynamics. 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 quasifrictionally 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 lengthscale 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 "CE2" or "SSST" theories), produces unrealistic belt-zone contrasts.

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.

In this paper and its companion, Part I, we explore the response of the atmosphere to sea surface temperature anomalies in different geographical locations and seasons. In Part I, we focused on Northern Hemisphere winter (DJF), whereas in this paper, Part II, we focus on summer (JJA) and interseasonal comparisons. We use two different configurations of the same idealized atmospheric model, constructed using two different configurations of continents and topography. These configurations give rise to slightly 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 characterize the types of responses that are found to SST anomalies in the midlatitudes and tropics in JJA and compare these with the two corresponding responses in DJF. We find that the responses to midlatitude SST anomalies in JJA are generally on a much smaller spatial scale than those in DJF. Responses in the tropical Pacific are much less dependent on season, although teleconnections between the tropical Pacific and the North Atlantic are not found in JJA as robustly as they are in DJF. Given insight from our model results, however, we do find some summer periods in reanalysis data where there is a strong association between the tropical Pacific and the summer North Atlantic Oscillation. We discuss the reasons for these effects and the implications for Northern Hemisphere seasonal prediction in summer.