Publications

Jupiter's equatorial eastward zonal flows reach wind velocities of ∼100 m s⁻¹, while on Saturn they are three times as strong and extend about twice as wide in latitude, despite the two planets being overall dynamically similar. Recent gravity measurements obtained by the Juno and Cassini spacecraft uncovered that the depth of zonal flows on Saturn is about three times greater than on Jupiter. Here we show, using 3D deep convection simulations, that the atmospheric depth is the determining factor controlling both the strength and latitudinal extent of the equatorial zonal flows, consistent with the measurements for both planets. We show that the atmospheric depth is proportional to the convectively driven eddy momentum flux, which controls the strength of the zonal flows. These results provide a mechanistic explanation for the observed differences in the equatorial regions of Jupiter and Saturn, and offer new understandings about the dynamics of gas giants beyond the Solar System.

Chang (2024, https://doi.org/10.1029/2024gl110011) challenged the methodology proposed recently by Okajima et al. for evaluating cyclonic and anticyclonic contributions to Eulerian eddy statistics and atmospheric energetics based on the local flow curvature. He argued that using the local wind curvature to separate energetic contributions from cyclones and anticyclones is not physically meaningful. Here we argue that his claims are based on an unrealistic assumption of monopolar relative vorticity in an entire storm-track domain and a meridionally uniform zonal background flow atypical to midlatitudes. We also demonstrate that the error in attributing eddy statistics to cyclones and anticyclones is significantly smaller than his estimation. Rather, we further demonstrate that the curvature-based methodology effectively eliminates the shear influence to identify cyclonic and anticyclonic regions, which is dismissed in his argument. We conclude that the curvature-based methodology is beneficial in evaluating distinct cyclonic and anticyclonic contributions to atmospheric energetics in realistic conditions.

Climate models generally predict a poleward shift of the midlatitude circulation in response to climate change induced by increased greenhouse gas concentration, but the intermodel spread of the eddy-driven jet shift is large and poorly understood. Recent studies point to the significance of midlatitude midtropospheric diabatic heating for the intermodel spread in the jet latitude. To examine the role of diabatic heating in the jet response to climate change, a series of simulations are performed using an idealized aquaplanet model. It is found that both increased CO₂ concentration and increased saturation vapor pressure induce a similar warming response, leading to a poleward and upward shift of the midlatitude circulation. An exception to this poleward shift is found for a certain range of temperatures, where the eddy-driven jet shifts equatorward, while the latitude of the eddy heat flux remains essentially unchanged. This equatorward jet shift is explained by the connection between the zonal-mean momentum and heat budgets: increased diabatic heating in the midlatitude midtroposphere balances the cooling by the Ferrel cell ascending branch, enabling an equatorward shift of the Ferrel cell streamfunction and eddy-driven jet, while the latitude of the eddy heat flux remains unchanged. The equatorward jet shift and the strengthening of the midlatitude diabatic heating are found to be sensitive to the model resolution. The implications of these results for a potential reduction in the jet shift uncertainty through the improvement of convective parameterizations are discussed.

Letter

Abstract Jupiter's atmosphere comprises several dynamical regimes: the equatorial eastward flows and surrounding retrograde jets; the midlatitudes, with the eddy-driven, alternating jet-streams and meridional circulation cells; and the jet-free turbulent polar region. Despite intensive research conducted on each of these dynamical regimes over the past decades, they remain only partially understood. Saturn's atmosphere also encompasses similar distinguishable regimes, but observational evidence for midlatitude deep meridional cells is lacking. Models offer a variety of explanations for each of these regions, but only a few are capable of simulating more than one of the regimes at once. This study presents new numerical simulations using a 3D deep anelastic model that can reproduce the equatorial flows as well as the midlatitudinal pattern of the mostly barotropic, alternating eddy-driven jets and the meridional circulation cells accompanying them. These simulations are consistent with recent Juno mission gravity and microwave data. We find that the vertical eddy momentum fluxes are as important as the meridional eddy momentum fluxes, which drive the midlatitudinal circulation on Earth. In addition, we discuss the parameters controlling the number of midlatitudinal jets/cells, their extent, strength, and location. We identify the strong relationship between meridional circulation and the zonal jets in a deep convection setup, and analyze the mechanism responsible for their generation and maintenance. The analysis presented here provides another step in the ongoing pursuit of understanding the deep atmospheres of gas giants.

ESAs Jupiter Icy Moons Explorer (JUICE) will provide a detailed investigation of the Jovian system in the 2030s, combining a suite of state-of-the-art instruments with an orbital tour tailored to maximise observing opportunities. We review the Jupiter science enabled by the JUICE mission, building on the legacy of discoveries from the Galileo, Cassini, and Juno missions, alongside ground- and space-based observatories. We focus on remote sensing of the climate, meteorology, and chemistry of the atmosphere and auroras from the cloud-forming weather layer, through the upper troposphere, into the stratosphere and ionosphere. The Jupiter orbital tour provides a wealth of opportunities for atmospheric and auroral science: global perspectives with its near-equatorial and inclined phases, sampling all phase angles from dayside to nightside, and investigating phenomena evolving on timescales from minutes to months. The remote sensing payload spans far-UV spectroscopy (50-210 nm), visible imaging (340-1080 nm), visible/near-infrared spectroscopy (0.49-5.56 μm), and sub-millimetre sounding (near 530-625 GHz and 1067-1275 GHz). This is coupled to radio, stellar, and solar occultation opportunities to explore the atmosphere at high vertical resolution; and radio and plasma wave measurements of electric discharges in the Jovian atmosphere and auroras. Cross-disciplinary scientific investigations enable JUICE to explore coupling processes in giant planet atmospheres, to show how the atmosphere is connected to (i) the deep circulation and composition of the hydrogen-dominated interior; and (ii) to the currents and charged particle environments of the external magnetosphere. JUICE will provide a comprehensive characterisation of the atmosphere and auroras of this archetypal giant planet.

The North Pacific storm-track activity is suppressed substantially under the excessively strong westerlies to form a distinct minimum in midwinter, which seems inconsistent with linear baroclinic instability theory. This \u201cmidwinter minimum\u201d of the storm-track activity has been intensively investigated for decades as a test case for storm-track dynamics. However, the mechanisms controlling it are yet to be fully unveiled and are still under debate. Here we investigate the detailed seasonal evolution of the climatological density of surface migratory anticyclones over the North Pacific, in comparison with its counterpart for cyclones, based on a Lagrangian tracking algorithm. We demonstrate that the frequency of surface cyclones over the North Pacific maximizes in midwinter, whereas that of anticyclones exhibits a distinct midwinter minimum under the upstream influence, especially from the Japan Sea region. In midwinter, it is only on such a rare occasion that prominent weakening of the East Asian winter monsoon allows a migratory surface anticyclone to form over the Japan Sea, despite the unfavorable climatological-mean conditions due to persistent monsoonal cold-air outbreaks and the excessively strong upper-tropospheric westerlies. The midwinter minimum of the North Pacific anticyclone density suggests that anticyclones are likely the key to understanding the midwinter minimum of the North Pacific storm-track activity as measured by Eulerian eddy statistics.

This chapter reviews the way the six key questions about planetary systems, from their origins to the way they work and their habitability, identified in Chapter 1 (Blanc et al., 2021), can be addressed by means of solar system exploration, and how one can find partial answers to these six questions by flying to the different provinces to the solar system: terrestrial planets, giant planets, small bodies, and up to its interface with the local interstellar medium. It derives from this analysis a synthetic description of the most important space observations to be performed at the different solar system objects by future planetary exploration missions. These \u201cobservation requirements\u201d illustrate the diversity of measurement techniques to be used as well as the diversity of destinations where these observations must be made. They constitute the base for the identification of the future planetary missions we need to fly by 2061, which are described in Chapter 4.

Context. The Juno mission has provided measurements of Jupiter's gravity field with an outstanding level of accuracy, leading to better constraints on the interior of the planet. Improving our knowledge of the internal structure of Jupiter is key to understanding its formation and evolution but is also important in the framework of exoplanet exploration. Aims. In this study, we investigated the differences between the state-of-the-art equations of state and their impact on the properties of interior models. Accounting for uncertainty on the hydrogen and helium equation of state, we assessed the span of the interior features of Jupiter. Methods. We carried out an extensive exploration of the parameter space and studied a wide range of interior models using Markov chain Monte Carlo simulations. To consider the uncertainty on the equation of state, we allowed for modifications of the equation of state in our calculations. Results. Our models harbour a dilute core and indicate that Jupiter's internal entropy is higher than what is usually assumed from the Galileo probe measurements. We obtain solutions with extended dilute cores, but contrary to other recent interior models of Jupiter, we also obtain models with small dilute cores. The dilute cores in such solutions extend to ∼20% of Jupiter's mass, leading to better agreement with formation-evolution models. Conclusions. We conclude that the equations of state used in Jupiter models have a crucial effect on the inferred structure and composition. Further explorations of the behaviour of hydrogen-helium mixtures at the pressure and temperature conditions in Jupiter will help to constrain the interior of the planet, and therefore its origin.

The large-scale Hadley circulation is a key element in the global heat and moisture transport. It is traditionally defined as the zonally averaged meridional circulation in the tropics, but was shown to have a strong longitudinal dependence, as seen in a decomposition of the three-dimensional atmospheric flow into spatially dependent meridional and zonal circulations. Recent studies provided a useful analysis of the regional strengthening/weakening of the decomposed circulation but not its patterns. Here, we study the interannual variability of the longitudinally dependent meridional circulation (LMC), with a focus on its spatial patterns. We use hierarchical clustering to objectively determine the four main modes of the LMC interannual variability, and apply a Lagrangian air parcel tracking method to reveal the full circulation patterns. While El Niño and La Niña are found, as in previous studies, to play a role in setting these patterns, we find the patterns are not uniquely characterized by standard El Niño-Southern Oscillation (ENSO) indices (Nino3.4 or Southern Oscillation Index). Instead, ENSO flavors (i.e., East Pacific vs. Central Pacific) have different effects on the LMC. The most prominent interannual variability of the LMC is an east-west shift. Latitudinal shifts, as well as contraction/expansion in both latitude and longitude are also identified. Multiple linear regression analysis shows that while a large fraction of the LMC variance is explained by Sea Surface Temperature, the Madden-Julian Oscillation makes a nonnegligible independent contribution. The clustering patterns are also used to study the remote precipitation and surface air temperature teleconnections.

The polar cyclone at Jupiter's south pole and the five cyclones surrounding it oscillate in position and interact. These cyclones, observed since 2016 by NASA's Juno mission, present a unique opportunity to study vortex dynamics and interactions on long time scales. The cyclones' position data, acquired by Juno's JIRAM instrument, is analyzed, showing dominant oscillations with ∼12 month periods and amplitudes of ∼400 km. Here, the mechanism driving these oscillations is revealed by considering vorticity-gradient forces generated by mutual interactions between the cyclones and the latitudinal variation in planetary vorticity. Data-driven estimation of these forces exhibits a high correlation with the measured acceleration of the cyclones. To further test this mechanism, a model is constructed, simulating how cyclones subject to these forces exhibit similar oscillatory motion.

The Juno spacecraft has been collecting data to shed light on the planets origin and characterize its interior structure. The onboard gravity science experiment based on X-band and Ka-band dual-frequency Doppler tracking precisely measured Jupiters zonal gravitational field. Here, we analyze 22 Junos gravity passes to investigate the gravity field. Our analysis provides evidence of new gravity field features, which perturb its otherwise axially symmetric structure with a time-variable component. We show that normal modes of the planet could explain the anomalous signatures present in the Doppler data better than other alternative explanations, such as localized density anomalies and non-axisymmetric components of the static gravity field. We explain Juno data by p-modes having an amplitude spectrum with a peak radial velocity of 1050 cm/s at 9001200 μHz (compatible with ground-based observations) and provide upper bounds on lower frequency f-modes (radial velocity smaller than 1 cm/s). The new Juno results could open the possibility of exploring the interior structure of the gas giants through measurements of the time-variable gravity or with onboard instrumentation devoted to the observation of normal modes, which could drive spacecraft operations of future missions.

Context. While Jupiter's massive gas envelope consists mainly of hydrogen and helium, the key to understanding Jupiter's formation and evolution lies in the distribution of the remaining (heavy) elements. Before the Juno mission, the lack of high-precision gravity harmonics precluded the use of statistical analyses in a robust determination of the heavy-element distribution in Jupiter's envelope. Aims. In this paper, we assemble the most comprehensive and diverse collection of Jupiter interior models to date and use it to study the distribution of heavy elements in the planet's envelope. Methods. We apply a Bayesian statistical approach to our interior model calculations, reproducing the Juno gravitational and atmospheric measurements and constraints from the deep zonal flows. Results. Our results show that the gravity constraints lead to a deep entropy of Jupiter corresponding to a 1 bar temperature that is 515 K higher than traditionally assumed. We also find that uncertainties in the equation of state are crucial when determining the amount of heavy elements in Jupiter's interior. Our models put an upper limit to the inner compact core of Jupiter of 7 MEarth, independently of the structure model (with or without a dilute core) and the equation of state considered. Furthermore, we robustly demonstrate that Jupiter's envelope is inhomogeneous, with a heavy-element enrichment in the interior relative to the outer envelope. This implies that heavy-element enrichment continued through the gas accretion phase, with important implications for the formation of giant planets in our Solar System and beyond.

Storm-track activity over the North Pacific (NP) climatologically exhibits a clear minimum in midwinter, when the westerly jet speed sharply maximizes. This counterintuitive phenomenon, referred to as the “midwinter minimum (MWM),” has been investigated from various perspectives, but the mechanisms are still to be unrevealed. Toward better understanding of this phenomenon, the present study delineates the detailed seasonal evolution of climatological-mean Eulerian statistics and energetics of migratory eddies along the NP storm track over 60 years. As a comprehensive investigation of the mechanisms for the MWM, this study has revealed that the net eddy conversion/generation rate normalized by the eddy total energy, which is independent of eddy amplitude, is indeed reduced in midwinter. The reduction from early winter occurs mainly due to the decreased effectiveness of the baroclinic energy conversion through seasonally weakened temperature fluctuations and the resultant poleward eddy heat flux. The reduced net normalized conversion/generation rate in midwinter is also found to arise in part from the seasonally enhanced kinetic energy conversion from eddies into the strongly diffluent Pacific jet around its exit. The seasonality of the net energy influx also contributes especially to the spring recovery of the net normalized conversion/generation rate. The midwinter reduction in the normalized rates of both the net energy conversion/generation and baroclinic energy conversion was more pronounced in the period before the late 1980s, during which the MWM of the storm-track activity was climatologically more prominent.

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 momentumbalance 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.

Interior modeling of Jupiter and Saturn has advanced to a state where thousands of models are generated that cover the uncertainty space of many parameters. This approach demands a fast method of computing their gravity field and shape. Moreover, the Cassini mission at Saturn and the ongoing Juno mission delivered gravitational harmonics up to J12. Here we report the expansion of the theory of figures, which is a fast method for gravity field and shape computation, to the seventh order (ToF7), which allows for computation of up to J14. We apply three different codes to compare the accuracy using polytropic models. We apply ToF7 to Jupiter and Saturn interior models in conjunction with CMS-19 H/He equation of state. For Jupiter, we find that J6 is best matched by a transition from an He-depleted to He-enriched envelope at 22.5 Mbar. However, the atmospheric metallicity reaches 1 × solar only if the adiabat is perturbed toward lower densities, or if the surface temperature is enhanced by ∼14 K from the Galileo value. Our Saturn models imply a largely homogeneous-in-Z envelope at 1.54 × solar atop a small core. Perturbing the adiabat yields metallicity profiles with extended, heavy-element-enriched deep interior (diffuse core) out to 0.4 RSat, as for Jupiter. Classical models with compact, dilute, or no core are possible as long as the deep interior is enriched in heavy elements. Including a thermal wind fitted to the observed wind speeds, representative Jupiter and Saturn models are consistent with all observed Jn values.

Jupiter's atmosphere is dominated by multiple jet streams which are strongly tied to its 3D atmospheric circulation. Lacking a rigid bottom boundary, several models exist for how the meridional circulation extends into the planetary interior. Here, we show, collecting evidence from multiple instruments of the Juno mission, the existence of midlatitudinal meridional circulation cells which are driven by turbulence, similar to the Ferrel cells on Earth. Different than Earth, which contains only one such cell in each hemisphere, the larger, faster rotating Jupiter can incorporate multiple cells. The cells form regions of upwelling and downwelling, which we show are clearly evident in Juno's microwave data between latitudes (Formula presented.) and (Formula presented.). The existence of these cells is confirmed by reproducing the ammonia observations using a simplistic model. This study solves a long-standing puzzle regarding the nature of Jupiter's subcloud dynamics and provides evidence for eight cells in each Jovian hemisphere.

The midlatitude storm tracks are among the most prominent features of extratropical climate. Despite the theoretical expectation, based on baroclinic instability theory, that baroclinic eddies strengthen with jet intensification, there is evidence that this relation breaks when the jet is intense. The most well-known case is the Pacific midwinter minimum in storm-track activity, where eddy activity is diminished in winter compared to fall and spring despite the jet being the strongest in winter. To isolate the effect of jet strength on storm activity, we conduct a series of idealized GCM experiments systematically varying jet intensity. The simulations are analyzed using Lagrangian tracking to understand the response from a single-eddy perspective. The Lagrangian analysis shows that while the response of upper-level eddies is dominated by a reduction in the amount of tracked features, the lower-level eddies' response is also affected by a reduction in their lifetime. Analyzing the jet strength effect on the pairing between the upper- and lower-level eddies, we find that the jet intensification increases the relative speed of the upper-level eddies, breaking the baroclinic wave structure and limiting its growth.We show that the Lagrangian response correlates with a shift in the midlatitude spectrum to lower wavenumbers. This shift settles these results with linear baroclinic instability theory, as under the stronger jet conditions synoptic-scale eddies are predicted to have a suboptimal growth rate. These results can explain the midwinter suppression of storm activity over the Pacific and the difference from the Atlantic response.

Migratory cyclones and anticyclones account for most of the day-to-day weather variability in the extratropics. These transient eddies act to maintain the midlatitude jet streams by systematically transporting westerly momentum and heat. Yet, little is known about the separate contributions of cyclones and anticyclones to their interaction with the westerlies. Here, using a novel methodology for identifying cyclonic and anticyclonic vortices based on curvature, we quantify their separate contributions to atmospheric energetics and their feedback on the westerly jet streams as represented in Eulerian statistics. We show that climatological westerly acceleration by cyclonic vortices acts to dominantly reinforce the wintertime eddy-driven near-surface westerlies and associated cyclonic shear. Though less baroclinic and energetic, anticyclones still play an important role in transporting westerly momentum toward midlatitudes from the upper-tropospheric thermally driven jet core and carrying eddy energy downstream. These new findings have uncovered essential characteristics of atmospheric energetics, storm track dynamics and eddy-mean flow interaction.

The structure and stability of Jupiter's atmosphere is analyzed using transformed Eulerian mean (TEM) theory. Utilizing the ammonia distribution derived from microwave radiometer measurements of the Juno orbiter, the latitudinal and vertical distribution of the vertical velocity in the interior of Jupiter's atmosphere is inferred. The resulting overturning circulation is then interpreted in the TEM framework to offer speculation of the vertical and meridional temperature distribution. At midlatitudes, the analyzed vertical velocity field shows Ferrel-cell-like patterns associated with each of the jets. A scaling analysis of the TEM overturning circulation equation suggests that in order for the Ferrel-cell-like patterns to be visible in the ammonia distribution, the static stability of Jupiter's weather layer should be on the order of 1 x 10(-2) s(-1). At low latitudes, the ammonia distribution suggests strong upward motion, which is reminiscent of the rising branch of the Hadley cell where the static stability is weaker. Taken together, the analysis suggests that the temperature lapse rate in the midlatitudes is markedly smaller than that in the low latitudes. Because the cloud-top temperature is nearly uniform across all latitudes, the analysis suggests that in the interior of the weather layer, there could exist a temperature gradient between the low- and midlatitude regions.

The nature and structure of the observed east-west flows on Jupiter and Saturn have been a long-standing mystery in planetary science. This mystery has been recently unraveled by the accurate gravity measurements provided by the Juno mission to Jupiter and the Grand Finale of the Cassini mission to Saturn. These two experiments, which coincidentally happened around the same time, allowed the determination of the overall vertical and meridional profiles of the zonal flows on both planets. This paper reviews the topic of zonal jets on the gas giants in light of the new data from these two experiments. The gravity measurements not only allow the depth of the jets to be constrained, yielding the inference that the jets extend to roughly 3000 and 9000 km below the observed clouds on Jupiter and Saturn, respectively, but also provide insights into the mechanisms controlling these zonal flows. Specifically, for both planets this depth corresponds to the depth where electrical conductivity is within an order of magnitude of 1 S m(-1), implying that the magnetic field likely plays a key role in damping the zonal flows. An intrinsic characteristic of any gravity inversion, as discussed here, is that the solutions might not be unique. We analyze the robustness of the solutions and present several independent lines of evidence supporting the results presented here.

Early gravity measurements performed by the Juno spacecraft determined Jupiter's lowdegree gravity harmonics, including the first estimate of the planet's northsouth asymmetric field. The retrieved information was used to infer that the strong zonal winds visible at the cloud tops must extend down a few thousand kilometers, where they are suppressed in the deep interior. The next frontier for the Juno gravity experiment includes, among other goals, the determination of Jupiter's smallscale gravity field with high accuracy, and its relation to atmospheric circulation at shorter length scales. The geometry of the Juno closest approaches to the planet poses a challenge to this task, as they span latitudes between 4∘N and 29∘N over the course of the nominal mission. Since Doppler measurements are the most sensitive to gravity anomalies when the spacecraft is close to the body, observations of Jupiter's gravity field are mostly concentrated in the northern hemisphere, while the traditional spherical harmonic functions are not orthonormal over a latitudinal subdomain. Here we define customized Slepian functions, which are orthogonal in a specific latitude range and are optimized to represent Jupiter's local surface gravity at north latitudes. We show that with the new functions, the shortscale latitudinal variability of the gravity field is resolved with high accuracy between 15∘S and 45∘N latitude. Furthermore, preliminary results show that the estimated values for the Slepian coefficients from the Juno data match the predictions obtained using thermal wind balance to relate the dynamical density anomalies and the winds with an optimized scale height.

The asymmetric gravity field measured by the Juno spacecraft has allowed the estimation of the depth of Jupiter's zonal jets, showing that the winds extend approximately 3,000 km beneath the cloud level. This estimate was based on an analysis using a combination of all measured odd gravity harmonics, J
₃, J
₅, J
₇, and J
₉, but the wind profile's dependence on each of them separately has yet to be investigated. Furthermore, these calculations assumed the meridional profile of the cloud-level wind extends to depth. However, it is possible that the interior jet profile varies somewhat from that of the cloud level. Here we analyze in detail the possible meridional and vertical structure of Jupiter's deep jet streams that can match the gravity measurements. We find that each odd gravity harmonic constrains the flow at a different depth, with J
₃ the most dominant at depths below 3,000 km, J
₅ the most restrictive overall, whereas J
₉ does not add any constraint on the flow if the other odd harmonics are considered. Interior flow profiles constructed from perturbations to the cloud-level winds allow a more extensive range of vertical wind profiles, yet when the meridional profiles differ substantially from the cloud level, the ability to match the gravity data significantly diminishes. Overall, we find that while interior wind profiles that do not resemble the cloud level are possible, they are statistically unlikely. Finally, inspired by the Juno microwave radiometer measurements, assuming the brightness temperature is dominated by the ammonia abundance, we find that depth-dependent flow profiles are still compatible with the gravity measurements.

Global warming projections show an anomalous temperature increase both at the Arctic surface and at lower latitudes in the upper troposphere. The Arctic amplification decreases the meridional temperature gradient, and simultaneously decreases static stability. These changes in the meridional temperature gradient and in the static stability have opposing effects on baroclinicity. The temperature increase at the upper tropospheric lower latitudes tends to increase the meridional temperature gradient and simultaneously increase static stability, which have opposing effects on baroclinicity as well. In this study, a dry idealized general circulation model with a modified Newtonian cooling scheme, which allows any chosen zonally symmetric temperature distribution to be simulated, is used to study the effect of Arctic amplification and lower-latitude upper-level warming on eddy activity. Due to the interplay between the static stability and meridional temperature gradient on atmospheric baroclinicity changes, and their opposing effect on atmospheric baroclinicity, it is found that both the Arctic amplification and lower-latitude upper-level warming could potentially lead to both decreases and increases in eddy activity, depending on the exact prescribed temperature modifications. Therefore, to understand the effect of global warming-like temperature trends on eddy activity, the zonally symmetric global warming temperature projections from state-of-the-art models are simulated. It is found that the eddy kinetic energy changes are dominated by the lower-latitude upper-level warming, which tends to weaken the eddy kinetic energy due to increased static stability. On the other hand, the eddy heat flux changes are dominated by the Arctic amplification, which tends to weaken the eddy heat flux at the lower levels.

The atmospheres of the four giant planets of our Solar System share a common and well-observed characteristic: they each display patterns of planetary banding, with regions of different temperatures, composition, aerosol properties and dynamics separated by strong meridional and vertical gradients in the zonal (i.e., east-west) winds. Remote sensing observations, from both visiting spacecraft and Earth-based astronomical facilities, have revealed the significant variation in environmental conditions from one band to the next. On Jupiter, the reflective white bands of low temperatures, elevated aerosol opacities, and enhancements of quasi-conserved chemical tracers are referred to as 'zones.' Conversely, the darker bands of warmer temperatures, depleted aerosols, and reductions of chemical tracers are known as 'belts.' On Saturn, we define cyclonic belts and anticyclonic zones via their temperature and wind characteristics, although their relation to Saturn's albedo is not as clear as on Jupiter. On distant Uranus and Neptune, the exact relationships between the banded albedo contrasts and the environmental properties is a topic of active study. This review is an attempt to reconcile the observed properties of belts and zones with (i) the meridional overturning inferred from the convergence of eddy angular momentum into the eastward zonal jets at the cloud level on Jupiter and Saturn and the prevalence of moist convective activity in belts; and (ii) the opposing meridional motions inferred from the upper tropospheric temperature structure, which implies decay and dissipation of the zonal jets with altitude above the clouds. These two scenarios suggest meridional circulations in opposing directions, the former suggesting upwelling in belts, the latter suggesting upwelling in zones. Numerical simulations successfully reproduce the former, whereas there is a wealth of observational evidence in support of the latter. This presents an unresolved paradox for our current understanding of the banded structure of giant planet atmospheres, that could be addressed via a multi-tiered vertical structure of "stacked circulation cells," with a natural transition from zonal jet pumping to dissipation as we move from the convectively-unstable mid-troposphere into the stably-stratified upper troposphere.

Uranus and Neptune, and their diverse satellite and ring systems, represent the least explored environments of our Solar System, and yet may provide the archetype for the most common outcome of planetary formation throughout our galaxy. Ice Giants will be the last remaining class of Solar System planet to have a dedicated orbital explorer, and international efforts are under way to realise such an ambitious mission in the coming decades. In 2019, the European Space Agency released a call for scientific themes for its strategic science planning process for the 2030s and 2040s, known as Voyage 2050. We used this opportunity to review our present-day knowledge of the Uranus and Neptune systems, producing a revised and updated set of scientific questions and motivations for their exploration. This review article describes how such a mission could explore their origins, ice- rich interiors, dynamic atmospheres, unique magnetospheres, and myriad icy satellites, to address questions at the heart of modern planetary science. These two worlds are superb examples of how planets with shared origins can exhibit remarkably different evolutionary paths: Neptune as the archetype for Ice Giants, whereas Uranus may be atypical. Exploring Uranus' natural satellites and Neptune's captured moon Triton could reveal how Ocean Worlds form and remain active, redefining the extent of the habitable zone in our Solar System. For these reasons and more, we advocate that an Ice Giant System explorer should become a strategic cornerstone mission within ESA's Voyage 2050 programme, in partnership with international collaborators, and targeting launch opportunities in the early 2030s.

The interior structure of Saturn, the depth of its winds, and the mass and age of its rings constrain its formation and evolution. In the final phase of the Cassini mission, the spacecraft dived between the planet and its innermost ring, at altitudes of 2600 to 3900 kilometers above the cloud tops. During six of these crossings, a radio link with Earth was monitored to determine the gravitational field of the planet and the mass of its rings. We find that Saturns gravity deviates from theoretical expectations and requires differential rotation of the atmosphere extending to a depth of at least 9000 kilometers. The total mass of the rings is (1.54 ± 0.49) × 10
¹⁹ kilograms (0.41 ± 0.13 times that of the moon Mimas), indicating that the rings may have formed 10
⁷ to 10
⁸ years ago.

Jupiter's internal flow structure is still not fully known, but can be now better constrained due to Juno's highprecision measurements. The recently published gravity and magnetic field measurements have led to new information regarding the planet and its internal flows, and future magnetic measurements will help to solve this puzzle. In this study, we propose a new method to better constrain Jupiter's internal flow field using the Juno gravity measurements combined with the expected measurements of magnetic secular variation. Based on a combination of hydrodynamical and magnetic field considerations we show that an optimized vertical profile of the zonal flows that fits both measurements can be obtained. Incorporating the magnetic field effects on the flow better constrains the flow decay profile. This will get us closer to answering the persistent question regarding the depth and nature of the flows on Jupiter.

Jupiter's atmosphere is rotating differentially, with zones and belts rotating at speeds that differ by up to 100 metres per second. Whether this is also true of the gas giant's interior has been unknown(1,2), limiting our ability to probe the structure and composition of the planet(3,4). The discovery by the Juno spacecraft that Jupiter's gravity field is north-south asymmetric(5) and the determination of its non-zero odd gravitational harmonics J(3), J(5), J(7) and J(9) demonstrates that the observed zonal cloud flow must persist to a depth of about 3,000 kilometres from the cloud tops(6). Here we report an analysis of Jupiter's even gravitational harmonics J(4), J(6), J(8) and J(10) as observed by Juno(5) and compared to the predictions of interior models. We find that the deep interior of the planet rotates nearly as a rigid body, with differential rotation decreasing by at least an order of magnitude compared to the atmosphere. Moreover, we find that the atmospheric zonal flow extends to more than 2,000 kilometres and to less than 3,500 kilometres, making it fully consistent with the constraints obtained independently from the odd gravitational harmonics. This depth corresponds to the point at which the electric conductivity becomes large and magnetic drag should suppress differential rotation(7). Given that electric conductivity is dependent on planetary mass, we expect the outer, differentially rotating region to be at least three times deeper in Saturn and to be shallower in massive giant planets and brown dwarfs.

The depth to which Jupiter's observed east-west jet streams extend has been a long-standing question(1,2). Resolving this puzzle has been a primary goal for the Juno spacecraft(3,4), which has been in orbit around the gas giant since July 2016. Juno's gravitational measurements have revealed that Jupiter's gravitational field is north-south asymmetric(5), which is a signature of the planet's atmospheric and interior flows(6). Here we report that the measured odd gravitational harmonics J(3), J(5), J(7) and J(9) indicate that the observed jet streams, as they appear at the cloud level, extend down to depths of thousands of kilometres beneath the cloud level, probably to the region of magnetic dissipation at a depth of about 3,000 kilometres(7,8). By inverting the measured gravity values into a wind field(9), we calculate the most likely vertical profile of the deep atmospheric and interior flow, and the latitudinal dependence of its depth. Furthermore, the even gravity harmonics J(8) and J(10) resulting from this flow profile also match the measurements, when taking into account the contribution of the interior structure(10). These results indicate that the mass of the dynamical atmosphere is about one per cent of Jupiter's total mass.

Hadley cells dominate the meridional circulation of terrestrial atmospheres. The solar system terrestrial atmospheres, Venus, Earth, Mars, and Titan, exhibit a large variety in the strength, width, and seasonality of their Hadley circulation. Despite the Hadley cell being thermally driven, in all planets, the ascending branch does not coincide with the warmest latitude, even in cases with very long seasonality (e.g., Titan) or very small thermal inertia (e.g., Mars). In order to understand the characteristics of the Hadley circulation in cases of extreme planetary characteristics, we show both theoretically, using axisymmetric theory, and numerically, using a set of idealized GCM simulations, that the thermal Rossby number dictates the character of the circulation. Given the possible variation of thermal Rossby number parameters, the rotation rate is found to be the most critical factor controlling the circulation characteristics. The results also explain the location of the Hadley cell ascending branch on Mars and Titan.Plain Language Summary The Hadley circulation is a thermally driven circulation, meaning that air raises at warm latitudes and descends at colder ones. As the solar forcing is seasonal this cell has a seasonal cycle as well, with typically the winter cell being stronger. Previous studies showed that under planetary conditions where the maximum temperature at solstice is at the summer pole, the ascending branch does not necessarily follow the maximum surface temperature; however, slowing down the rotation rate allows the ascending branch to follow the warmest latitude. In this study, we aim to explain this rotation rate dependence and the Hadley circulation on planets that exhibit strong seasonality like Titan and Mars, by using both theoretical arguments based on angular momentum conservation and idealized 3D model simulations. We find that the rotation rate is the main factor controlling the ascending branch of the circulation.

The atmosphere exhibits two distinct types of jets: the thermally driven subtropical jet and the more poleward eddy-driven jet. Depending on location and season, these jets are often merged or separated, and their position, structure, and intensity strongly influence the eddy fields. Here, the authors study the sensitivity of eddies to changes in the jets' amplitudes and positions in an idealized general circulation model. A modified Newtonian relaxation scheme that has a very short relaxation time for the mean state and a long relaxation time for eddies is used. This scheme makes it possible to obtain any zonally symmetric temperature distribution and is used to systematically modify the jets' amplitudes and locations. It is found that eddies are more sensitive to changes in the amplitude of the eddy-driven jet than to changes in the amplitude of the subtropical jet. Furthermore, when the eddy-driven jet is shifted poleward, eddies tend to intensify. These results are tested for robustness in two different reference simulations: one resembling a situation where the subtropical and eddy-driven jets are nearly merged and one when they are separated.

The Juno spacecraft has measured Jupiter's low-order, even gravitational moments, J₂J₈, to an unprecedented precision, providing important constraints on the density profile and core mass of the planet. Here we report on a selection of interior models based on ab initio computer simulations of hydrogen-helium mixtures. We demonstrate that a dilute core, expanded to a significant fraction of the planet's radius, is helpful in reconciling the calculated J_n with Juno's observations. Although model predictions are strongly affected by the chosen equation of state, the prediction of an enrichment of Z in the deep, metallic envelope over that in the shallow, molecular envelope holds. We estimate Jupiter's core to contain a 725 Earth mass of heavy elements. We discuss the current difficulties in reconciling measured J_n with the equations of state and with theory for formation and evolution of the planet.

Deciphering the flow below the cloud-level of Jupiter remains a critical milestone in understanding Jupiter's internal structure and dynamics. The expected high-precision Juno measurements of both the gravity field and the magnetic field might help to reach this goal. Here we propose a method that combines both fields to constrain the depth-dependent flow field inside Jupiter. This method is based on a mean-field electrodynamic balance that relates the flow field to the anomalous magnetic field, and geostrophic balance that relates the flow field to the anomalous gravity field. We find that the flow field has two distinct regions of influence: an upper region in which the flow affects mostly the gravity field and a lower region in which the flow affects mostly the magnetic field. An optimization procedure allows to reach a unified flow structure that is consistent with both the gravity and the magnetic fields.

The nature of the flow below the cloud level on Jupiter and Saturn is still unknown. Relating the flow on these planets to perturbations in their density field is key to the analysis of the gravity measurements expected from both the Juno (Jupiter) and Cassini (Saturn) spacecrafts during 2016-2018. Both missions will provide latitude-dependent gravity fields, which in principle could be inverted to calculate the vertical structure of the observed cloud-level zonal flow on these planets. Theories to date connecting the gravity field and the flow structure have been limited to potential theories under a barotropic assumption, or estimates based on thermal wind balance that allow baroclinic wind structures to be analysed, but have made simplifying assumptions that neglected several physical effects. These include the effects of the deviations from spherical symmetry, the centrifugal force due to density perturbations and self-gravitational effects of the density perturbations. Recent studies attempted to include some of these neglected terms, but lacked an overall approach that is able to include all effects in a self-consistent manner. The present study introduces such a self-consistent perturbation approach to the thermal wind balance that incorporates all physical effects, and applies it to several example wind structures, both barotropic and baroclinic. The contribution of each term is analysed, and the results are compared in the barotropic limit with those of potential theory. It is found that the dominant balance involves the original simplified thermal wind approach. This balance produces a good order-of-magnitude estimate of the gravitational moments, and is able, therefore, to address the order one question of how deep the flows are given measurements of gravitational moments. The additional terms are significantly smaller yet can affect the gravitational moments to some degree. However, none of these terms is dominant so any approximation attempting to improve over the simplified thermal wind approach needs to include all other terms.

The sole in situ measurement of a giant planet atmosphere comes from the Galileo probe, which plunged through Jupiter's weather layer at 6.5°N and measured a remarkably stable atmospheric temperature profile. Horizontal winds were observed to substantially increase from 1 to 3 bars, in a region of relatively low static stability. We show that this high-shear region indicates the best possibility of zero potential vorticity and resulting slantwise convection and suggest that the fluid here could potentially be adiabatic. We generalize an expression to determine lapse rates along constant angular momentum surfaces for deep atmospheres at any latitude.

The upcoming Juno spacecraft measurements have the potential of improving our knowledge of Jupiter's gravity field. The analysis of the Juno Doppler data will provide a very accurate reconstruction of spatial gravity variations, but these measurements will be very accurate only over a limited latitudinal range. In order to deduce the full gravity field of Jupiter, additional information needs to be incorporated into the analysis, especially regarding the Jovian flow structure and its depth, which can influence the measured gravity field. In this study we propose a new iterative method for the estimation of the Jupiter gravity field, using a simulated Juno trajectory, a trajectory estimation model, and an adjoint-based inverse model for the flow dynamics. We test this method both for zonal harmonics only and with a full gravity field including tesseral harmonics. The results show that this method can fit some of the gravitational harmonics better to the "measured" harmonics, mainly because of the added information from the dynamical model, which includes the flow structure. Thus, it is suggested that the method presented here has the potential of improving the accuracy of the expected gravity harmonics estimated from the Juno and Cassini radio science experiments.

The Cassini measurements of Saturn's gravity field during its Grand Finale might shed light on a long-standing question regarding the flow on Saturn. While the cloud-level winds are well known, little is known about whether these winds are confined to the outer layers of the planet or penetrate deep into the interior. An additional complexity is added by the uncertainty in the exact rotation period of Saturn, a key factor in determining the cloud-level winds, with an effect on the north-south symmetric part of the winds. Using Saturn's cloud-level winds we relate the flow to the gravity harmonics. We give a prediction for the odd harmonics as a function of the flow depth, identifying three ranges of depths. Since the odd harmonics depend solely on the flow, and are not influenced by Saturn's shape and static density distribution, any measured value of the odd harmonics by Cassini can be used to uniquely determine the depth of the flow. We also discuss the flow-induced even harmonics ΔJ₂ ΔJ₄⋯ ,ΔJ₁₂ that are affected by Saturn's rotation period. While the high-degree even harmonics might also be used to determine the flow depth, the lower-degree even harmonics serve as uncertainties for analysis of the planet's interior structure and composition. Thus, the gravity harmonics measured during the Cassini Grand Finale may be used to get a first-order estimate of the flow structure and to better constrain the planet's density structure and composition.

The Atlantic and Pacific storm tracks in the Northern Hemisphere are characterized by a downstream poleward deflection, which has important consequences for the distribution of heat, wind, and precipitation in the midlatitudes. In this study, the spatial structure of the storm tracks is examined by tracking transient cyclones in an idealized GCM with a localized ocean heat flux. The localized atmospheric response is decomposed in terms of a time- and zonal-mean background flow, a stationary wave, and a transient eddy field. The Lagrangian tracks are used to construct cyclone composites and perform a spatially varying PV budget. Three distinct mechanisms that contribute to the poleward tilt emerge: transient nonlinear advection, latent heat release, and stationary advection. The downstream evolution of the PV composites shows the different role played by the stationary wave in each region. In the region where the tilt is maximized, all three mechanisms contribute to the poleward propagation of the low-level PV anomaly associated with the cyclone. Upstream of that region, the stationary wave is opposing the former two, and the poleward tendency is therefore reduced. Finally, through repeated experiments with enhanced strength of the heating source, it is shown that the poleward deflection of the storms enhances when the amplitude of the stationary wave increases.

Observations of the flow on Jupiter exists essentially only for the cloud-level, which is dominated by strong east-west jet-streams. These have been suggested to result from dynamics in a superficial thin weather-layer, or alternatively be a manifestation of deep interior cylindrical flows. However, it is possible that the observed wind is indeed superficial, yet there exists a completely decoupled deep flow. To date, all models linking the wind, via the induced density anomalies, to the gravity field, to be measured by Juno, consider only flow that is a projection of the observed cloud-level wind. Here we explore the possibility of complex wind dynamics that include both the shallow weather-layer wind, and a deep flow that is decoupled from the flow above it. The upper flow is based on the observed cloud-level flow and is set to decay with depth. The deep flow is constructed to produce cylindrical structures with variable width and magnitude, thus allowing for a wide range of possible scenarios for the unknown deep flow. The combined flow is then related to the density anomalies and gravitational moments via a dynamical model. An adjoint inverse model is used for optimizing the parameters controlling the setup of the deep and surface-bound flows, so that these flows can be reconstructed given a gravity field. We show that the model can be used for examination of various scenarios, including cases in which the deep flow is dominating over the surface wind, and discuss the uncertainties associated with the model solution. The flexibility of the adjoint method allows for a wide range of dynamical setups, so that when new observations and physical understanding will arise, these constraints could be easily implemented and used to better decipher Jupiter flow dynamics. (C) 2017 Elsevier Inc. All rights reserved.

Jupiter's Great Red Spot (GRS) is the most dominant and long-lived feature in Jupiter's atmosphere. However, whether this is a shallow atmospheric feature or a deeply rooted vortex has remained an open question. In this study, we assess the possibility of inferring the depth of the GRS by the upcoming Juno gravity experiment. This is achieved by an exploration of the possible gravitational signature of the vortex by systematically extending the surface winds into the interior and analyzing the resulting gravity signal. The gravity anomaly is then compared to the expected accuracy in the retrieval of the surface gravity at the GRS location obtained with numerical simulations of the Doppler data inversion based on the expected trajectory of the spacecraft. Starting from observations of the atmospheric velocity at the cloud level, we project the wind using a decay scale height along coaxial cylinders parallel to the spin axis and explore a wide range of decay scale heights in the radial direction. Assuming the large scale vortex dynamics are geostrophic, and therefore thermal wind balance holds, the density anomaly distribution due to Jupiter's winds can be derived from the velocity maps. The novelty of this approach is in the integration of thermal wind relations over a three-dimensional grid, and in the inclusion of the observed meridional velocity as measured during the Cassini flyby of Jupiter. The perturbations in the mean zonal flow give rise to non zero tesseral spherical harmonics in Jupiter's gravitational potential. We provide an estimate of this asymmetric gravity coefficients for different values of the wind decay scale height. We conclude that the mass anomaly associated with the GRS is detectable by the Juno gravity experiment if the vortex is deep, characterized by a vertical height larger than 2,000 km below the cloud level of Jupiter, and that the large mass involved with deep winds does not render much the ability to measure the feature.

The relation between the mean meridional temperature gradient and eddy fluxes has been addressed by several eddy flux closure theories. However, these theories give little information on the dependence of eddy fluxes on the vertical structure of the temperature gradient. The response of eddies to changes in the vertical structure of the temperature gradient is especially interesting since global circulation models suggest that as a result of greenhouse warming, the lower-tropospheric temperature gradient will decrease whereas the upper-tropospheric temperature gradient will increase. The effects of the vertical structure of baroclinicity on atmospheric circulation, particularly on the eddy activity, are investigated. An idealized global circulation model with a modified Newtonian relaxation scheme is used. The scheme allows the authors to obtain a heating profile that produces a predetermined mean temperature profile and to study the response of eddy activity to changes in the vertical structure of baroclinicity. The results indicate that eddy activity is more sensitive to temperature gradient changes in the upper troposphere. It is suggested that the larger eddy sensitivity to the upper-tropospheric temperature gradient is a consequence of large baroclinicity concentrated in upper levels. This result is consistent with a 1D Eady-like model with nonuniform shear showing more sensitivity to shear changes in regions of larger baroclinicity. In some cases, an increased temperature gradient at lower-tropospheric levels might decrease the eddy kinetic energy, and it is demonstrated that this might be related to the midwinter minimum in eddy kinetic energy observed above the northern Pacific.

The upcoming Juno and Cassini gravity measurements of Jupiter and Saturn, respectively, will allow probing the internal dynamics of these planets through accurate analysis of their gravity spectra. To date, two general approaches have been suggested for relating the flow velocities and gravity fields. In the first, barotropic potential surface models, which naturally take into account the oblateness of the planet, are used to calculate the gravity field. However, barotropicity restricts the flows to be constant along cylinders parallel to the rotation axis. The second approach, calculated in the reference frame of the rotating planet, assumes that due to the large scale and rapid rotation of these planets, the winds are to leading order in geostrophic balance. Therefore, thermal wind balance relates the wind shear to the density gradients. While this approach can take into account any internal flow structure, it is limited to only calculating the dynamical gravity contributions, and has traditionally assumed spherical symmetry. This study comes to relate the two approaches both from a theoretical perspective, showing that they are analytically identical in the barotropic limit, and numerically, through systematically comparing the different model solutions for the gravity harmonics. For the barotropic potential surface models we employ two independent solution methods - the potential-theory and Maclaurin spheroid methods. We find that despite the sphericity assumption, in the barotropic limit the thermal wind solutions match well the barotropic oblate potential-surface solutions.

The midlatitude atmosphere is characterized by turbulent eddies that act to produce a depth-independent (barotropic) mean flow. Using the NCEP (National Centers for Environmental Prediction) Reanalysis 2 data, the latitudinal dependence of barotropic kinetic energy and enstrophy are investigated. Most of the barotropization takes place in the extratropics with a maximum value at midlatitudes, due to the latitudinal variations of the static stability, tropopause height, and sphericity of the planet. Barotropic advection transfers the eddy kinetic energy to the zonal mean flow and thus maintains the barotropic component of the eddy-driven jet. The classic description of geostrophic turbulence exists only at high latitudes, where the quasi-geostrophic flow is supercritical to baroclinic instability; the eddy-eddy interactions carry both the barotropization of eddy kinetic energy upscale to the Rhines scale and the barotropization of eddy potential enstrophy downscale.

Slantwise convection should be ubiquitous in the atmospheres of rapidly rotating fluid planets. We argue that convectively adjusted lapse rates should be interpreted along constant angular momentum surfaces instead of lines parallel to the local gravity vector. Using Cassini wind observations of Jupiter and different lapse rates to construct toy atmospheres, we explore parcel paths in symmetrically stable and unstable weather layers by the numerically modeled insertion of negatively buoyant bubbles. Low-Richardson number atmospheres are very susceptible to transient symmetric instability upon local diabatic forcing, even outside of the tropics. We explore parcel paths in symmetrically stable and unstable weather layer environments, the latter by adding thermal bubbles to the weather layer. Parcels that cool in Jupiter's belt regions have particularly horizontal paths, with implications for jetward angular momentum fluxes. These considerations may be relevant to the interpretation of Juno's ongoing observations of Jupiter's weather layer.

The recent discoveries of terrestrial exoplanets and super-Earths extending over a broad range of orbital and physical parameters suggest that these planets will span a wide range of climatic regimes. Characterization of the atmospheres of warm super-Earths has already begun and will be extended to smaller and more distant planets over the coming decade. The habitability of these worlds may be strongly affected by their three-dimensional atmospheric circulation regimes, since the global climate feedbacks that control the inner and outer edges of the habitable zone - including transitions to Snowball-like states and runaway-greenhouse feedbacks - depend on the equator-to-pole temperature differences, patterns of relative humidity, and other aspects of the dynamics. Here, using an idealized moist atmospheric general circulation model including a hydrological cycle, we study the dynamical principles governing the atmospheric dynamics on such planets. We show how the planetary rotation rate, stellar flux, atmospheric mass, surface gravity, optical thickness, and planetary radius affect the atmospheric circulation and temperature distribution on such planets. Our simulations demonstrate that equator-to-pole temperature differences, meridional heat transport rates, structure and strength of the winds, and the hydrological cycle vary strongly with these parameters, implying that the sensitivity of the planet to global climate feedbacks will depend significantly on the atmospheric circulation. We elucidate the possible climatic regimes and diagnose the mechanisms controlling the formation of atmospheric jet streams, Hadley and Ferrel cells, and latitudinal temperature differences. Finally, we discuss the implications for understanding how the atmospheric circulation influences the global climate.

Poleward migration of eddy-driven jets is found to occur in the extratropics when the subtropical and eddy-driven jets are clearly separated, as achieved by simulations at high-rotation rates. The poleward migration of these eddy-driven baroclinic jets over time is consistent with variation of eddy momentum flux convergence and baroclinicity across the width of the jet. We demonstrate this using a high-resolution idealized GCM where we systematically examine the eddy-driven jets over a wide range of rotation rates (up to 16 times the rotation rate of Earth). At the flanks of the jets, the poleward migration is caused by a poleward bias in baroclinicity across the width of the jet, estimated through measures such as Eady growth rate and supercriticality. The poleward biased baroclinicity is due to the meridional variation of the Coriolis parameter, which causes a poleward bias of the eddy momentum flux convergence. At the core of the jets, the poleward biased eddy momentum flux convergence relative to the mean jet deflects over time the baroclinicity and the jets poleward. As the rotation rate is increased, and more (narrower) jets emerge the migration rate becomes smaller due to less eddy momentum flux convergence over the narrower baroclinic zones. We find a linear relation between the migration rate of the jets and the net eddy momentum flux convergence across the jets. This poleward migration might be related to the slow poleward propagation of temporal anomalies of zonal winds observed in the upper troposphere.

This study demonstrates that water vapor transport and precipitation are largely modulated by the intensity of the subtropical jet, transient eddies, and the location of wave breaking events during the different phases of ENSO. Clear differences are found in the potential vorticity (PV), meteorological fields, and trajectory pathways between the two different phases. Rossby wave breaking events have cyclonic and anticyclonic regimes, with associated differences in the frequency of occurrence and the dynamic response. During La Ni~na, there is a relatively weak subtropical jet allowing PV to intrude into lower latitudes over the western United States. This induces a large amount of moisture transport inland ahead of the PV intrusions, as well as northward transport to the west of a surface anticyclone. During El Ni~no, the subtropical jet is relatively strong and is associated with an enhanced cyclonic wave breaking. This is accompanied by a time-mean surface cyclone, which brings zonal moisture transport to the western United States. In both (El Ni~no and La Nĩna) phases, there is a high correlation (0.3-0.7) between upper-level PV at 250 hPa and precipitation over the west coast of the United States with a time lag of 0-1 days. Vertically integrated water vapor fluxes during El Nĩno areup to 70kgm^-1s^-1larger than those during La Nĩna along the west coast of the United States. The zonal and meridional moist static energy flux resembles wave vapor transport patterns, suggesting that they are closely controlled by the large-scale flows and location of wave breaking events during the different phase of ENSO.

The investigation of planets around other stars began with the study of gas giants, but is now extending to the discovery and characterization of super-Earths and terrestrial planets. Motivated by this observational tide, we survey the basic dynamical principles governing the atmospheric circulation of terrestrial exoplanets, and discuss the interaction of their circulation with the hydrological cycle and global-scale climate feedbacks. Terrestrial exoplanets occupy a wide range of physical and dynamical conditions, only a small fraction of which have yet been explored in detail. Our approach is to lay out the fundamental dynamical principles governing the atmospheric circulation on terrestrial planetsbroadly definedand show how they can provide a foundation for understanding the atmospheric behavior of these worlds. We first survey basic atmospheric dynamics, including the role of geostrophy, baroclinic instabilities, and jets in the strongly rotating regime (the \u201cextratropics\u201d) and the role of the Hadley circulation, wave adjustment of the thermal structure, and the tendency toward equatorial superrotation in the slowly rotating regime (the \u201ctropics\u201d). We then survey key elements of the hydrological cycle, including the factors that control precipitation, humidity, and cloudiness. Next, we summarize key mechanisms by which the circulation affects the global-mean climate, and hence planetary habitability. In particular, we discuss the runaway greenhouse, transitions to snowball states, atmospheric collapse, and the links between atmospheric circulation and CO2 weathering rates. We finish by summarizing the key questions and challenges for this emerging field in the future.

The observed cloud-level atmospheric circulation on the outer planets of the Solar System is dominated by strong east-west jet streams. The depth of these winds is a crucial unknown in constraining their overall dynamics, energetics and internal structures. There are two approaches to explaining the existence of these strong winds. The first suggests that the jets are driven by shallow atmospheric processes near the surface, whereas the second suggests that the atmospheric dynamics extend deeply into the planetary interiors. Here we report that on Uranus and Neptune the depth of the atmospheric dynamics can be revealed by the planets' respective gravity fields. We show that the measured fourth-order gravity harmonic, J 4, constrains the dynamics to the outermost 0.15 per cent of the total mass of Uranus and the outermost 0.2 per cent of the total mass of Neptune. This provides a stronger limit to the depth of the dynamical atmosphere than previously suggested, and shows that the dynamics are confined to a thin weather layer no more than about 1, 000 kilometres deep on both planets.

Three dimensional studies of convection in deep spherical shells have been used to test the hypothesis that the strong jet streams on Jupiter, Saturn, Uranus, and Neptune result from convection throughout the molecular envelopes. Due to computational limitations, these simulations must be performed at parameter settings far from jovian values and generally adopt heat fluxes 5-10 orders of magnitude larger than the planetary values. Several numerical investigations have identified trends for how the mean jet speed varies with heat flux and viscosity in these models, but no previous theories have been advanced to explain these trends. Here, we show using simple arguments that if convective release of potential energy pumps the jets and viscosity damps them, the mean jet speeds split into two regimes. When the convection is weakly nonlinear, the equilibrated jet speeds should scale approximately with Fly. where F is the convective heat flux and v is the viscosity. When the convection is strongly nonlinear, the jet speeds are faster and should scale approximately as (F/v)(1/2). We demonstrate how this regime shift can naturally result from a shift in the behavior of the jet-pumping efficiency with heat flux and viscosity. Moreover, both Boussinesq and anelastic simulations hint at the existence of a third regime where, at sufficiently high heat fluxes or sufficiently small viscosities, the jet speed becomes independent of the viscosity. We show based on mixing-length estimates that if such a regime exists, mean jet speeds should scale as heat flux to the 1/4 power. Our scalings provide a good match to the mean jet speeds obtained in previous Boussinesq and anelastic, three-dimensional simulations of convection within giant planets over a broad range of parameters. When extrapolated to the real heat fluxes, these scalings suggest that the mass-weighted jet speeds in the molecular envelopes of the giant planets are much weaker-by an order of magnitude or more-than the speeds measured at cloud level. (C) 2010 Elsevier Inc. All rights reserved.

In winter, northeastern North America and northeastern Asia are both colder than other regions at similar latitudes. This has been attributed to the effects of stationary weather systems set by elevated terrain (orography)(1), and to a lack of maritime influences from the prevailing westerly winds(2). However, the differences in extent and orography between the two continents suggest that further mechanisms are involved. Here we show that this anomalous winter cold can result in part from westward radiation of large-scale atmospheric waves-nearly stationary Rossby waves-generated by heating of the atmosphere over warm ocean waters. We demonstrate this mechanism using simulations with an idealized general circulation model(3-5), with which we show that the extent of the cold region is controlled by properties of Rossby waves, such as their group velocity and its dependence on the planetary rotation rate. Our results show that warm ocean waters contribute to the contrast in mid-latitude winter temperatures between eastern and western continental boundaries not only by warming western boundaries, but also by cooling eastern boundaries.

The giant gas planets have hot convective interiors, and therefore a common assumption is that these deep atmospheres are close to a barotropic state. Here we show using a new anelastic general circulation model that baroclinic vorticity contributions are not negligible, and drive the system away from an isentropic and therefore barotropic state. The motion is still aligned with the direction of the axis of rotation as in a barotropic rotating fluid, but the wind structure has a vertical shear with stronger winds in the atmosphere than in the interior. This shear is associated with baroclinic compressibility effects. Most previous convection models of giant planets have used the Boussinesq approximation, which assumes the density is constant in depth; however, Jupiter's actual density varies by four orders of magnitude through its deep molecular envelope. We therefore developed a new general circulation model (based on the MITgcm) that is anelastic and thereby incorporates this density variation. The model's geometry is a full 3D sphere down to a small inner core. It is nonhydrostatic, uses an equation of state suitable for hydrogen-helium mixtures (SCVH), and is driven by an internal heating profile. We demonstrate the effect of compressibility by comparing anelastic and Boussinesq cases. The simulations develop a mean state that is geostrophic and hydrostatic including the often neglected, but significant, vertical Coriolis contribution. This leads to modification of the standard thermal wind relation for a deep compressible atmosphere. The interior flow organizes in large cyclonically rotating columnar eddies parallel to the rotation axis, which drive upgradient angular momentum eddy fluxes, generating the observed equatorial superrotation. Heat fluxes align with the axis of rotation, and provide a mechanism for the transport of heat poleward, which can cause the observed flat meridional emission. We address the issue of over-forcing which is common in such convection models and analyze the dependence of our results on this; showing that the vertical wind structure is not very sensitive to the Rayleigh number. We also study the effect of rotation, showing how the transition from a rapidly to a slowly rotating system affects the dynamics.

In this paper it is proposed that baroclinic instability of even a weak shear may play an important role in the generation and stability of the strong zonal jets observed in the atmospheres of the giant planets. The atmosphere is modeled as a two-layer structure, where the upper layer is a standard quasigeostrophic layer on a beta plane and the lower layer is parameterized to represent a deep interior convective columnar structure using a negative beta plane as in Ingersoll and Pollard. Linear stability theory predicts that the high wave-number perturbations will be the dominant unstable modes for a small vertical wind shear like that inferred from observations. Here a nonlinear analytical model is developed that is truncated to one growing mode that exhibits a multiple jet meridional structure, driven by the nonlinear interaction between the eddies. In the weakly supercritical limit, this model agrees with previous weakly nonlinear theory, but it can be explored beyond this limit allowing the multiple jet-induced zonal flow to be stronger than the eddy field. Calculations with a fully nonlinear pseudospectral model produce stable meridional multijet structures when beginning from a random potential vorticity perturbation field. The instability removes energy-from the background weak baroclinic shear and generates turbulent eddies that undergo an inverse energy cascade and form multijet zonal winds. The jets are the dominant feature in the instantaneous upper-layer flow. with the eddies being relatively weak. The jets scale with the Rhines length, but are strong enough to violate the barotropic stability criterion. It is shown that the basic physical mechanism for the generation and stability of the jets in the full numerical model is similar to that of the truncated model.