Publications

Projected tropical precipitation changes by the end of the century include increased net precipitation over the Pacific Ocean and drying over the Indian Ocean, prompting ongoing debate about the underlying mechanisms. Previous studies argued for the importance of the zonal circulation in the longitudinally dependent tropical precipitation response, as the meridional circulation is often defined and analyzed as the zonal mean. Here we show that the projected changes in the meridional circulation are highly longitudinally dependent, and explain the zonally dependent changes in net precipitation. Our analysis exposes a zonal shift in the ascending branch of the meridional circulation, associated with a strengthened net precipitation over the central Pacific and weakened precipitation in the Indo Pacific. The zonal circulation has minor influence on these projected tropical precipitation changes. These results point to the importance of monitoring the longitudinal changes in the meridional circulation for improving our preparedness for climate change impacts.

Dynamical understandings of midlatitude transient eddy activity, especially its midwinter minimum over the North Pacific, are still limited, partly because conventional Eulerian eddy statistics are incapable of separating cyclonic and anticyclonic contributions. Here we evaluate the two contributions separately based on local curvature of instantaneous flow fields to compare their seasonality between the North Pacific and North Atlantic storm-tracks. The anticyclonic contribution is found crucial for the midwinter minimum of the North Pacific transient eddy activity. Eddy energetics reveals that the net efficiency of the anticyclonic contribution in replenishing total transient eddy energy over the North Pacific exhibits a pronounced midwinter minimum leading to net energy loss, while that of its cyclonic counterpart does not, in harmony with a precipitation peak around midwinter. This study suggests that more attention should be paid to anticyclones in studying midlatitude storm-track dynamics.

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.

ESA\u2019s 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 “observation requirements” 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 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 planet\u2019s origin and characterize its interior structure. The onboard gravity science experiment based on X-band and Ka-band dual-frequency Doppler tracking precisely measured Jupiter\u2019s zonal gravitational field. Here, we analyze 22 Juno\u2019s 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 10\u201350 cm/s at 900\u20131200 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.

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.

Juno microwave radiometer (MWR) observations of Jupiter's midlatitudes reveal a strong correlation between brightness temperature contrasts and zonal winds, confirming that the banded structure extends throughout the troposphere. However, the microwave brightness gradient is observed to change sign with depth: the belts are microwave-bright in the p10 bar range. The transition level (which we call the "jovicline") is evident in the MWR 11.5 cm channel, which samples the 5-14 bar range when using the limb-darkening at all emission angles. The transition is located between 4 and 10 bars, and implies that belts change with depth from being NH3-depleted to NH3-enriched, or from physically warm to physically cool, or more likely a combination of both. The change in character occurs near the statically stable layer associated with water condensation. The implications of the transition are discussed in terms of ammonia redistribution via meridional circulation cells with opposing flows above and below the water condensation layer, and in terms of the "mushball" precipitation model, which predicts steeper vertical ammonia gradients in the belts versus the zones. We show via the moist thermal wind equation that both the temperature and ammonia interpretations can lead to vertical shear on the zonal winds, but the shear is similar to 50x weaker if only NH3 gradients are considered. Conversely, if MWR observations are associated with kinetic temperature gradients then it would produce zonal winds that increase in strength down to the "jovicline", consistent with Galileo probe measurements; then decay slowly at higher pressures.

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 mid-latitudinal 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 60◦S and 60◦N. 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 sub-cloud dynamics and provides evidence for 8 cells in each Jovian hemisphere.

Jupiter’s atmosphere has a system of zones and belts punctuated by small and large vortices, the largest being the Great Red Spot. How these features change with depth is unknown, with theories of their structure ranging from shallow meteorological features to surface expressions of deep-seated convection. We present observations of atmospheric vortices using the Juno spacecraft’s Microwave Radiometer. We find vortex roots that extend deeper than the altitude at which water is expected to condense, and identify density inversion layers. Our results constrain the 3-dimensional structure of Jupiter’s vortices and their extension below the clouds.

The midlatitude storm tracks are of 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 particularly strong. The most known case is the Pacific midwinter minimum in storm track activity. To isolate the effect of jet strength on storm activity, we conduct a series of 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 low wavenumbers. The shift settles these results with linear baroclinic instability theory, as under the stronger jet conditions synoptic-scale eddies are predicted to have a sub-optimal growth rate. These results can potentially explain the midwinter suppression of storm activity over the Pacific and the difference from the Atlantic response.

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 2–2.5 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 1.5–4 × 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.

Uranus and Neptune are the archetypes of “ice giants”, a class of planets that may be among the most common in the Galaxy. They are the last unexplored planets of the Solar System, yet they hold the keys to understand the atmospheric dynamics and structure of planets with hydrogen atmospheres inside and outside the solar system.

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 low\u2010degree gravity harmonics, including the first estimate of the planet's north\u2010south 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 small\u2010scale 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 short\u2010scale 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 insolation a planet receives from its parent star is the main driver of the climate and depends on the planet's orbital configuration. Planets with nonzero obliquity and eccentricity experience variations in seasonal insolation. As a result, the climate exhibits a seasonal cycle, with its strength depending on the orbital configuration and atmospheric characteristics. In this study, using an idealized general circulation model, we examine the climate response to changes in eccentricity for both zero and nonzero obliquity planets. In the zero obliquity case, a comparison between the seasonal response to changes in eccentricity and perpetual changes in the solar constant shows that the seasonal response strongly depends on the orbital period and radiative timescale. More specifically, using a simple energy balance model, we show the importance of the latitudinal structure of the radiative timescale in the climate response. We also show that the response strongly depends on the atmospheric moisture content. The combination of an eccentric orbit with nonzero obliquity is complex, as the insolation also depends on the perihelion position. Although the detailed response of the climate to variations in eccentricity, obliquity, and perihelion is involved, the circulation is constrained mainly by the thermal Rossby number and the maximum temperature latitude. Finally, we discuss the importance of different planetary parameters that affect the climate response to orbital configuration variations.

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.

Thousands of exoplanets have been detected to date, and with future planned missions this tally will increase. Understanding the climate dependence on the planetary parameters is vital for the study of terrestrial exoplanet habitability. Using an idealized general circulation model with a seasonal cycle, we study the seasonal response of the surface temperature and Hadley circulation to changes in the orbital, rotational, and radiative timescales. Analyzing the climate's seasonal response to variations in these timescales, we find a regime transition between planets controlled by the annual mean insolation to planets controlled by the seasonal variability depending on the relation between the length of the orbital period, obliquity, and radiative timescale. Consequently, planets with obliquity greater than 54° and a short orbital period will have a minimum surface temperature at the equator. We also show that in specific configurations, mainly high atmospheric mass and short orbital periods, high obliquity planets can still have an equable climate. Based on the model results, we suggest an empirical power law for the ascending and descending branches of the Hadley circulation and its strength. These power laws show that the Hadley circulation becomes wider and stronger by increasing the obliquity and orbital period or by decreasing the atmospheric mass and rotation rate. Consistent with previous studies, we show that the rotation rate plays an essential role in dictating the width of the seasonally dependent Hadley circulation.

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.

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.

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.

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.

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.

On 27 August 2016, the Juno spacecraft acquired science observations of Jupiter, passing less than 5000 kilometers above the equatorial cloud tops. Images of Jupiter's poles show a chaotic scene, unlike Saturn's poles. Microwave sounding reveals weather features at pressures deeper than 100 bars, dominated by an ammonia-rich, narrow low-latitude plume resembling a deeper, wider version of Earth's Hadley cell. Near-infrared mapping reveals the relative humidity within prominent downwelling regions. Juno's measured gravity field differs substantially from the last available estimate and is one order of magnitude more precise. This has implications for the distribution of heavy elements in the interior, including the existence and mass of Jupiter's core. The observed magnetic field exhibits smaller spatial variations than expected, indicative of a rich harmonic content.

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.

During 2016-17, the Juno and Cassini spacecraft will both perform close eccentric orbits of Jupiter and Saturn, respectively, obtaining high-precision gravity measurements for these planets. These data will be used to estimate the depth of the observed surface flows on these planets. All models to date, relating the winds to the gravity field, have been in the forward direction, thus only allowing the calculation of the gravity field from given wind models. However, there is a need to do the inverse problem since the new observations will be of the gravity field. Here, an inverse dynamical model is developed to relate the expected measurable gravity field, to perturbations of the density and wind fields, and therefore to the observed cloud-level winds. In order to invert the gravity field into the 3D circulation, an adjoint model is constructed for the dynamical model, thus allowing backward integration. This tool is used for the examination of various scenarios, simulating cases in which the depth of the wind depends on latitude. We show that it is possible to use the gravity measurements to derive the depth of the winds, both on Jupiter and Saturn, also taking into account measurement errors. Calculating the solution uncertainties, we show that the wind depth can be determined more precisely in the low-to-mid-latitudes. In addition, the gravitational moments are found to be particularly sensitive to flows at the equatorial intermediate depths. Therefore, we expect that if deep winds exist on these planets they will have a measurable signature by Juno and Cassini.

Geostrophic turbulence theory predicted already a few decades ago an inverse energy cascade in the barotropic mode, yet there has been limited evidence for it in the ocean. In this study, the latitudinal behavior of the oceanic barotropic energy balance and macroturbulent scales is studied using the ECCO2 (Estimating the Circulation and Climate of the Ocean) state estimate, which synthesizes satellite data and in situ measurements with a high-resolution general circulation model containing realistic bathymetry and wind forcing. It is found that inverse energy cascade occurs at high latitudes, as eddy-eddy interactions spread the conversion of eddy kinetic energy from the baroclinic to the barotropic mode, both upscale and downscale. At these latitudes, the conversion scale of baroclinic eddy kinetic energy and the energy-containing scale follow the most unstable and Rhines scales, respectively. Even though an inverse energy cascade occurs at high latitudes, the energy spectrum follows a steeper slope than the -5/3 slope. Different than classic arguments, the Rossby deformation radius does not follow the baroclinic conversion and most unstable scales.

Observations suggest that Earth's early atmospheric mass differed from the present day. The effects of a different atmospheric mass on radiative forcing have been investigated in climate models of variable sophistication, but a mechanistic understanding of the thermodynamic component of the effect of atmospheric mass on early climate is missing. Using a 3-D idealized global circulation model (GCM), we systematically examine the thermodynamic effect of atmospheric mass on near-surface temperature. We find that higher atmospheric mass tends to increase the near-surface temperature mostly due to an increase in the heat capacity of the atmosphere, which decreases the net radiative cooling effect in the lower layers of the atmosphere. Additionally, the vertical advection of heat by eddies decreases with increasing atmospheric mass, resulting in further near-surface warming. As both net radiative cooling and vertical eddy heat fluxes are extratropical phenomena, higher atmospheric mass tends to flatten the meridional temperature gradient.

The effect of eddy-eddy interactions on zonal and meridional macroturbulent scales is investigated over a wide range of eddy scales, using high-resolution idealized GCM simulations with and without eddy-eddy interactions. The wide range of eddy scales is achieved through systematic variation of the planetary rotation rate and thus multiple-jet planets. It is found that not only are eddy-eddy interactions not essential for the formation of jets, but the existence of eddy-eddy interactions decreases the number of eddy-driven jets in the atmosphere. The eddy-eddy interactions have little effect on the jet scale, which in both types of simulations coincides with the Rhines scale through all latitudes. The decrease in the number of jets in the presence of eddy-eddy interactions occurs because of the narrowing of the latitudinal region where zonal jets appear. This narrowing occurs because eddy-eddy interactions are mostly important at latitudes poleward of where the Rhines scale is equal to the Rossby deformation radius. Thus, once eddy-eddy interactions are removed, the conversion from baroclinic to barotropic eddy kinetic energy increases, and eddy-mean flow interactions intrude into these latitudes and maintain additional jets there. The eddy-eddy interactions are found to increase the energy-containing zonal scale so it coincides with the jets' scale and thus make the flow more isotropic. While the conversion scale coincides with the most unstable scale, the Rossby deformation radius does not provide a good indication to these scales in both types of simulations.

The poleward propagation of midlatitude storms is studied using a potential vorticity (PV) tendency analysis of cyclone-tracking composites, in an idealized zonally symmetric moist GCM. A detailed PV budget reveals the important role of the upper-level PV and diabatic heating associated with latent heat release. During the growth stage, the classic picture of baroclinic instability emerges, with an upper-level PV to the west of a low-level PV associated with the cyclone. This configuration not only promotes intensification, but also a poleward tendency that results from the nonlinear advection of the low-level anomaly by the upper-level PV. The separate contributions of the upper- and lower-level PV as well as the surface temperature anomaly are analyzed using a piecewise PV inversion, which shows the importance of the upper-level PV anomaly in advecting the cyclone poleward. The PV analysis also emphasizes the crucial role played by latent heat release in the poleward motion of the cyclone. The latent heat release tends to maximize on the northeastern side of cyclones, where the warm and moist air ascends. A positive PV tendency results at lower levels, propagating the anomaly eastward and poleward. It is also shown here that stronger cyclones have stronger latent heat release and poleward advection, hence, larger poleward propagation. Time development of the cyclone composites shows that the poleward propagation increases during the growth stage of the cyclone, as both processes intensify. However, during the decay stage, the vertical alignment of the upper and lower PV anomalies implies that these processes no longer contribute to a poleward tendency.

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.

Transient and stationary eddies shape the extratropical climate through their transport of heat, moisture, and momentum. In the zonal mean, the transports by transient eddies dominate over those by stationary eddies, but this is not necessarily the case locally. In particular, in storm-track entrance and exit regions during winter, stationary eddies and their interactions with the mean flow dominate the atmospheric energy transport. Here it is shown that stationary eddies can shape storm tracks and control where they terminate by modifying local baroclinicity. Simulations with an idealized aquaplanet GCM show that zonally localized surface heating alone (e.g., ocean heat flux convergence) gives rise to storm tracks, which have a well-defined length scale that is similar to that of Earth's storm tracks. The storm tracks terminate downstream of the surface heating even in the absence of continents, at a distance controlled by the stationary Rossby wavelength scale. Stationary eddies play a dual role: within about half a Rossby wavelength downstream of the heating region, stationary eddy energy fluxes increase the baroclinicity and therefore contribute to energizing the storm track; farther downstream, enhanced poleward and upward energy transport by stationary eddies reduces the baroclinicity by reducing the meridional temperature gradients and enhancing the static stability. Transports both of sensible and latent heat (water vapor) play important roles in determining where storm tracks terminate.

NASA's Juno spacecraft will make microwave and gravity measurements of Jupiter. These can reveal information about the composition of Jupiter's atmosphere and about the temperature and density structure below the visible clouds, which is in balance with the structure of the zonal winds. Here we show that there exist strong physical constraints on the structure of the off-equatorial deep zonal winds, and that these imply dynamical constraints on the thermal and gravitational signals Juno will measure. The constraints derive from the facts that Jupiter is rapidly rotating, has nearly inviscid flow, and has strong intrinsic heat fluxes emanating from the deep interior. Because of the strong intrinsic heat fluxes, Jupiter's interior is convecting, but the rapid rotation and weak viscosity constrain the convective motions away from the equator to occur primarily along cylinders parallel to the planet's spin axis. As a consequence, convection is expected to approximately homogenize entropy along the spin axis, thereby adjusting the interior to a convectively and inertially nearly neutral state. In this state, entropy gradients perpendicular to the spin axis are constant but generally not zero on cylinders concentric with the spin axis. Additionally, thermal wind balance relates entropy gradients perpendicular to the spin axis to the zonal wind shear between the observed cloud-level winds and winds in the deep interior (pressures of order 10⁶bar), which must be much weaker because otherwise the Ohmic energy dissipation produced by the interaction of the zonal winds with the planetary magnetic field would exceed the planetary luminosity. Combining these physical constraints with thermal and electrical properties of the atmosphere, we obtain that zonal winds away from the equator likely extend deeply into Jupiter (to a depth between about 0.84R_J and 0.94R_J with Jupiter radius R_J) but have strengths similar to cloud level winds only within the outer few percent of Jupiter's radius. Meridional equator-to-pole temperature contrasts in thermal wind balance with the zonal winds increase with depth and reach ∼1-2K at 50bar; they would reach O(10K) if the winds were shallowly confined, as has been proposed previously. Such temperature contrasts will be detectable by Juno's microwave instrument and are expected to be much larger than those associated with variations in water vapor abundance. The associated gravitational signals of the zonal winds will also be detectable by Juno, but they will be more difficult to distinguish from those implied by other flow models with deep zonal flows. The combination of Juno's gravity and microwave instruments should be able to distinguish deep flows (detectable gravitational signals) from shallow flows (detectable thermal signals), providing strong constraints on the penetration depth of substantial zonal winds.

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.

The Northern Hemisphere storm tracks have maximum intensity over the Pacific and Atlantic basins; their intensity is reduced over the continents downstream. Here, simulations with an idealized aquaplanet general circulation model are used to demonstrate that even without continents, storm tracks have a self-determined longitudinal length scale. Their length is controlled primarily by the planetary rotation rate and is similar to that of Earth's storm tracks for Earth's rotation rate. Downstream, storm tracks self-destruct: the downstream eddy kinetic energy is lower than it would be without the zonal asymmetries that cause localized storm tracks. Likely involved in the downstream self-destruction of storm tracks are the energy fluxes associated with them. The zonal asymmetries that cause localized storm tracks enhance the energy transport through the generation of stationary eddies, and this leads to a reduced baroclinicity that persists far downstream of the eddy kinetic energy maxima.

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.