Publications

The unresolved recent multidecadal cooling of the eastern Pacic and its uncertain future remain key puzzles in climate dynamics. We propose the land-sea heating contrast as a potential driver of this phenomenon. To test this, we amplify the land-sea contrast by quadrupling CO2 only over land in a global coupled climate model. We show that this causes a pronounced transient cooling of the eastern and equatorial Pacic. A transient 1%-per-year CO2 increase over land produces decadal cooling, suggesting relevance to the observed eastern Pacic cooling. Targeted simulations with locally increased CO2 concentrations over dierent land regions reveal three drivers of this cooling: a northward ITCZ shift, a westward convection shift over the Western Pacic, and strengthened Subtropical Highs from Rossby waves. These insights recast the commonly employed ocean dynamical thermostat mechanism as a feedback rather than a driver of transient cooling, and contribute to understanding and predicting Pacic temperature patterns.

Jupiter, the fastest-rotating planet in the Solar System, exhibits a pronounced equatorial bulge, with its equatorial radius exceeding the polar radius by approximately 7%. This oblate shape reflects the combined effects of rapid rotation, complex internal structure and atmospheric winds. Existing estimates of Jupiters shape, with uncertainties of about 4 km, are based on a single analysis of Voyager and Pioneer radio occultations from nearly five decades ago and do not account for the influence of Jupiters strong winds. The Juno spacecraft has recently returned numerous high-precision radio-occultation measurements, enabling a more accurate determination. Here, incorporating the effects of zonal winds, we derive Jupiters shape with an order-of-magnitude reduction in uncertainty. At the 1-bar pressure level, we find a polar radius of 66,842 ± 0.4 km, an equatorial radius of 71,488 ± 0.4 km and a mean radius of 69,886 ± 0.4 km, which are 12 km, 4 km and 8 km smaller than previous estimates, respectively. The results indicate that winds above the visible cloud tops are largely barotropic, showing minimal vertical variation. The updated shape has important implications for interior structure models, supporting a metal-enriched and cooler atmosphere, thereby helping reconcile discrepancies between models, Galileo probe measurements and Voyager-derived temperatures. The refined radius profile also improves spatial referencing for pressure-dependent measurements, offering a more precise context for interpreting Jupiters atmospheric dynamics.

The eddy-driven jet meandering has been hypothesized to increase due to climate change. This meandering frequently relates to slow-moving patterns of surface low and high pressure anomalies, potentially causing extreme weather events such as droughts, flooding, heat waves, and cold spells. However, the quantitative link between jet meandering and storm development is still lacking, as well as a conclusive mechanism for the effect of climate change on jet meandering. In this study, we separate the physical components in the atmospheric complex system using an idealized moist global circulation model (GCM), performing a series of global warming simulations. We outline the connection between the decreasing equator-to-pole temperature gradient due to Arctic amplification and the meandering of the jet. We derive a theoretical relation between the 500-hPa geopotential height meridional gradient and jet meandering, indicating that the leading reason for the increase in jet meandering is the flattening of the midatmospheric meridional temperature gradient, constraining the variations of the streamfunction anomaly with latitude. As it decreases, the eddy-driven jet slows and its meridional layout widens. By Lagrangian tracking of cyclones and anticyclones, we link the jet meandering to the development of cyclones and anticyclones, showing that intense storms are more likely to be associated with jet meanders during their development stage. Furthermore, jet stream meanders with greater meridional extent will be more likely associated with intense storms. In simulations with increased mean surface temperature, our results demonstrate an increase in both jet meandering and storm intensity.

Context. Jupiter's polar upper troposphere and stratosphere host a persistent cold vortex poleward of 65 degrees N, but its detailed structure and dynamics have remained difficult to resolve.Aims. The goal is to characterize the thermal structure and dynamics of the polar vortex using new and complementary remote sensing techniques. Methods. We used a combination of high-resolution vertical profiles derived from Juno's recent radio occultation measurements and mid-infrared imaging from the VLT/VISIR instrument. The former provided direct retrievals of temperature and density near and within the vortex, while VISIR imaging revealed spatial thermal contrasts across the region. Results. Our analysis confirms the presence of a steep meridional temperature jump at 65 degrees N, of about 7 +/- 1 K at 100 mbar, which is consistent with a strong vertical wind shear and a prograde polar stratospheric jet reaching up to 80 ms(-1) at the 10 mbar level. We find the atmosphere to be thermally stable above 0.55 bar, reaching a Brunt-Vaisala frequency of 0.025 s(-1) in the mid-stratosphere. Thermal contrasts observed in the infrared data align with the vertical structures inferred from radio occultations, which validates the presence and extent of the cold vortex. Conclusions. These findings offer a quantitative analysis of the thermal structure and the dynamical behavior of Jupiter's polar atmosphere and demonstrate the diagnostic power of combining radio occultation and thermal infrared techniques in planetary atmospheric studies.

Jupiter's poles feature striking polygons of cyclones that drift westward over time, a motion governed by [Formula: see text]-drift (vortex motion caused by the latitudinal variation of the Coriolis force). This study investigates how [Formula: see text]-drift and the resulting westward motion depend on the depth of these cyclones. Counterintuitively, shallower cyclones drift more slowly, a consequence of stronger vortex stretching. By employing a 2D quasi-geostrophic model of Jupiter's polar regions, we constrain the cyclones' deformation radius, a key parameter that serves as a proxy for their vertical extent, required to replicate the observed westward drift. We then explore possible vertical structures and the static stability of the poles by solving the eigenvalue problem that links the 2D model to a 3D framework, matching the constrained deformation radius. These findings provide a foundation for interpreting upcoming Juno microwave measurements of Jupiter's north pole, offering insights into the static stability and vertical structure of the polar cyclones. Thus, by leveraging long-term motion as a constraint on vertical dynamics, this work sets the stage for advancing our understanding of the formation and evolution of Jupiter's enigmatic polar cyclones.

The equatorial jets dominating the dynamics of the Jovian planets exhibit two distinct types of zonal flows: strongly eastward in the gas giants (superrotation) and strongly westward in the ice giants (subrotation). Existing theories propose different mechanisms for these patterns, but no single mechanism has successfully explained both. However, the planetary parameters of the four Solar System giant planets suggest that a fundamentally different mechanism is unlikely. In this study, we show that convection-driven columnar structures can account for both eastward and westward equatorial jets, framing the phenomenon as a bifurcation. Consequently, both superrotation and subrotation emerge as stable branches of the same mechanistic solution. Our analysis of these solutions uncovers similarities in the properties of equatorial waves and the leading-order momentum balance. This study suggests that the fundamental dynamics governing equatorial jet formation may be more broadly applicable across the Jovian planets than previously believed, offering a unified explanation for their two distinct zonal wind patterns.

This paper presents an analysis of Juno's first radio occultation experiments. Relying on two-way radio links in the X- and Ka-bands, we processed data from NASA's Deep Space Network antennas through a ray-tracing inversion algorithm. By effectively isolating dispersive effects, we obtained measurements of the neutral atmosphere's characteristics. This enabled the derivation of pressure and temperature profiles from the recorded frequencies. These results complement prior data from Voyager occultations and CIRS observations, providing valuable contributions to our understanding of Jupiter's atmospheric dynamics.

Europa, Jupiters second Galilean moon, is believed to host a subsurface ocean in contact with a rocky mantle, where hydrothermal activity may drive the synthesis of organic molecules. Among these possible organic molecules, abiotic synthesis of aromatic amino acids is unlikely, so their detection on planetary surfaces such as Europa suggests that they could be considered a potential biosignature. Fluorescence from aromatic amino acids, with characteristic emissions in the 200-400 nm wavelength range, can be induced by a laser and may be detectable where ocean material has been relatively recently emplaced on Europas surface, as indicated by geologically young terrain and surface features. However, surface bombardment by charged particles from the jovian magnetosphere and solar ultraviolet (UV) radiation degrades organic molecules and limits their longevity. We model radiolysis and photolysis of aromatic amino acids embedded in ice. Our model shows dependencies on hemispheric and latitudinal patterns of charged particle bombardment and ice phase. We demonstrate that such molecules contained within freshly deposited ice in high-latitude regions on the surface of Europa are detectable using laser-induced UV fluorescence, even from an orbiting spacecraft.

Extratropical storms dominate midlatitude climate and weather and are known to grow baroclinically and decay barotropically. Traditionally, quantitative climatic measures of storm activity have been mostly based on Eulerian measures, taking into account the mean state of the atmosphere and how those affect Eulerian eddy activity, but they do not consider the Lagrangian growth of the storms themselves. Here, using ERA-5 reanalysis data and tracking all extratropical storms (cyclones and anticyclones) from 83 years of data, we examine the actual growth of the storms and compare it to the Eulerian characteristics of the background state as the storms develop. In the limit of weak baroclinicity, we find that baroclinicity provides a good measure for storm maximum intensity. However, this monotonic relationship breaks for high baroclinicity levels. We show that although the actual growth rate of individual storms monotonically increases with baroclinicity, the reduction in maximum intensity at high baroclinicity is caused by a decrease in storm growth time. Based on the Lagrangian analysis, we suggest a nonlinear correction to the traditional linear connection between baroclinicity and storms' activity. Then, we show that a simplified model of storm growth, incorporating the baroclinicity effect on the vertical tilt of anomalies, reproduces the observed nonlinear relationship. Expanding the analysis to include the mean flow's barotropic properties highlights their marginal effect on storm growth rate, but the crucial impact on growth time. Our results emphasize the potential of Lagrangianly studying storm dynamics to advance understanding of the midlatitude climate.

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.

Context. The internal structure of Jupiter is constrained by the precise gravity field measurements by NASAa's Juno mission, atmospheric data from the Galileo entry probe, and Voyager radio occultations. Not only are these observations few compared to the possible interior setups and their multiple controlling parameters, but they remain challenging to reconcile. As a complex, multidimensional problem, characterizing typical structures can help simplify the modeling process. Aims. We explored the plausible range of Jupitera's interior structures using a coupled interior and wind model, identifying key structures and effective parameters to simplify its multidimensional representation. Methods. We used NeuralCMS, a deep learning model based on the accurate concentric Maclaurin spheroid (CMS) method, coupled with a fully consistent wind model to efficiently explore a wide range of interior models without prior assumptions. We then identified those consistent with the measurements and clustered the plausible combinations of parameters controlling the interior. Results. We determine the plausible ranges of internal structures and the dynamical contributions to Jupitera's gravity field. Four typical interior structures are identified, characterized by their envelope and core properties. This reduces the dimensionality of Jupitera's interior to only two effective parameters. Within the reduced 2D phase space, we show that the most observationally constrained structures fall within one of the key structures, but they require a higher 1 bar temperature than the observed value. Conclusions. We provide a robust framework for characterizing giant planet interiors with consistent wind treatment, demonstrating that for Jupiter, wind constraints strongly impact the gravity harmonics while the interior parameter distribution remains largely unchanged. Importantly, we find that Jupitera's interior can be described by two effective parameters that clearly distinguish the four characteristic structures and conclude that atmospheric measurements may not fully represent the entire envelope.

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.

The shape of the two gas giants, Jupiter and Saturn, is determined primarily by their rotation rate, and interior density distribution. It is also affected by their zonal winds, causing an anomaly of O(10 km) at low latitudes. However, uncertainties in the observed cloud-level wind and the polar radius, translate to an uncertainty in the shape with the same order of magnitude. The Juno (Jupiter) and Cassini (Saturn) missions gave unprecedented accurate gravity measurements, constraining better the uncertainty in the wind structure. Using an accurate shape calculation, and a joint optimization, given both gravity and radio-occultation measurements, we calculate the possible range of dynamical height for both planets. We find that for Saturn there is an excellent match to the radio-occultation measurements, while at Jupiter such a match is not achieved. This may point to deviations from a barotropic flow above the cloud level, which might be tested with the forthcoming radio-occultation measurements by Juno.

Letter

The atmospheric dynamics of Jupiter are dominated by strong zonal winds engulfing the planet. Since the first gravity measurements taken by Juno at Jupiter, the low-degree gravity harmonics (J ₃J ₁₀) have been used to determine the depth and structure of the zonal winds observed at the cloud level, limiting inferences on the deep flows to the wide latitudinal structure of these harmonics. Here, using constraints on the dynamical contribution to gravity at high latitude, we present the gravity harmonics up to J _40. We find an excellent correlation between these measurements and the gravity harmonics resulting from the observed cloud-level winds extending inwards cylindrically to depths of ~10⁵ bar (3,000 km). These measurements provide direct evidence that the flows penetrate inwards along the direction of the spin axis, confirming the cylindrical nature of the flow, which has been postulated theoretically since the 1970s. Furthermore, this detailed new gravity spectrum allows us to quantify the contribution of the various jets to the gravity signal, showing the dominance of the strong zonal flows around 20° latitude in both hemispheres.

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.

The Northern and Southern Hemispheres reflect on average almost equal amounts of sunlight due to compensating hemispheric asymmetries in clear-sky and cloud albedo. Recent work indicates that the cloud albedo asymmetry is largely due to clouds in extratropical oceanic regions. Here, we investigate the proximate causes of this extratropical cloud albedo asymmetry using a cloud-controlling factor (CCF) approach. We develop a simple index that measures the skill of CCFs, either individually or in combination, in predicting the asymmetry. The index captures the contribution to the asymmetry due to interhemispheric differences in the probability distribution function of daily CCF values. Cloud albedo is quantified using daily MODIS satellite retrievals, and is related to range of CCFs derived from the ERA5 product. We find that sea surface temperature is the CCF that individually explains the largest fraction of the asymmetry, followed by surface wind. The asymmetry is predominantly due to low clouds, and our results are consistent with prior local-scale modeling work showing that marine boundary layer clouds become thicker and more extensive as surface wind increases and surface temperature cools. The asymmetry is consistent with large-scale control of storm-track intensity and surface winds by meridional temperature gradients: persistently cold and windy conditions in the Southern Hemisphere keep cloud albedo high year-round. Our results have important implications for global-scale cloud feedbacks and contribute to efforts to develop a theory for planetary albedo and its symmetry.

Several different factors influence the seasonal cycle of a planet. This study uses a general circulation model and an energy balance model (EBM) to assess the parameters that govern the seasonal cycle. We define two metrics to describe the seasonal cycle, ϕ_s, the latitudinal shift of the maximum temperature, and ΔT, the maximum seasonal temperature variation amplitude. We show that alongside the expected dependence on the obliquity and orbital period, where seasonality generally strengthens with an increase in these parameters, the seasonality depends in a nontrivial way on the rotation rate. While the seasonal amplitude decreases as the rotation rate slows down, the latitudinal shift, ϕ_s, shifts poleward. A similar result occurs in a diffusive EBM with increasing diffusivity. These results suggest that the influence of the rotation rate on the seasonal cycle stems from the effect of the rotation rate on the atmospheric heat transport.

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.

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 measured Jupiters gravity field and determined the even and odd zonal harmonics, J_n, with unprecedented precision. However, interpreting these observations has been a challenge because it is difficult to reconcile the unexpectedly small magnitudes of the moments J₄ and J₆ with conventional interior models that assume a large, distinct core of rock and ice. Here we show that the entire set of gravity harmonics can be matched with models that assume an ab initio equation of state, wind profiles, and a dilute core of heavy elements that are distributed as far out as 63% of the planets radius. In the core region, heavy elements are predicted to be distributed uniformly and make up only 18% by mass because of dilution with hydrogen and helium. Our models are consistent with the existence of primary and secondary dynamo layers that will help explain the complexity of the observed magnetic field.

Polar vortices are common planetary-scale flows that encircle the pole in the middle or high latitudes and are observed in most of the solar systems planetary atmospheres. The polar vortices on Earth, Mars, and Titan are dynamically related to the mean meridional circulation and exhibit a significant seasonal cycle. However, the polar vortexs characteristics vary between the three planets. To understand the mechanisms that influence the polar vortexs dynamics and dependence on planetary parameters, we use an idealized general circulation model with a seasonal cycle in which we vary the obliquity, rotation rate, and orbital period. We find that there are distinct regimes for the polar vortex seasonal cycle across the parameter space. Some regimes have similarities to the observed polar vortices, including a weakening of the polar vortex during midwinter at slow rotation rates, similar to Titans polar vortex. Other regimes found within the parameter space have no counterpart in the solar system. In addition, we show that for a significant fraction of the parameter space, the vortexs potential vorticity latitudinal structure is annular, similar to the observed structure of the polar vortices on Mars and Titan. We also find a suppression of storm activity during midwinter that resembles the suppression observed on Mars and Earth, which occurs in simulations where the jet velocity is particularly strong. This wide variety of polar vortex dynamical regimes that shares similarities with observed polar vortices, suggests that among exoplanets there can be a wide variability of polar vortices.

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.

Uranus and Neptune are the archetypes of \u201cice giants\u201d, 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.

Jupiters 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 spacecrafts Microwave Radiometer. We found vortex roots that extend deeper than the altitude at which water is expected to condense, and we identified density inversion layers. Our results constrain the three-dimensional structure of Jupiters vortices and their extension below the clouds.

During the past few years, both the Cassini mission at Saturn and the Juno mission at Jupiter provided measurements with unprecedented accuracy of the gravity and magnetic fields of the two gas giants. Using the gravity measurements, it was found that the strong zonal flows observed at the cloud level of the gas giants are likely to extend thousands of kilometres deep into the planetary interior. However, the gravity measurements alone, which are by definition an integrative measure of mass, cannot constrain with high certainty the exact vertical structure of the flow. Taking into account the recent Cassini magnetic field measurements of Saturn, and past secular variations of Jupiter's magnetic field, we obtain an additional physical constraint on the vertical decay profile of the observed zonal flows on these planets. Our combined gravity-magnetic analysis reveals that the cloud-level winds on Saturn (Jupiter) extend with very little decay, i.e. barotropically, down to a depth of around 7000 km (2000 km) and then decay rapidly in the semiconducting region, so that within the next 1000 km (600 km) their value reduces to about 1 per cent of that at the cloud level. These results indicate that there is no significant mechanism acting to decay the flow in the outer neutral region, and that the interaction with the magnetic field in the semiconducting region might play a central role in the decay of the flows.

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.

The observed zonal winds at Jupiter's cloud tops have been shown to be closely linked to the asymmetric part of the planet's measured gravity field. Here, we examine to what extent, and at which latitudes, must the flows at depth resemble those at the cloud level to match the gravity signal. We show, using both the symmetric and asymmetric parts of the measured gravity field, that the observed cloud-level wind profile between 25°S and 25°N must extend unaltered to depths of thousands of kilometers. Poleward, the midlatitude deep jets also contribute to the gravity signal, but might differ somewhat from the cloud-level winds. We analyze the likelihood of this difference and give bounds to its strength. We also find that to match the gravity measurements, the winds must project inward in the direction parallel to Jupiter's spin axis, and decay inward in the radial direction.

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 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 Saturns 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 \u201cstacked circulation cells,\u201d 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.

Early gravity measurements performed by the Juno spacecraft determined Jupiter's low-degree gravity harmonics, including the first estimate of the planet's north-south 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-scale 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-scale 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.

We evaluate a method for determining the depth of Jupiter's Great Red Spot (GRS) with two Jima overflights dedicated to gravity science. The small-scale, localized nature of the anticyclone requires a detection method where the gravity perturbations are regional. To this end, we model the GRS as a dipole of flat disk mass concentrations (mascons), separated along the radial direction of Jupiter. Thermal wind theory predicts such structure composed of two equal and opposite masses below the visible cloud tops, condition that is used to constrain our estimation algorithm. Furthermore, Juno radiometer observations of the GRS brightness temperature indicate the presence of anomalies of opposite sign at different depths. We present the results of both a covariance and recovery analyses of the simulated data, in terms of accuracy in the estimation of the GRS mass and depth of winds. The analyses are carried out using precise Doppler tracking of the Juno spacecraft and by posing constraints on the interior model of the vortex from theory. We find that, if the surface dynamics extend at least several hundred kilometers below the cloud tops, it is possible to resolve the mass concentrations using both gravity passes, and tie the mass to the vortex's depth through thermal wind.

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.

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) × 10¹⁹ kilograms (0.41 ± 0.13 times that of the moon Mimas), indicating that the rings may have formed 10⁷ to 10⁸ years ago.

One of Jupiter's most prominent atmospheric features, the Great Red Spot (GRS), has been observed for more than two centuries, yet little is known about its structure and dynamics below its observed cloud level. While its anticyclonic vortex appearance suggests it might be a shallow weather-layer feature, the very long time span for which it was observed implies it is likely deeply rooted, otherwise it would have been sheared apart by Jupiter's turbulent atmosphere. Determining the GRS depth will shed light not only on the processes governing the GRS, but on the dynamics of Jupiter's atmosphere as a whole. The Juno mission single flyby over the GRS (PJ7) discovered using microwave radiometer measurements that the GRS is at least a couple hundred kilometers deep. The next flybys over the GRS (PJ18 and PJ21), will allow high-precision gravity measurements that can be used to estimate how deep the GRS winds penetrate below the cloud level. Here we propose a novel method to determine the depth of the GRS based on the new gravity measurements and a Slepian function approach that enables an effective representation of the wind-induced spatially confined gravity signal, and an efficient determination of the GRS depth given the limited measurements. We show that with this method the gravity signal of the GRS should be detectable for wind depths deeper than 300 km, with reasonable uncertainties that depend on depth (e.g., ±100 km for a GRS depth of 1000 km).

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, limiting our ability to probe the structure and composition of the planet. The discovery by the Juno spacecraft that Jupiter's gravity field is north-south asymmetric 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. Here we report an analysis of Jupiter's even gravitational harmonics J 4, J 6, J 8 and J 10 as observed by Juno 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. 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 gravity harmonics of a fluid, rotating planet can be decomposed into static components arising from solid-body rotation and dynamic components arising from flows. In the absence of internal dynamics, the gravity field is axially and hemispherically symmetric and is dominated by even zonal gravity harmonics J 2n that are approximately proportional to q n, where q is the ratio between centrifugal acceleration and gravity at the planet's equator. Any asymmetry in the gravity field is attributed to differential rotation and deep atmospheric flows. The odd harmonics, J 3, J 5, J 7, J 9 and higher, are a measure of the depth of the winds in the different zones of the atmosphere. Here we report measurements of Jupiter's gravity harmonics (both even and odd) through precise Doppler tracking of the Juno spacecraft in its polar orbit around Jupiter. We find a north-south asymmetry, which is a signature of atmospheric and interior flows. Analysis of the harmonics, described in two accompanying papers, provides the vertical profile of the winds and precise constraints for the depth of Jupiter's dynamical atmosphere.

This study relates the occurrence of the midwinter minimum in eddy activity over the North Pacific with the seasonality in jet characteristics. During winter, the Pacific jet core is typically around latitude 32°N and has features of a merged subtropical eddy-driven jet. On the other hand, during transition seasons, the jet is at higher latitudes (≈40°N) and resembles more an eddy-driven jet. We find that these differences in jet characteristics play a role in the occurrence of the midwinter minimum. It is found that a midwinter minimum-like behavior in eddy activity, as observed, is obtained in idealized simulations where zonally symmetric temperature profiles are adjusted to mimic the seasonality of longitudinally averaged temperature observed across the North Pacific. Furthermore, we find in both reanalysis data and the idealized simulations that a poleward shift of the January jet leads to an increase in eddy kinetic energy.

Over the past decade, the Cassini spacecraft has provided an unprecedented observational record of the atmosphere of Saturn, which in many ways now surpasses Jupiter as the best-observed giant planet. These observations, along with data from the Voyager spacecraft and Earth-based telescopes, demonstrate that Saturn, like Jupiter, has an atmospheric circulation dominated by zonal (east-west) jet streams, including a broad, fast eastward equatorial jet and numerous weaker jets at higher latitudes. Imaging from Voyager, Cassini and ground-based telescopes also document a wide range of tropospheric features, including vortices, waves, turbulence and moist convective storms. At large scales, the clouds, ammonia gas and other chemical tracers exhibit a zonally banded pattern whose relationships to the zonal jets remain poorly understood. Infrared observations constrain the stratospheric thermal structure and allow the derivation of stratospheric temperatures; these exhibit not only the expected seasonal changes, but also a wealth of variations that are likely dynamical in origin and highlight dynamical coupling between the stratosphere and the underlying troposphere. In parallel to these observational developments, significant advances in theory and modeling have occurred over the past decade, especially regarding the dynamics of zonal jets, and we survey these new developments in the context of both Jupiter and Saturn. Highly idealized two-dimensional models illuminate the dynamics that give rise to zonal jets in rapidly rotating atmospheres stirred by convection or other processes, while more realistic three-dimensional models of the atmosphere and interior are starting to identify the particular conditions under which Jupiter- and Saturn-like flows including the fast equatorial superrotation, multiple jets at higher latitudes, storms and vortices can occur. Future data analysis and models have the potential to greatly increase our understanding over the next decade.

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.

Motivated by the expectation that under global warming upper-level meridional temperature gradients will increase while lower-level temperature gradients will decrease, the relations between the vertical structure of baroclinicity and eddy fields are investigated. The sensitivity of eddies and the relation between the mean available potential energy and eddy quantities are studied for cases where the vertical structure of the lapse rate and meridional temperature gradient are modified. To investigate this systematically, an idealized general circulation model with a Newtonian cooling scheme that has a very short relaxation time for the mean state and a long relaxation time for eddies is used. This scheme allows for any chosen zonally mean state to be obtained with good precision. The results indicate that for similar change in the lapse rate or meridional temperature gradient, eddies are more sensitive to changes in baroclinicity where it is already large. Furthermore, when the vertical structure of the lapse rate or the meridional temperature gradient is modified, there is no universal linear relation between the mean available potential energy and eddy quantities.

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.

This study investigates the occurrence of a midwinter suppression in synoptic eddy activity within the North Atlantic storm track. It is found that eddy kinetic energy over the Atlantic is reduced during winter relative to fall and spring, despite the stronger wintertime jet and enhanced baroclinicity. This behavior is similar to the well-known Pacific midwinter minimum, yet the reduction over the Atlantic is smaller and persists for a shorter period. To examine the conditions favorable for this phenomenon, we present an analysis of years with stronger jet intensity versus years of weaker jets over the Atlantic and Pacific basins. When the wintertime jet is stronger, the midwinter suppression of eddy activity is more pronounced, and the jet is more equatorward. Since the climatological Atlantic jet is weaker relative to the Pacific jet, the conditions for a midwinter suppression in the Atlantic are generally less favorable, yet a midwinter suppression often occurs in years of a strong jet.

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.

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.

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 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.

The latitudinal width of atmospheric eddy-driven jets and scales of macroturbulence are examined latitude by latitude over a wide range of rotation rates using a high-resolution idealized GCM. It is found that for each latitude, through all rotation rates, the jet spacing scales with the Rhines scale. These simulations show the presence of a "supercriticality latitude" within the baroclinic zone, where poleward (equatorward) of this latitude, the Rhines scale is larger (smaller) than the Rossby deformation radius. Poleward of this latitude, a classic geostrophic turbulence picture appears with a - spectral slope of inverse cascade from the deformation radius up to the Rhines scale. A shallower slope than the -3 slope of enstrophy cascade is found from the deformation radius down to the viscosity scale as a result of the broad input of baroclinic eddy kinetic energy. At these latitudes, eddy-eddy interactions transfer barotropic eddy kinetic energy from the input scales of baroclinic eddy kinetic energy up to the jet scale and down to smaller scales. For the Earth case, this latitude is outside the baroclinic zone and therefore an inverse cascade does not appear. Equatorward of the supercriticality latitude, the - slope of inverse cascade vanishes, eddy-mean flow interactions play an important role in the balance, and the spectrum follows a -3 slope from the Rhines scale down to smaller scales, similar to what is observed on Earth. Moreover, the length scale of the energy-containing zonal wavenumber is equal to (larger than) the jet scale poleward (equatorward) of the supercriticality latitude.

The alignment of Saturn's magnetic pole with its rotation axis precludes the use of magnetic field measurements to determine its rotation period. The period was previously determined from radio measurements by the Voyager spacecraft to be 10 h 39 min 22.4 s (ref. 2). When the Cassini spacecraft measured a period of 10 h 47 min 6 s, which was additionally found to change between sequential measurements, it became clear that the radio period could not be used to determine the bulk planetary rotation period. Estimates based upon Saturn's measured wind fields have increased the uncertainty even more, giving numbers smaller than the Voyager rotation period, and at present Saturn's rotation period is thought to be between 10 h 32 min and 10 h 47 min, which is unsatisfactory for such a fundamental property. Here we report a period of 10 h 32 min 45 s ± 46 s, based upon an optimization approach using Saturn's measured gravitational field and limits on the observed shape and possible internal density profiles. Moreover, even when solely using the constraints from its gravitational field, the rotation period can be inferred with a precision of several minutes. To validate our method, we applied the same procedure to Jupiter and correctly recovered its well-known rotation period.

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.

A variety of observations provide evidence for vigorous motion in the atmospheres of brown dwarfs and directly imaged giant planets. Motivated by these observations, we examine the dynamical regime of the circulation in the atmospheres and interiors of these objects. Brown dwarfs rotate rapidly, and for plausible wind speeds, the flow at large scales will be rotationally dominated. We present three-dimensional, global, numerical simulations of convection in the interior, which demonstrate that at large scales, the convection aligns in the direction parallel to the rotation axis. Convection occurs more efficiently at high latitudes than low latitudes, leading to systematic equator-to-pole temperature differences that may reach ∼1 K near the top of the convection zone. The interaction of convection with the overlying, stably stratified atmosphere will generate a wealth of atmospheric waves, and we argue that, as in the stratospheres of planets in the solar system, the interaction of these waves with the mean flow will cause a significant atmospheric circulation at regional to global scales. At large scales, this should consist of stratified turbulence (possibly organizing into coherent structures such as vortices and jets) and an accompanying overturning circulation. We present an approximate analytic theory of this circulation, which predicts characteristic horizontal temperature variations of several to ∼50 K, horizontal wind speeds of ∼10-300 m s^-1, and vertical velocities that advect air over a scale height in ∼10⁵-10⁶ s. This vertical mixing may help to explain the chemical disequilibrium observed on some brown dwarfs. Moreover, the implied large-scale organization of temperature perturbations and vertical velocities suggests that near the L/T transition, patchy clouds can form near the photosphere, helping to explain recent observations of brown-dwarf variability in the near-IR.

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 low-order even gravity harmonics J₂, J₄, and J₆ are well constrained for Jupiter and Saturn from spacecraft encounters over the past few decades. These gravity harmonics are dominated by the oblate shape and radial density distribution of these gaseous planets. In the lack of any north-south asymmetry, odd gravity harmonics will be zero. However, the winds on these planets are not hemispherically symmetric, and therefore can contribute to the odd gravity harmonics through dynamical variations to the density field. Here it is shown that even relatively shallow winds (reaching ~ 40 bars) can cause considerable odd gravity harmonics that can be detectable by NASA's Juno and Cassini missions to Jupiter and Saturn. Moreover, these measurements will have better sensitivity to the odd harmonics than to the high-order even harmonics, which have been previously proposed as a proxy for deep winds. Determining the odd gravity harmonics will therefore help constrain the depth of the jets on these planets, and may provide valuable information about the planet's core and structure. Key Points Measurable odd harmonics due to atmospheric circulation exist on giant planets Odd harmonics provide a pure dynamical gravity signal (no solid-body component) Juno/Cassini will have better sensitivity to odd harmonics than high even ones

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.

Telescopic observations and space missions to Jupiter have provided vast information about Jupiter's cloud level winds, but the depth to which these winds penetrate has remained an ongoing mystery. Scheduled to be launched in 2011, the Jupiter orbiter Juno will make high-resolution observations of Jupiter's gravity field. In this paper we show that these measurements are sensitive to the depth of the internal winds. We use dynamical models ranging from an idealized thermal wind balance analysis, using the observed cloud-top winds, to a full general circulation model (GCM). We relate the depth of the dynamics to the external gravity spectrum for different internal wind structure scenarios. In particular, we predict that substantial Jovian winds below a depth of 500 km would lead to detectable (milligal-level) gravity anomalies with respect to the expected gravity for a planet in solid body rotation.

This thesis studies the dynamics of a rotating compressible gas sphere, driven by internal convection, as a model for the dynamics on the giant planets. We develop a new general circulation model for the Jovian atmosphere, based on the MITgcm dynamical core augmenting the nonhydrostatic model. The grid extends deep into the planet's interior allowing the model to compute the dynamics of a whole sphere of gas rather than a spherical shell (including the strong variations in gravity and the equation of state). Different from most previous 3D convection models, this model is anelastic rather than Boussinesq and thereby incorporates the full density variation of the planet. We show that the density gradients caused by convection drive the system away from an isentropic and therefore barotropic state as previously assumed, leading to significant baroclinic shear. This shear is concentrated mainly in the upper levels and associated with baroclinic compressibility effects. 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, contributing to the observed flat meridional emission. We show the transition from weak convection cases with symmetric spiraling columnar modes similar to those found in previous analytic linear theory, to more turbulent cases which exhibit similar, though less regular and solely cyclonic, convection columns which manifest on the surface in the form of waves embedded within the superrotation. We develop a mechanical understanding of this system and scaling laws by studying simpler configurations and the dependence on physical properties such as the rotation period, bottom boundary location and forcing structure. These columnar cyclonic structures propagate eastward, driven by dynamics similar to that of a Rossby wave except that the restoring planetary vorticity gradient is in the opposite direction, due to the spherical geometry in the interior.

We further study these interior dynamics using a simplified barotropic annulus model, which shows that the planetary vorticity radial variation causes the eddy angular momentum flux divergence, which drives the superrotating equatorial flow. In addition we study the interaction of the interior dynamics with a stable exterior weather layer, using a quasigeostrophic two layer channel model on a beta plane, where the columnar interior is therefore represented by a negative beta effect. We find that baroclinic instability of even a weak shear can drive strong, stable multiple zonal jets. For this model we find an analytic nonlinear solution, truncated to one growing mode, that exhibits a multiple jet meridional structure, driven by the nonlinear interaction between the eddies. Finally, given the density field from our 3D convection model we derive the high order gravitational spectra of Jupiter, which is a measurable quantity for the upcoming JUNO mission to Jupiter.

Abrupt, switch-like, changes in sea ice cover are proposed as a mechanism for the large-amplitude abrupt warming that seemed to have occurred after each Heinrich event. Sea ice changes are also used to explain the colder-than-ambient glacial conditions around the time of the glacier discharge. The abrupt warming events occur in this mechanism, owing to rapid sea ice melting which warmed the atmosphere via the strong sea ice albedo and insulating feedbacks. Such abrupt sea ice changes can also account for the warming observed during Dansgaard-Oeschger events. The sea ice changes are caused by a weak (order of 5 Sv) response of the thermohaline circulation (THC) to glacier discharges. The main point of this work is therefore that sea ice may be thought of as a very effective amplifier of a weak THC variability, explaining the abrupt temperature changes over Greenland. Synchronous ice sheet collapses from different ice sheets around the North Atlantic, indicated by some proxy records, are shown to be possible via the weak coupling between the different ice sheets by the atmospheric temperature changes caused by the sea ice changes. This weak coupling can lead to a "nonlinear phase locking'' of the different ice sheets which therefore discharge synchronously. It is shown that the phase locking may also lead to "precursor'' glacier discharge events from smaller ice sheets before the Laurentide Ice Sheet discharges. The precursor events in this mechanism are the result rather than the cause of the major glacier discharges from the Laurentide Ice Sheet.