Selected publications

Observational evidence for cylindrically oriented zonal flows on Jupiter
Kaspi Y., Galanti E., Park R. S., Duer K., Gavriel N., Durante D., Iess L., Parisi M., Buccino D. R., Guillot T., Stevenson D. J., Bolton S. J. et al. (2023) Nature Astronomy. 7, 12, p. 1463-1472 Abstract
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 ₃–J 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.

[All authors]
The role of baroclinic activity in controlling Earth’s albedo in the present and future climates
Hadas O., Datseris G., Blanco J., Bony S., Caballero R., Stevens B. & Kaspi Y. (2023) Proceedings of the National Academy of Sciences - PNAS. 120, 5, e220877812. Abstract
Clouds are one of the most influential components of Earth’s climate system. Specifically, the midlatitude clouds play a vital role in shaping Earth’s albedo. This study investigates the connection between baroclinic activity, which dominates the midlatitude climate, and cloud-albedo and how it relates to Earth’s existing hemispheric albedo symmetry. We show that baroclinic activity and cloud-albedo are highly correlated. By using Lagrangian tracking of cyclones and anticyclones and analyzing their individual cloud properties at different vertical levels, we explain why their cloud-albedo increases monotonically with intensity. We find that while for anticyclones, the relation between strength and cloudiness is mostly linear, for cyclones, in which clouds are more prevalent, the relation saturates with strength. Using the cloud-albedo strength relationships and the climatology of baroclinic activity, we demonstrate that the observed hemispheric difference in cloud-albedo is well explained by the difference in the population of cyclones and anticyclones, which counter-balances the difference in clear-sky albedo. Finally, we discuss the robustness of the hemispheric albedo symmetry in the future climate. Seemingly, the symmetry should break, as the northern hemisphere’s storm track response differs from that of the southern hemisphere due to Arctic amplification. However, we show that the saturation of the cloud response to storm intensity implies that the increase in the skewness of the southern hemisphere storm distribution toward strong storms will decrease future cloud-albedo in the southern hemisphere. This complex response explains how albedo symmetry might persist even with the predicted asymmetric hemispheric change in baroclinicity under climate change.
The depth of Jupiter’s Great Red Spot constrained by Juno gravity overflights
Parisi M., Kaspi Y., Galanti E., Durante D., Bolton S. J., Levin S. M., Buccino D. R., Fletcher L. N., Folkner W. M., Guillot T., Helled R., Iess L., Li C., Oudrhiri K., Wong M. H. et al. (2021) Science. 374, p. 964–968 eabf1396. Abstract
Jupiter’s Great Red Spot (GRS) is the largest atmospheric vortex in the Solar System and has been observed for at least two centuries. It has been unclear how deep the vortex extends beneath its visible cloud tops. We examine the gravity signature of the GRS using data from twelve encounters of the Juno spacecraft with the planet, including two direct overflights of the vortex. We identify localized density anomalies due to the presence of the GRS, which cause a shift in the spacecraft line-of-sight velocity. Using two different approaches to infer the GRS depth, which yield consistent results, we find that the GRS is contained within the upper 500 km of Jupiter’s atmosphere.

[All authors]
The number and location of Jupiter’s circumpolar cyclones explained by vorticity dynamics
Gavriel N. and Kaspi Y. (2021) Nature Geoscience. 14, p. 559–563 Abstract
The Juno mission observed that both poles of Jupiter have polar cyclones that are surrounded by a ring of circumpolar cyclones (CPCs). The north pole holds eight CPCs and the south pole possesses five, with both circumpolar rings positioned along latitude ~84° N/S. Here we explain the location, stability and number of the Jovian CPCs by establishing the primary forces that act on them, which develop because of vorticity gradients in the background of a cyclone. In the meridional direction, the background vorticity varies owing to the planetary sphericity and the presence of the polar cyclone. In the zonal direction, the vorticity varies by the presence of adjacent cyclones in the ring. Our analysis successfully predicts the latitude and number of circumpolar cyclones for both poles, according to the size and spin of the respective polar cyclone. Moreover, the analysis successfully predicts that Jupiter can hold circumpolar cyclones, whereas Saturn currently cannot. The match between the theory and observations implies that vortices in the polar regions of the giant planets are largely governed by barotropic dynamics, and that the movement of other vortices at high latitudes is also driven by interaction with the background vorticity.
Microwave observations reveal the deep extent and structure of Jupiter’s atmospheric vortices
Bolton S. J., Levin S. M., Guillot T., Li C., Kaspi Y., Orton G., Wong M. H., Oyafuso F., Allison M., Arballo J., Atreya S., Becker H. N., Bloxham J., Brown S., Fletcher L. N., Galanti E., Gulkis S., Janssen M., Ingersoll A., Lunine J. L., Misra S., Steffes P., Stevenson D., Waite J. H., Yadav R. K., Zhang Z. et al. (2021) Science . 374, p. 968-972 eabf1015. Abstract
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.

[All authors]
Measurement and implications of Saturn's gravity field and ring mass
Iess L., Militzer B., Kaspi Y., Nicholson P., Durante D., Racioppa P., Anabtawi A., Galanti E., Hubbard W., Mariani M. J., Tortora P., Wahl S., Zannoni M. et al. (2019) Science. 364, 6445, eaat2965. Abstract
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 Saturn’s gravity deviates from theoretical expectations and requires differential rotation of the atmosphere extending to a depth of at least 9000 kilometers. The total mass of the rings is (1.54 ± 0.49) × 10
¹⁹ kilograms (0.41 ± 0.13 times that of the moon Mimas), indicating that the rings may have formed 10
⁷ to 10
⁸ years ago.

[All authors]
Jupiter's atmospheric jet streams extend thousands of kilometres deep
Kaspi Y., Galanti E., Hubbard W. B., Stevenson D. J., Bolton S. J., Less L., Guillot T., Bloxham J., Connerney J. E. P., Cao H., Durante D., Folkner W. M., Helled R., Ingersoll A. P., Levin S. M., Lunine J. I., Miguel Y., Militzer B., Parisi M., Wahl S. M. et al. (2018) Nature. 555, 7695, p. 223-226 Abstract
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.

[All authors]
Enhanced poleward propagation of storms under climate change
Tamarin-Brodsky T. & Kaspi Y. (2017) Nature Geoscience. 10, 12, p. 908-913 Abstract
Earth's midlatitudes are dominated by regions of large atmospheric weather variability-often referred to as storm tracks-which influence the distribution of temperature, precipitation and wind in the extratropics. Comprehensive climate models forced by increased greenhouse gas emissions suggest that under global warming the storm tracks shift poleward. While the poleward shift is a robust response across most models, there is currently no consensus on what the underlying dynamical mechanism is. Here we present a new perspective on the poleward shift, which is based on a Lagrangian view of the storm tracks. We show that in addition to a poleward shift in the genesis latitude of the storms, associated with the shift in baroclinicity, the latitudinal displacement of cyclonic storms increases under global warming. This is achieved by applying a storm-tracking algorithm to an ensemble of CMIP5 models. The increased latitudinal propagation in a warmer climate is shown to be a result of stronger upper-level winds and increased atmospheric water vapour. These changes in the propagation characteristics of the storms can have a significant impact on midlatitude climate.
Jupiter's interior and deep atmosphere: The initial pole-To-pole passes with the Juno spacecraft
Bolton S. J., Adriani A., Adumitroaie V., Allison M., Anderson J., Atreya S., Bloxham J., Brown S., Connerney J. E. P., DeJong E., Folkner W., Gautier D., Grassi D., Gulkis S., Guillot T., Hansen C., Hubbard W. B., Iess L., Ingersoll A., Janssen M., Jorgensen J., Kaspi Y., Levin S. M., Li C., Lunine J., Miguel Y., Mura A., Orton G., Owen T., Ravine M., Smith E., Steffes P., Stone E., Stevenson D., Thorne R., Waite J., Durante D., Ebert R. W., Greathouse T. K., Hue V., Parisi M., Szalay J. R., Wilson R. et al. (2017) Science. 356, 6340, p. 821-825 Abstract
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.

[All authors]
Saturn's fast spin determined from its gravitational field and oblateness
Helled R., Galanti E. & Kaspi Y. (2015) Nature. 520, 7546, p. 202-204 Abstract
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.
Atmospheric confinement of jet streams on Uranus and Neptune
Kaspi Y., Showman A. P., Hubbard W. B., Aharonson O. & Helled R. (2013) Nature. 497, 7449, p. 344-347 Abstract
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.
Inferring the depth of the zonal jets on Jupiter and Saturn from odd gravity harmonics
Kaspi Y. (2013) Geophysical Research Letters. 40, 4, p. 676-680 Abstract
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
Winter cold of eastern continental boundaries induced by warm ocean waters
Kaspi Y. & Schneider T. (2011) Nature. 471, 7340, p. 621-624 Abstract
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.