Before Cassini, scientists viewed Saturn’s unique features only from Earth and from three spacecraft flying by. During more than a decade orbiting the gas giant, Cassini studied the planet from its ...interior to the top of the atmosphere. It observed the changing seasons, provided up-close observations of Saturn’s exotic storms and jet streams, and heard Saturn’s lightning, which cannot be detected from Earth. During the Grand Finale orbits, it dove through the gap between the planet and its rings and gathered valuable data on Saturn’s interior structure and rotation. Key discoveries and events include: watching the eruption of a planet-encircling storm, which is a 20- or 30-year event, detection of gravity perturbations from winds 9000 km below the tops of the clouds, demonstration that eddies are supplying energy to the zonal jets, which are remarkably steady over the 25-year interval since the Voyager encounters, re-discovery of the north polar hexagon after 25 years, determination of elemental abundance ratios He/H, C/H, N/H, P/H, and As/H, which are clues to planet formation and evolution, characterization of the semiannual oscillation of the equatorial stratosphere, documentation of the mysteriously high temperatures of the thermosphere outside the auroral zone, and seeing the strange intermittency of lightning, which typically ceases to exist on the planet between outbursts every 1–2 years. These results and results from the Jupiter flyby are all discussed in this review.
The Madden-Julian oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics. Despite its primary importance, a generally accepted theory that accounts for fundamental ...features of the MJO, including its propagation speed, planetary horizontal scale, multiscale features, and quadrupole structures, remains elusive. In this study, the authors use a shallow-water model to simulate the MJO. In this model, convection is parameterized as a short-duration localized mass source and is triggered when the layer thickness falls below a critical value. Radiation is parameterized as a steady uniform mass sink. The following MJO-like signals are observed in the simulations: 1) slow eastward-propagating large-scale disturbances, which show up as low-frequency, low-wavenumber features with eastward propagation in the spectral domain, 2) multiscale structures in the time-longitude (Hovmoller) domain, and 3) quadrupole vortex structures in the longitude-latitude (map view) domain. The authors propose that the simulated MJO signal is an interference pattern of westward and eastward inertia-gravity (WIG and EIG) waves. Its propagation speed is half of the speed difference between the WIG and EIG waves. The horizontal scale of its large-scale envelope is determined by the bandwidth of the excited waves, and the bandwidth is controlled by the number density of convection events. In this model, convection events trigger other convection events, thereby aggregating into large-scale structures, but there is no feedback of the large-scale structures onto the convection events. The results suggest that the MJO is not so much a low-frequency wave, in which convection acts as a quasi-equilibrium adjustment, but is more a pattern of high-frequency waves that interact directly with the convection.
From its pole-to-pole orbit, the Juno spacecraft discovered arrays of cyclonic vortices in polygonal patterns around the poles of Jupiter. In the north, there are eight vortices around a central ...vortex, and in the south there are five. The patterns and the individual vortices that define them have been stable since August 2016. The azimuthal velocity profile vs. radius has been measured, but vertical structure is unknown. Here, we ask, what repulsive mechanism prevents the vortices from merging, given that cyclones drift poleward in atmospheres of rotating planets like Earth? What atmospheric properties distinguish Jupiter from Saturn, which has only one cyclone at each pole? We model the vortices using the shallow water equations, which describe a single layer of fluid that moves horizontally and has a free surface that moves up and down in response to fluid convergence and divergence. We find that the stability of the pattern depends mostly on shielding—an anticyclonic ring around each cyclone, but also on the depth. Too little shielding and small depth lead to merging and loss of the polygonal pattern. Too much shielding causes the cyclonic and anticyclonic parts of the vortices to fly apart. The stable polygons exist in between. Why Jupiter’s vortices occupy this middle range is unknown. The budget—how the vortices appear and disappear—is also unknown, since no changes, except for an intruder that visited the south pole briefly, have occurred at either pole since Juno arrived at Jupiter in 2016.
Polar vortex crystals: Emergence and structure Siegelman, Lia; Young, William R; Ingersoll, Andrew P
Proceedings of the National Academy of Sciences - PNAS,
04/2022, Letnik:
119, Številka:
17
Journal Article
Recenzirano
Odprti dostop
Vortex crystals are quasiregular arrays of like-signed vortices in solid-body rotation embedded within a uniform background of weaker vorticity. Vortex crystals are observed at the poles of Jupiter ...and in laboratory experiments with magnetized electron plasmas in axisymmetric geometries. We show that vortex crystals form from the free evolution of randomly excited two-dimensional turbulence on an idealized polar cap. Once formed, the crystals are long lived and survive until the end of the simulations (300 crystal-rotation periods). We identify a fundamental length scale, Lγ=(U/γ)1/3, characterizing the size of the crystal in terms of the mean-square velocity U of the fluid and the polar parameter γ=fp/a2p, with fp the Coriolis parameter at the pole and ap the polar radius of the planet.
•We model dynamics of Enceladus plumes inside of cracks.•We find that a narrower and deeper crack produces a larger plume mass flux.•The ideal crack width is 0.05–0.75 m and total crack length is 1.7 ...× 500 km.•Our model can explain observed hot spots on Enceladus.
Plumes of water vapor and ice particles have been observed from the so-called tiger stripes at the south polar terrain (SPT) of Saturn’s satellite, Enceladus. The observed high salinity (∼0.5–2%) of the ice particles in the plumes may indicate that the plumes originate from a subsurface liquid ocean. Additionally, the SPT is the source of strong infrared radiation (∼4.2 GW), which is especially intense near (within tens of meters) the tiger stripes. This could indicate that the radiation is associated with plume activity, but the connection remains unclear. Here we investigate the constraints that plume observations place on the widths of the cracks, the depth to the liquid-vapor interface, and the mechanisms controlling plume variability. We solve the fluid dynamics of the flow in the crack and the interaction between the flow and ice walls assuming that the flows of water vapor and ice particles originate from a few kilometers deep liquid ocean. For a crack with a uniform width, we find that our model could explain the observed vapor mass flow rate of the plumes when the crack width is 0.05–0.075 m. A wider crack is not favorable because it would produce a higher vapor mass flow rate than the observed value, but it may be allowed if there are some flows that do not reach the surface of Enceladus either due to condensation on the icy walls or the tortuosity of the crack. The observed heat flow can be explained if the total crack length is approximately 1.7 × 500 km. A tapering crack (a crack which is ∼1 m wide at the bottom of the flow and sharply becomes 0.05–0.075 m at shallower depths) can also explain the observed vapor mass flow rate and heat flow. Widths of 1 m or more are necessary to avoid freezing at the liquid-vapor interface, as shown in our paired paper (Ingersoll and Nakajima 2016 Icarus). The observed intense heat flow along the tiger stripes can be explained by the latent heat release due to vapor condensation onto the ice walls near the surface. The resulting buildup of ice causes the vents to seal themselves on time scales less than a year. We also find that the ice to vapor ratio of the plumes is sensitive to the ice mass fraction at the bottom of the flow (liquid–vapor interface). We find that the total mass flow rate of the plumes becomes larger when the crack width is larger, which is consistent with the observation that the flow rate increases near the orbital apocenter, where the crack is expected to be widest.
A theory of the MJO horizontal scale Yang, Da; Ingersoll, Andrew P.
Geophysical research letters,
16 February 2014, Letnik:
41, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Here we ask, what controls the horizontal scale of the Madden‐Julian Oscillation, i.e., what controls its zonal wave number k? We present a new one‐dimensional (1D) β‐plane model that successfully ...simulates the MJO with the same governing mechanism as the 2D shallow water model of Yang and Ingersoll (2013). Convection is parameterized as a short‐duration localized mass source that is triggered when the layer thickness falls below a critical value. Radiation is parameterized as a steady uniform mass sink. Both models tend toward a statistically steady state—a state of radiative‐convective equilibrium, not just on a global scale but also on the scale of each MJO event. This gives k ~ (Sc/c)1/2, where Sc is the spatial‐temporal frequency of convection events and c is the Kelvin wave speed. We offer this scaling as a prediction of how the MJO would respond to climate change.
Key Points
Triggered convection is essential for simulating the MJO
Interaction of convection with gravity waves is the other essential element
The scale of the MJO is controlled by the number density of convective events
•The liquid on Enceladus is boiling if it is vented to space.•Controlled boiling occurs if the vent is long and narrow.•There is no need for a large vapor chamber above the evaporating liquid.•Crack ...widths greater than ∼1m can bring liquid water close to the surface.•Salinities greater than ∼20gkg−1 are necessary to avoid freezing.
Controlled boiling will occur on Enceladus whenever a long, narrow conduit connects liquid water to the vacuum of space. In a companion paper we focus on the upward flow of the vapor and show how it controls the evaporation rate through backpressure, which arises from friction on the walls. In this paper we focus on the liquid and show how it flows through the conduit up to its level of neutral buoyancy. For an ice shell 20km thick, the liquid water interface could be 2km below the surface. We find that the evaporating surface can be narrow. There is no need for a large vapor chamber that acts as a plume source. Freezing on the icy walls and the evaporating surface is avoided if the crack width averaged over the length of the tiger stripes is greater than 1m and the salinity of the liquid is greater than 20gkg−1. Controlled boiling plays a crucial role in our model, which makes it different from earlier published models. The liquids on Enceladus are boiling because there is no overburden pressure—the saturation vapor pressure is equal to the total pressure. Salinity plays a crucial role in preventing freezing, and we argue that the subsurface oceans of icy satellites can have water vapor plumes only if their salinities are greater than about 20gkg−1.
Previous observations and simulations suggest that an approximate 3°–5°C warming occurred at intermediate depths in the North Atlantic over several millennia during Heinrich stadial 1 (HS1), which ...induces warm salty water (WSW) lying beneath surface cold freshwater. This arrangement eventually generates ocean convective available potential energy (OCAPE), the maximum potential energy releasable by adiabatic vertical parcel rearrangements in an ocean column. The authors find that basin-scale OCAPE starts to appear in the North Atlantic (∼67.5°–73.5°N) and builds up over decades at the end of HS1 with amagnitude of about 0.05 J kg−1. OCAPE provides a key kinetic energy source for thermobaric cabbeling convection (TCC). Using a high-resolution TCC-resolved regionalmodel, it is found that this decadal-scale accumulation of OCAPE ultimately overshoots its intrinsic threshold and is released abruptly (∼1 month) into kinetic energy of TCC, with further intensification from cabbeling. TCC has convective plumes with approximately 0.2–1-km horizontal scales and large vertical displacements (∼1 km),which make TCC difficult to be resolved or parameterized by current general circulation models. The simulation herein indicates that these local TCC events are spread quickly throughout the OCAPE-contained basin by internal wave perturbations. Their convective plumes have large vertical velocities (∼8–15 cm s−1) and bring the WSW to the surface, causing an approximate 2°C sea surface warming for the whole basin (∼700 km) within a month. This exposes a huge heat reservoir to the atmosphere, which helps to explain the abrupt Bølling–Allerød warming.
Abstract
Exactly solving the absolute minimum potential energy state (Lorenz reference state) is a difficult problem because of the nonlinear nature of the equation of state of seawater. This problem ...has been solved recently but the algorithm comes at a high computational cost. As the first part of this study, the authors develop an algorithm that is ~10
3
–10
5
times faster, making it useful for energy diagnosis in ocean models. The second part of this study shows that the global patterns of Lorenz available potential energy (APE) density are distinct from those of eddy kinetic energy (EKE). This is because the Lorenz APE density is based on the entire domainwide parcel rearrangement, while mesoscale eddies, if related to baroclinic instability, are typically generated through local parcel rearrangement approximately around the eddy size. Inspired by this contrast, this study develops a locally defined APE framework: the eddy size–constrained APE density based on the strong constraint that the parcel rearrangement/displacement to achieve the minimum potential energy state should not exceed the local eddy size horizontally. This concept typically identifies baroclinically unstable regions. It is shown to be helpful to detect individual eddies/vortices and local EKE patterns, for example, around the Southern Ocean fronts and subtropical western boundary currents. This is consistent with the physical picture that mesoscale eddies are associated with a strong signature in both the velocity field (i.e., EKE) and the stratification (i.e., local APE). The new APE concept may be useful in parameterizing mesoscale eddies in ocean models.
•The plumes decreased by a factor of ∼2 from 2005 to 2015.•Decadal trend roughly matches a decrease in eccentricity.•Interannual stochasic variability of plumes is likely.•Launch velocity is less at ...apocenter, greater at pericenter.•A secondary maximum occurs at 90 deg mean anomaly.
The brightness of the Enceladus plumes varies with position in the satellite's eccentric orbit, with altitude above the surface, and with time from one year to the next. Hedman et al. (2013, hereinafter H13) were the first to report these variations. They used data from Cassini's Visible and Infrared Mapping Spectrometer (VIMS). Here we present brightness observations from Cassini's Imaging Science Subsystem (ISS), which has 40 times higher spatial resolution than VIMS. Our unit of measure is slab density, the total mass of particles in a horizontal slab per unit thickness of the slab. Using slab density is one approach to correcting for the variation of brightness with wavelength and scattering angle. Approaches differ mainly by a multiplicative scaling factor that depends on particle density, which is uncertain. All approaches lead to the same qualitative conclusions and agree with the conclusions from VIMS. We summarize our conclusions as follows: At all altitudes between 50 and 200 km, the corrected brightness is 4–5 times greater when Enceladus is farther from Saturn (near apocenter) than when it is closer (near pericenter). A secondary maximum occurs after pericenter and before apocenter. Corrected brightness vs. altitude is best described as a power law whose negative exponent is greatest in magnitude at apocenter, indicating a slower launch speed for the particles at apocenter than at other points in the orbit. Corrected brightness decreased by roughly a factor of two during much of the period 2005–2015. The last is our principal result, and we offer three hypotheses to explain it. One is a long-period tide—the decreasing phase of an 11-year cycle in orbital eccentricity; another is buildup of ice at the throats of the vents; and the third is seasonal change—the end of summer at the south pole.
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