arXiv subjects

Malte F. Jansen

Publications and source records attributed to Malte F. Jansen.

10 recordsLinked to original sources

arXiv2026

Parameterizing slantwise convection in icy moon oceans

Convection in icy moon oceans is strongly influenced by rotation, organizing into slantwise columnar structures aligned with the planetary rotation axis. They generate significant meridional heat transport, which can affect the ice shell topography, a primary observable of these moons. However, global ocean simulations cannot resolve convection under realistic icy moon conditions, and traditional convection schemes cannot represent slantwise convection. Here, we develop a slantwise convection scheme and implement it in a global ocean model. We perform benchmark tests in a global spherical shell by comparing parameterized fluxes with convection-resolving simulations. The scheme reproduces the meridional heat transport inside the tangent cylinder, where slantwise convection dominates. The resulting meridional heat transport significantly modifies the surface heat flux, producing variations comparable to the imposed bottom heating magnitude. Although the simulations with parameterized convection cannot fully reproduce the temperature structure, likely due to an inability to reproduce the temperature gradients near the boundaries, they capture the bulk interior vertical temperature gradient. The new scheme allows unresolved slantwise convection to be represented in global ocean simulations for icy moons. It is also applicable to other rapidly rotating oceans with small natural Rossby number ($\mathrm{Ro}^* \ll 1$), including deep ocean worlds on exoplanets.

astro-ph.EP↗

arXiv2025

Slantwise convection and heat transport in icy moon oceans

Ocean heat transport on icy moons shapes the ice shell topography, a primary observable of these moons. Two key processes control the heat transport: baroclinic instability driven by surface buoyancy contrasts and convective instability driven by heating from the core. However, global ocean simulations cannot accurately resolve convection under realistic icy moon conditions and instead often use Earth-based convective parameterizations, which capture only vertical convective mixing and cannot represent rotation-aligned slantwise convection on icy moons. We use high-resolution convection-resolving simulations to investigate ocean heat transport by slantwise convection in a parameter regime relevant to icy moons, isolated from baroclinic instability. Total heat transport follows the Coriolis-Inertial-Archimedean scaling with an added latitude dependence. The vertical transport increases with latitude, and the meridional transport is poleward. These results indicate that slantwise convection redistributes heat toward the poles, favoring a poleward-thinning ice shell, qualitatively consistent with Enceladus's observed ice thickness distribution.

astro-ph.EP↗

arXiv2025

Symmetric instability in a Boussinesq fluid on a rotating planet

Symmetric instability has broad applications in geophysical and planetary fluid dynamics. It plays a crucial role in the formation of mesoscale rainbands at mid-latitudes on Earth, instability in the ocean's mixed layer, and slantwise convection on gas giants and icy moon oceans. Here, we apply linear instability analysis to an arbitrary zonally symmetric Boussinesq flow on a rotating spherical planet, with applicability to icy moon oceans. We divide the instabilities into three types: (1) gravitational instability, occurring when stratification is unstable along angular momentum surfaces, (2) inertial instability, occurring when angular momentum shear is unstable along buoyancy surfaces, and (3) a mixed symmetric instability, occurring when neither of the previous conditions are fulfilled, but the potential vorticity has the opposite sign to planetary rotation. We note that $N^2<0$ where $N$ is the Brunt--Väisälä frequency -- a typical criterion used to trigger convective adjustment in global ocean models -- is neither necessary nor sufficient for instability. Instead, $b_z \sin{θ_0}<0$, where $b_z$ is the stratification along the planetary rotation axis and $θ_0$ is the local latitude, is always sufficient for instability and also necessary in the low Rossby number limit. In this limit, relevant for deep convection in icy moon oceans, the most unstable mode is slantwise convection parallel to the planetary rotation axis. This slantwise convection differs from the parameterized convection in existing general circulation models, whose convection schemes parameterize convection in the direction of gravity. Our results suggest that convection schemes in global ocean models must be revised before being applied to icy moon oceans.

physics.flu-dyn↗

arXiv2023

Why are Mountaintops Cold? The Transition of Surface Lapse Rate on Dry Planets

Understanding surface temperature is important for habitability. Recent work on Mars has found that the dependence of surface temperature on elevation (surface lapse rate) converges to zero in the limit of a thin CO2 atmosphere. However, the mechanisms that control the surface lapse rate are still not fully understood. It remains unclear how the surface lapse rate depends on both greenhouse effect and surface pressure. Here, we use climate models to study when and why "mountaintops are cold". We find the tropical surface lapse rate increases with the greenhouse effect and with surface pressure. The greenhouse effect dominates the surface lapse rate transition and is robust across latitudes. The pressure effect is important at low latitudes in moderately opaque atmospheres. A simple model provides insights into the mechanisms of the transition. Our results suggest that topographic cold-trapping may be important for the climate of arid planets.

astro-ph.EP↗

arXiv2023

The effect of salinity on ocean circulation and ice-ocean interaction on Enceladus

Observational data suggest that the ice shell on Enceladus is thicker at the equator than at the pole, indicating an equator-to-pole ice flow. If the ice shell is in an equilibrium state, the mass transport of the ice flow must be balanced by the freezing and melting of the ice shell, which in turn is modulated by the ocean heat transport. Here we use a numerical ocean model to study the ice-ocean interaction and ocean circulation on Enceladus with different salinities. We find that salinity fundamentally determines the ocean stratification. A stratified layer forms in the low salinity ocean, affecting the ocean circulation and heat transport. However, in the absence of tidal heating in the ice shell, the ocean heat transport is found to always be towards lower latitudes, resulting in freezing at the poles, which cannot maintain the ice shell geometry against the equator-to-pole ice flow. The simulation results suggest that either the ice shell on Enceladus is not in an equilibrium state, or tidal dissipation in the ice shell is important in maintaining the ice shell geometry. The simulations also suggest that a positive feedback between cross-equatorial ocean heat transport and ice melting results in spontaneous symmetry breaking between the two hemispheres. This feedback may play a role in the observed interhemispheric asymmetry in the ice shell.

astro-ph.EP↗

arXiv2023

Energetic constraints on ocean circulations of icy ocean worlds

Globally ice-covered oceans have been found on multiple moons in the solar system and may also have been a feature of Earth's past. However, relatively little is understood about the dynamics of these ice-covered oceans, which affect not only the physical environment but also any potential life and its detectability. A number of studies have simulated the circulation of icy-world oceans, but have come to seemingly widely different conclusions. To better understand and narrow down these diverging results, we discuss energetic constraints for the circulation on ice-covered oceans, focusing in particular on Snowball Earth, Europa, and Enceladus. Energy input that can drive ocean circulation on ice-covered bodies can be associated with heat and salt fluxes at the boundaries as well as ocean tides and librations. We show that heating from the solid core balanced by heat loss through the ice sheet can drive an ocean circulation, but the resulting flows would be relatively weak and strongly affected by rotation. Salt fluxes associated with freezing and melting at the ice sheet boundary are unlikely to energetically drive a circulation, although they can shape the large-scale circulation when combined with turbulent mixing. Ocean tides and librations may provide an energy source for such turbulence, but the magnitude of this energy source remains highly uncertain for the icy moons, which poses a major obstacle to predicting the ocean dynamics of icy worlds and remains as an important topic for future research.

astro-ph.EP↗

arXiv2022

The Effect of Ocean Salinity on Climate and Its Implications for Earth's Habitability

The influence of atmospheric composition on the climates of present-day and early Earth has been studied extensively, but the role of ocean composition has received less attention. We use the ROCKE-3D ocean-atmosphere general circulation model to investigate the response of Earth's present-day and Archean climate system to low vs. high ocean salinity. We find that saltier oceans yield warmer climates in large part due to changes in ocean dynamics. Increasing ocean salinity from 20 g/kg to 50 g/kg results in a 71% reduction in sea ice cover in our present-day Earth scenario. This same salinity change also halves the pCO$_2$ threshold at which Snowball glaciation occurs in our Archean scenarios. In combination with higher levels of greenhouse gases such as CO$_2$ and CH$_4$, a saltier ocean may allow for a warm Archean Earth with only seasonal ice at the poles despite receiving 20% less energy from the Sun.

astro-ph.EP↗

arXiv2021

Ocean Circulation on Enceladus With a High Versus Low Salinity Ocean

Previous studies that have considered the ocean circulation on Enceladus have generally assumed the salinity to be Earth-like. However, according to observations and geochemical constraints, the salinity of Enceladus' ocean is likely to be lower, and importantly, it is probably low enough to reverse the sign of thermal expansivity. We investigate the ocean circulation and stratification of Enceladus' ocean using a combination of theoretical arguments and simulations using the MITgcm. We find that, if the salinity is high, the whole ocean is unstratified, and convection dominates the entire ocean. However, if the salinity is low enough, there exists a stratified layer in the upper ocean, whose thickness depends on the magnitude of the turbulent vertical diffusivity, which remains poorly constrained. Such a layer can suppress the vertical flux of heat and tracers, thereby affecting the heat flux to the ice shell and leading to a vertical tracer mixing time scale across the stratified layer of at least hundreds of years. This time scale is inconsistent with a previous estimate of vertical ocean mixing of several years, based on the size of detected silica nanoparticles in the plumes, leading us to conclude that either the salinity of Enceladus' ocean is higher than previously suggested or the interpretation of silica nanoparticle observations has to be reconsidered.

astro-ph.EP↗

arXiv2019

No Snowball on Habitable Tidally Locked Planets with a Dynamic Ocean

Terrestrial planets orbiting within the habitable zones of M-stars are likely to become tidally locked in a 1:1 spin:orbit configuration and are prime targets for future characterization efforts. An issue of importance for the potential habitability of terrestrial planets is whether they could experience snowball events (periods of global glaciation). Previous work using an intermediate complexity atmospheric Global Climate Model (GCM) with no ocean heat transport suggested that tidally locked planets would smoothly transition to a snowball, in contrast with Earth, which has bifurcations and hysteresis in climate state associated with global glaciation. In this paper, we use a coupled ocean-atmosphere GCM (ROCKE-3D) to model tidally locked planets with no continents. We chose this configuration in order to consider a case that we expect to have high ocean heat transport. We show that including ocean heat transport does not reintroduce the snowball bifurcation. An implication of this result is that a tidally locked planet in the habitable zone is unlikely to be found in a snowball state for a geologically significant period of time.

astro-ph.EP↗

arXiv2019

Scaling Relations for Terrestrial Exoplanet Atmospheres from Baroclinic Criticality

The macroturbulent atmospheric circulation of Earth-like planets mediates their equator-to-pole heat transport. For fast-rotating terrestrial planets, baroclinic instabilities in the mid-latitudes lead to turbulent eddies that act to transport heat poleward. In this work, we derive a scaling theory for the equator-to-pole temperature contrast and bulk lapse rate of terrestrial exoplanet atmospheres. This theory is built on the work of Jansen & Ferrari (2013), and determines how unstable the atmosphere is to baroclinic instability (the baroclinic "criticality") through a balance between the baroclinic eddy heat flux and radiative heating/cooling. We compare our scaling theory to General Circulation Model (GCM) simulations and find that the theoretical predictions for equator-to-pole temperature contrast and bulk lapse rate broadly agree with GCM experiments with varying rotation rate and surface pressure throughout the baroclincally unstable regime. Our theoretical results show that baroclinic instabilities are a strong control of heat transport in the atmospheres of Earth-like exoplanets, and our scalings can be used to estimate the equator-to-pole temperature contrast and bulk lapse rate of terrestrial exoplanets. These scalings can be tested by spectroscopic retrievals and full-phase light curves of terrestrial exoplanets with future space telescopes.

astro-ph.EP↗