Stratified Region Research Articles

XMM/HST monitoring of the ultra-soft highly accreting narrow-line Seyfert 1 RBS 1332

Ultra-soft narrow-line Seyfert 1s (US-NLSy 1s) are a poorly observed class of active galactic nuclei characterised by significant flux changes and an extreme soft X-ray excess. This peculiar spectral shape represents a golden opportunity to test whether the standard framework commonly adopted for modelling local AGNs is still valid. We thus present the results of the joint XMM-Newton and HST monitoring campaign of the highly accreting US-NLSy RBS,1332. The optical-to-UV spectrum of RBS,1332 exhibits evidence for both a stratified narrow-line region and an ionised outflow that produces absorption troughs over a wide range of velocities (from ∼--1500 km s^-1 to ∼1700 km s^-1) in several high-ionisation transitions (Lyα, N V C IV ). From a spectroscopic point of view, the optical/UV/FUV/X-ray emission of this source is due to the superposition of three distinct components that are best modelled in the context of the two-coronae framework in which the radiation of RBS,1332 can be ascribed to a standard outer disc, a warm Comptonisation region, and a soft coronal continuum. The present dataset is not compatible with a pure relativistic reflection scenario. Finally, the adoption of the novel model reXcor allowed us to determine that the soft X-ray excess in RBS,1332 is dominated by the emission of the optically thick and warm Comptonising medium, and only a marginal contribution is expected from relativistic reflection from a lamppost-like corona.

Simultaneous Evolutionary Fits for Jupiter and Saturn Incorporating Fuzzy Cores

Abstract With the recent realization that there likely are stably stratified regions in the interiors of both Jupiter and Saturn, we construct new nonadiabatic, inhomogeneous evolutionary models with the same microphysics for each that result at the present time in respectable fits for all major bulk observables for both planets. These include the effective temperature, radius, atmospheric heavy-element and helium abundances (including helium rain), and the lower-order gravity moments J 2 and J 4. The models preserve from birth most of an extended “fuzzy” heavy-element core. Our predicted atmospheric helium mass fraction for Saturn is ∼0.2, close to some measured estimates but in disagreement with some published predictions. To preserve a fuzzy core from birth, the interiors of both planets must start out at lower entropies than would be used for traditional “hot start” adiabatic models, though the initial exterior mantle entropies can range from hot to warm start values. We do not see a helium ocean in Saturn’s interior, and both models have inner envelopes with significant Brunt–Väisälä frequencies; this region for Saturn at the current epoch is more extended, and in it, the Brunt is larger. The total heavy-element mass fraction in Jupiter and in Saturn is determined to be ∼14% and ∼26%, respectively, though there is some play in these determinations.

Jupiter Evolutionary Models Incorporating Stably Stratified Regions

Abstract We address the issue of which broad set of initial conditions for the planet Jupiter best matches the current presence of a “fuzzy core” of heavy elements, while at the same time comporting with measured parameters such as its effective temperature, atmospheric helium abundance, radius, and atmospheric metallicity. Our focus is on the class of fuzzy cores that can survive convective mixing to the present day and on the unique challenges of an inhomogeneous Jupiter with stably stratified regions now demanded by the Juno gravity data. Hence, using the new code APPLE, we attempt to put a nonadiabatic Jupiter into an evolutionary context. This requires not only a mass density model, the major relevant byproduct of the Juno data, but a thermal model that is subject to interior heat transport, a realistic atmospheric flux boundary, a helium rain algorithm, and the latest equation of state. The result is a good fit to most major thermal, compositional, and structural constraints that still preserve a fuzzy core and that should inform future more detailed models of the current Jupiter in the context of its evolution from birth.

Improving tropical cyclone rapid intensification forecasts with satellite measurements of sea surface salinity and calibrated machine learning

Abstract Forecasting rapid intensification (RI) of tropical cyclones (TC) is a mission known for large errors. One under-researched factor that affects TC intensification is salinity, which is important for density stratification in certain ocean regions and can affect the surface enthalpy flux under a strengthening hurricane. To investigate the impact and efficacy of using salinity information in state-of-the-art forecasting, we use a statistical model consisting of a variety of machine learning (ML) methods. For salinity data, we use satellite measurements of pre-storm sea surface salinity (SSS) as a proxy for the salinity stratification. We train and test the model on various ocean basins, including the Atlantic, northern east Pacific, northern west Pacific. A calibrator was trained on top of the ML models to correct and enhance probability forecasts. The calibrator significantly improves probability forecasts relative to recent works. The ML model performance is improved with the addition of SSS in the Eastern North Pacific, western North Pacific, and the Caribbean subregion of the North Atlantic, and the overall model performance is better than previous studies. SSS decreases model skill for a model trained on the full Atlantic basin. In the Indian Ocean, SSS is also notably correlated with RI occurrence, but the TC samples are not sufficient to train ML models.

Diatom phytochromes integrate the underwater light spectrum to sense depth.

Aquatic life is strongly structured by the distribution of light, which, besides attenuation in intensity, exhibits a continuous change in the spectrum with depth1. The extent to which these light changes are perceived by phytoplankton through photoreceptors is still inadequately known. We addressed this issue by integrating functional studies of diatom phytochrome (DPH) photoreceptors in model species2 with environmental surveys of their distribution and activity. Here, by developing an in vivo dose-response assay to light spectral variations mediated by DPH, we show that DPH can trigger photoreversible responses across the entire light spectrum, resulting in a change in DPH photoequilibrium with depth. By generating dph mutants in the diatom Thalassiosira pseudonana, we also demonstrate that under simulated low-blue-light conditions of ocean depth, DPH regulates photosynthesis acclimation, thus linking optical depth detection with a functional response. The latitudinal distribution of DPH-containing diatoms from permanently stratified regions to seasonally mixed regions suggests an adaptive value of DPH functions in coping with vertical displacements in the water column. By establishing DPH as a detector of optical depth, this study provides a new view of how information embedded in the underwater light field can be exploited by diatoms to modulate their physiology throughout the photic zone.

Velocity-resolved Reverberation Mapping of Changing-look Active Galactic Nucleus NGC 4151 during Outburst Stage. II. Results of Four Seasons of Observation

We present the results of a four-year velocity-resolved reverberation mapping (RM) campaign of the changing-look active galactic nucleus (CL-AGN) NGC 4151 during its outburst phase. By measuring the time lags of the Hα, Hβ, Hγ, He i, and He ii emission lines, we confirm a stratified broad-line region (BLR) structure that aligns with predictions from photoionization models. Intriguingly, we observed an “anti-breathing” phenomenon, where the lags of broad emission lines decreased with increasing luminosity, contrary to the typical expectation. This anomaly may be attributed to the influence of the ultraviolet-optical lag or nonvirialized motions in the BLR gas. Velocity-resolved RM and ionization mapping analyses revealed rapid and significant changes in the BLR geometry and kinematics on timescales of less than a year, which cannot be interpreted by any single mechanism, such as an inhomogeneous BLR, variations in radiation pressure, or changes in the illuminated ionizing field. Additionally, the Hβ lags of NGC 4151 and other CL-AGNs agree with the radius–luminosity relationship established for AGNs with low accretion rates, implying that the CL phenomenon is more likely driven by intrinsic changes in the accretion rate rather than obscuration. These findings provide new insights into the complex internal processes of CL-AGNs and highlight the importance of long-term, multiline RM for understanding BLR structures, geometry, and kinematics.

Multidecadal decline in sea ice meltwater volume and Pacific Winter Water salinity in the Bering Sea revealed by ocean observations

Large amounts of freshwater and nutrients pass through the Bering Strait to the Arctic Ocean, making the Bering Sea a crucial marginal sea of the North Pacific Ocean. The hydrography and biological production of the Bering Sea are strongly influenced by the amount of sea ice produced and melted. The sea ice extent and production exhibited large interannual variability but no visible trend until 2016 when a strong decrease began. However, records of sea ice before 1979 and the beginning of satellite-based estimates do not exist. In this paper, we devised a methodology using historical temperature and salinity data, supplemented by historical oxygen isotope (δ18O) data, to estimate sea ice melt and its temporal variability in the Bering Sea from 1950 onward. Our results, consistent with estimates of sea ice thickness, indicate that the sea ice melt volume has declined significantly —following lower sea ice extent and production— with a decrease between 35 and 50 km3 (from 442 km3) between pre-1980 and post-1980 climatologies. In particular, our meltwater time series reveals a decline of 160 km3 between 2012 and 2018, which also reflects the strong decrease in sea ice volume between 2016 and 2018 that numerous previous studies have highlighted. We also evaluated the change in the salinity of the Pacific Winter Water (PWW), whose formation is also related to sea ice production. The time series of PWW salinity exhibits a strong decreasing trend, with a freshening of about 0.3 between the mid-1950s and the mid-2010s, that we attribute to a combination of a reduced sea ice production and the freshening of the Alaskan Coastal Current water. The decline in meltwater volume and PWW salinity that we observed strongly influences the stratification over the Bering shelf, with a significant weakening of the stratification in coastal polynya regions, and a stronger and increasingly temperature-controlled stratification in the rest of the shelf. These changes could have adverse consequences on the biological productivity of the northern Bering Sea.

Simultaneous X-Ray and Optical Polarization Observations of the Blazar Mrk 421

We present near-simultaneous X-ray and optical polarization measurements in the high synchrotron peaked (HSP) blazar Mrk 421. The X-ray polarimetric observations were carried out using Imaging X-ray Polarimetry Explorer (IXPE) on 2023 December 6. During IXPE observations, we also carried out optical polarimetric observations using 104 cm Sampurnanand telescope at Nainital and multiband optical imaging observations using 2 m Himalayan Chandra Telescope at Hanle. From model-independent analysis of IXPE data, we detected X-ray polarization with degree of polarization (ΠX) of 8.5% ± 0.5% and an electric vector position angle (ΨX) of 10.°6 ± 1.°7 in the 2−8 keV band. From optical polarimetry on 2023 December 6, in B, V, and R bands, we found values of Π B = 4.27% ± 0.32%, Π V = 3.57% ± 0.31%, and Π R = 3.13% ± 0.25%. The value of Π B is greater than that observed at longer optical wavelengths, with the degree of polarization suggesting an energy-dependent trend, gradually decreasing from higher to lower energies. This is consistent with that seen in other HSP blazars and favors a stratified emission region encompassing a shock front. The emission happening in the vicinity of the shock front will be more polarized due to the ordered magnetic field resulting from shock compression. The X-ray emission, involving high-energy electrons, originates closer to the shock front than the optical emission. The difference in the spatial extension could plausibly account for the observed variation in polarization between X-ray and optical wavelengths. This hypothesis is further supported by the broadband spectral energy distribution modeling of the X-ray and optical data.

Spatial and Seasonal Controls on Eddy Subduction in the Southern Ocean

AbstractCarbon export driven by submesoscale, eddy‐associated vertical velocities (“eddy subduction”), and particularly its seasonality, remains understudied, leaving a gap in our understanding of ocean carbon sequestration. Here, we assess mechanisms controlling eddy subduction's spatial and seasonal patterns using 15 years of observations from BGC‐Argo floats in the Southern Ocean. We identify signatures of eddy subduction as subsurface anomalies in temperature‐salinity and oxygen. The anomalies are spatially concentrated near weakly stratified areas and regions with strong lateral buoyancy gradients diagnosed from satellite altimetry, particularly in the Antarctic Circumpolar Current's standing meanders. We use bio‐optical ratios, specifically the chlorophyll a to particulate backscatter ratio (Chl/bbp) to find that eddy subduction is most active in the spring and early summer, with freshly exported material associated with seasonally weak vertical stratification and increasing surface biomass. Climate change is increasing ocean stratification globally, which may weaken eddy subduction's carbon export potential.

Local stability of differential rotation in magnetized radiation zones and the solar tachocline

ABSTRACT We study local magnetohydrodynamical instabilities of differential rotation in magnetized, stably stratified regions of stars and planets using a Cartesian Boussinesq model. We consider arbitrary latitudes and general shears (with gravity direction misaligned from this by an angle $\phi$), to model radial ($\phi =0$), latitudinal ($\phi =\pm 90^\circ$), and mixed differential rotations, and study both non-diffusive [including magnetorotational instability (MRI) and Solberg–Høiland instability] and diffusive instabilities [including Goldreich–Schubert–Fricke (GSF) and MRI with diffusion]. These instabilities could drive turbulent transport and mixing in radiative regions, including the solar tachocline and the cores of red giant stars, but their dynamics are incompletely understood. We revisit linear axisymmetric instabilities with and without diffusion and analyse their properties in the presence of magnetic fields, including deriving stability criteria and computing growth rates, wave vectors, and energetics, both analytically and numerically. We present a more comprehensive analysis of axisymmetric local instabilities than prior work, exploring arbitrary differential rotations and diffusive processes. The presence of a magnetic field leads to stability criteria depending upon angular velocity rather than angular momentum gradients. We find MRI operates for much weaker differential rotations than the hydrodynamic GSF instability, and that it typically prefers much larger length-scales, while the GSF instability is impeded by realistic strength magnetic fields. We anticipate MRI to be more important for turbulent transport in the solar tachocline than the GSF instability when $\phi \gt 0$ in the Northern (and vice versa in the Southern) hemisphere, though the latter could operate just below the convection zone when MRI is absent for $\phi \lt 0$.

Seasonal forecasting of the European North-West shelf seas: limits of winter and summer sea surface temperature predictability

The European North-West shelf seas (NWS) support economic interests and provide environmental services to adjacent countries. Expansion of offshore activities, such as renewable energy infrastructure, aquaculture, and growth of international shipping, will place increasingly complex demands on the marine environment over the coming decades. Skilful forecasting of NWS properties on seasonal timescales will help to effectively manage these activities. Here we quantify the skill of an operational large-ensemble ocean-atmosphere coupled global forecasting system (GloSea), as well as benchmark persistence forecasts, for predictions of NWS sea surface temperature (SST) at 2–4 months lead time in winter and summer. We identify sources of and limits to SST predictability, considering what additional skill may be available in the future. We find that GloSea NWS SST skill is generally high in winter and low in summer. GloSea outperforms simple persistence forecasts by adding information about atmospheric variability, but only to a modest extent as persistence of anomalies in the initial conditions contributes substantially to predictability. Where persistence is low – for example in seasonally stratified regions – GloSea forecasts show lower skill. GloSea skill can be degraded by model deficiencies in the relatively coarse global ocean component, which lacks dynamic tides and subsequently fails to robustly represent local circulation and mixing. However, “atmospheric mode matched” tests show potential for improving prediction skill of currently low performing regions if atmospheric circulation forecasts can be improved. This underlines the importance of coupled atmosphere-ocean model development for NWS seasonal forecasting applications.

X-ray Polarization of Blazars and Radio Galaxies Measured by the Imaging X-ray Polarimetry Explorer

X-ray polarization, which now can be measured by the Imaging X-ray Polarimetry Explorer (IXPE), is a new probe of jets in the supermassive black hole systems of active galactic nuclei (AGNs). Here, we summarize IXPE observations of radio-loud AGNs that have been published thus far. Blazars with synchrotron spectral energy distributions (SEDs) that peak at X-ray energies are routinely detected. The degree of X-ray polarization is considerably higher than at longer wavelengths. This is readily explained by energy stratification of the emission regions when electrons lose energy via radiation as they propagate away from the sites of particle acceleration as predicted in shock models. However, the 2–8 keV polarization electric vector is not always aligned with the jet direction as one would expect unless the shock is oblique. Magnetic reconnection may provide an alternative explanation. The rotation of the polarization vector in Mrk421 suggests the presence of a helical magnetic field in the jet. In blazars with lower-frequency peaks and the radio galaxy Centaurus A, the non-detection of X-ray polarization by IXPE constrains the X-ray emission mechanism.

APPLE: An Evolution Code for Modeling Giant Planets

We introduce APPLE, a novel planetary evolution code designed specifically for the study of giant exoplanet and Jovian planet evolution in the era of Galileo, Juno, and Cassini. With APPLE, state-of-the-art equations of state for hydrogen, helium, ice, and rock are integrated with advanced features to treat ice/rock cores and metals in the gaseous envelope; models for helium rain and hydrogen/helium immiscibility; detailed atmosphere boundary tables that also provide self-consistent albedos and spectra; and options to address envelope metal gradients and stably stratified regions. Our hope is that these purpose-built features of APPLE will help catalyze the development of the next generation of giant exoplanet and Jovian planet evolutionary models.

Oscillatory onset of rotating thermal convection subject to spatially varying magnetic fields and stable stratification

Convective fluid motions in deep planetary cores induce spatially and temporally varying magnetic fields observable as secular variation. Oscillatory instabilities occurring as onset modes in rapidly rotating thermal convection influenced by heterogeneous magnetic fields and stratification, investigated in this study, can be potentially linked to such observations. A plane layer approximation to near-polar and near-equatorial regions of the Earth's outer core is considered. In addition to penetrative convection and flow suppression effects, the simultaneous interaction of convective instabilities with asymmetrical magnetic fields and stable stratification is the focus of this study. The fundamental modes of magnetoconvection onset are preserved even under stable stratification, although the convection threshold is lowered irrespective of the parameter regime. The axial invariance of rapidly rotating columnar convection loses its midplane symmetry with intense localization of kinetic energy in the unstably stratified regions. The parameter regimes supporting the onset of the magnetically modified viscous oscillatory (mVO) modes are further extended toward Earthlike conditions such as high thermal conductivity (low Prandlt number, Pr) and magnetic dissipation (low Roberts number, q). Moreover, compared to fully unstable stratification, partial stable stratification enhances the range of imposed magnetic field length scale (δ) over which the onset of mVO modes is favored. The critical field length scale (δ⋆), above which the onset regime of the mVO modes is bounded by a minimum q, is determined. Irrespective of the rotation rate, below δ=δ⋆, the onset of the mVO mode is supported for asymptotically small q for Pr values close to the Earth's outer core.

The benthic nepheloid layer in the offshore waters of the Great Lakes and its post-dreissenid disappearance

Prior to the appearance of Dreissena, pronounced benthic nepheloid layers (BNL) near the bottom characterized by elevated levels of both turbidity and total phosphorus (TP), were a consistent and extensive feature of the offshore, stratified waters of all the Laurentian Great Lakes, except Lake Superior. In recent (2010–2019) years, the BNL has disappeared from all areas except for central Lake Erie, where only a small decrease in bottom turbidity has occurred. All stratified regions which exhibited a pre-Dreissena BNL, including central Lake Erie, experienced substantial post-Dreissena reductions in near-bottom TP, although the forms of phosphorus (particulate or soluble) responsible for these reductions have varied from lake to lake. Notably, the arrival of Dreissena at offshore sites was not accompanied by an increase in soluble phosphorus. Initiation of changes in the BNL almost invariably preceded appearance of Dreissena in the offshore, suggesting both that dreissenid impacts on the reductions in the BNL were largely remote, and by extension that the source of the BNL was also at least in part remote. Previous researchers’ estimates of the importance of the benthic pool of phosphorus to offshore water column concentration suggest that the post-invasion reductions in bottom phosphorus during the stratified season could be contributing to the offshore oligotrophication of the lakes.

Analysis of nitrogen migration and transformation in the typical deep and large reservoir of the Lancang River — Evidence from nitrogen and oxygen isotopes

The environmental effect resulting from dam construction and the formation of large reservoirs has long been a topic of significant concern. While current research at a macro scale provides a well understanding regarding the evolution of nitrogen nutrient status in river-reservoir ecosystems, the vertical migration and transformation characteristics of nitrogen in reservoirs under changing runoff remain less defined. This study centers on a typical deep reservoir along the Lancang River-Nuozhadu (NZD) Reservoir in April and December. The monitoring reveals a distinct vertical stratification phenomenon in the NZD Reservoir in the vertical profile in April, which can be categorized into a mixed layer (ML) (0–5 m), a thermocline layer (TL) (5–40 m), and an isothermal layer (IL) (>40 m) based on the water temperature. However, little stratification in the profile is occurred in December. The results of nitrogen and oxygen isotopes in April indicate that the biochemical process of nitrogen in the NZD Reservoir is predominantly driven by nitrification. However, different nitrogen biochemical processes are coupled in various stratified regions in the vertical direction. In the ML, simultaneous assimilation of ammonia nitrogen by phytoplankton occurs. In the TL, there is a process of oxidation and decomposition of particulate organic nitrogen due to a significant stratification effect and an extended hydraulic retention time. In the region of minimal dissolved oxygen (30–40 m), simultaneous denitrification may take place. In December, the main process is nitrogen assimilation in reservoir surface (ML). Statistical analysis of environmental factors highlights water temperature (T) and dissolved oxygen (DO) as crucial environmental factors influencing nitrogen migration and transformation within the reservoir. This study suggests that in deep reservoirs experiencing vertical thermal stratification, the characteristics of DO and nitrogen migration and transformation exhibit significant vertical spatial heterogeneity. This phenomenon may lead to diverse ecological environmental effects. Future efforts should focus on enhancing the fine monitoring of stratified water bodies in deep and large reservoirs and analyzing the ecological environmental effects of material cycling in greater detail.

Effects of stratification on overshooting and waves atop the convective core of M⊙ main-sequence stars

ABSTRACT As a massive star evolves along the main sequence, its core contracts, leaving behind a stable stratification in helium. We simulate two-dimensional convection in the core at three different stages of evolution of a $5\,\mathrm{ M}_{\odot }$ star, with three different stratifications in helium atop the core. We study the propagation of internal gravity waves in the stably stratified envelope, along with the overshooting length of convective plumes above the convective boundary. We find that the stratification in helium in evolved stars hinders radial motions and effectively shields the radiative envelope against plume penetration. This prevents convective overshooting from being an efficient mixing process in the radiative envelope. In addition, internal gravity waves are less excited in evolved models compared to the zero-age-main-sequence model, and are also more damped in the stratified region above the core. As a result, the wave power is several orders of magnitude lower in mid- and terminal-main-sequence models compared to zero-age-main-sequence stars.

On the Penetration of Large-scale Flows into Stellar Radiative Zones

The propagation of meridional circulation below the base of the convection zone (CZ) of low-mass stars may play a crucial role in the transport of angular momentum and also significantly contribute to the transport of chemical species and magnetic fields within their stable radiative zone (RZ). We systematically study these large-scale mean flows by performing three-dimensional global numerical simulations in a spherical shell that consists of a convective region overlying a stably stratified region. We find that the meridional flows can penetrate distances as large as ∼0.21r o (where r o is the outer radius) below the base of the CZ, provided that the Eddington–Sweet timescale t ES is much shorter than the viscous timescale t ν , as measured by the parameter σ=(tES/tν)1/2 . In the solar-like regime, where σ ≲ 1 in the upper RZ, we find that the angular momentum transport in the deep RZ is determined primarily by the action of the Coriolis force on meridional flows. In contrast, in models run in the σ > 1 regime, the meridional flows become weaker and the viscous effects dominate. We find that the penetration lengthscale δ MC of these mean flows when σ ≲ 1 is proportional to σ −0.22. Our findings may provide a better understanding of the role of the meridional flows in the dynamics of the solar interior and inform future numerical studies that are focused on capturing solar-like dynamics self-consistently.

Surface Heat Fluxes Drive a Two‐Phase Response in Southern Ocean Mode Water Stratification

AbstractSubantarctic mode waters have low stratification and are formed through subduction from thick winter mixed layers in the Southern Ocean. To investigate how surface forcing affects the stratification in mode water formation regions in the Southern Ocean, a set of adjoint sensitivity experiments are conducted. The objective function is the annual‐average stratification over the mode water formation region, which is evaluated from potential temperature and salinity adjoint sensitivity experiments. The analysis of impacts, from the product of sensitivities and forcing variability, identifies the separate effects of the wind stress, heat flux, and freshwater flux, revealing that the dominant control on stratification is from surface heat fluxes, as well as a smaller effect from zonal wind stress. The adjoint sensitivities of stratification to surface heat flux reveal a surprising change in sign over 2 years lead time: surface cooling leads to the expected initial local decrease in stratification, but there is a delayed response leading to an increase in stratification. This delayed response in stratification involves effective atmospheric damping of the surface thermal contribution, so that eventually the oppositely‐signed advective haline contribution dominates. This two‐phase response of stratification is found to hold over mode water formation regions in the South Indian and Southeast Pacific sectors of the Southern Ocean, where there are strong advective flows linked to the Antarctic Circumpolar Current.

On the impact of net-zero forcing Q-flux change

Numerical climate model simulations suggest that global warming is enhanced or hampered by the spatial pattern of the warming itself. This phenomenon is known as the “pattern effect” and has in recent years become the most promising explanation for the change over time of climate sensitivity in climate models. Under historical global warming, different patterns of surface-temperature change have emerged, notably a yet unexplained cooling in the Southern Ocean and the East Pacific. Historical climate model simulations notoriously fail to reproduce this cooling, which may contribute to the deviation of the simulated global-mean warming from the observed record. Here we qualitatively investigate the potential impact of historical and other surface-temperature pattern changes by changing the ocean heat transport convergence (Q-flux) in a slab-ocean model. The Q-flux changes are always implemented such that in the global mean they impose no net forcing. Consistent with earlier studies we find that the impact of a negative Q-flux change in the Southern Ocean has a stronger effect than in other regions because of a feedback loop between sea-surface temperatures (SSTs) and clouds in the Southern Ocean and the stably stratified regions in the tropics. The SST-cloud feedback loop facilitates the expansion of the Antarctic sea ice, indeed taking the model into a Snowball-Earth state. The intensity of this effect is found to be model dependent, especially due to differences in the cloud parametrisation. In experiments with deactivated sea ice the impact of the negative Q-flux change is much weaker.