arXiv subjects

Eli Tziperman

Publications and source records attributed to Eli Tziperman.

At least 19 recordsLinked to original sources

arXiv2025

Using a rare event sampling technique to quantify extreme El Ni\~no event statistics

Extreme El Ni\~no events, such as occurred in 1997--1998, can induce severe weather on a global scale, with significant socioeconomic impacts that motivate efforts to understand them better. However, extreme El Ni\~no events are rare, and even in a very long direct numerical simulation (DNS) occur too infrequently for robust statistical characterization. This study seeks to generate extreme El Ni\~no event model data at a lower cost, while preserving statistical fidelity, using a rare event sampling technique, which preferentially devotes computational resources toward extreme events by generating a large, branched ensemble of interrelated trajectories through successive targeted perturbations. We specifically use the ``trying-early adaptive multi-level splitting'' (TEAMS) algorithm, which is well-suited for El Ni\~no's relative timescales of predictability and event duration. We apply TEAMS to the Zebiak-Cane model, an intermediate-complexity ENSO model for which it is feasible to run a long DNS (500000 years) for validation. We compare extreme El Ni\~no event return time estimates from TEAMS to those from the long DNS to assess TEAMS' accuracy and efficiency. We find that TEAMS accurately reproduces the return time estimates of the DNS at about one fifth the computational cost. Therefore, TEAMS is an efficient approach to study rare ENSO events that can be plausibly applied to full-complexity climate models.

physics.ao-ph↗

arXiv2025

Climate change alters teleconnections

Internal modes of climate variability, such as El Ni\~no and the North Atlantic Oscillation, can have strong influences upon distant weather patterns, effects that are referred to as "teleconnections". The extent to which anthropogenic climate change has and will continue to affect these teleconnections, however, remains uncertain. Here, we employ a covariance fingerprinting approach to demonstrate that shifts in teleconnection patterns affecting monthly temperatures between the periods 1960-1990 and 1990-2020 are attributable to anthropogenic forcing. We further apply multilinear regression to assess the regional contributions and statistical significance of changes in five key climate modes: the El Ni\~no-Southern Oscillation, North Atlantic Oscillation, Southern Annular Mode, Indian Ocean Dipole, and the Pacific Decadal Oscillation. In many regions, observed changes exceed what would be expected from natural variability alone, further implicating an anthropogenic influence. Finally, we provide projections of how these teleconnections will alter in response to further changes in climate.

physics.ao-ph↗

arXiv2025

An improved noise model for representing westerly wind bursts in the recharge oscillator model of ENSO

Westerly wind bursts (WWBs) have long been known to have a major impact on the development of El Ni\~no events. In particular, they amplify these events, with stronger events associated with a higher number and stronger WWBs. We consider here a noise-driven recharge oscillator model of ENSO. Commonly, WWBs are represented by a state-dependent Gaussian noise that naturally reproduces the amplification of warm events. However, we show that many properties of WWBs and their effects on sea surface temperature (SST) are better captured by a conditional additive and multiplicative (CAM) noise, which presents a promising alternative to represent WWBs. In addition to recovering the sporadic nature of WWBs, CAM noise leads to an asymmetry between El Ni\~no and La Ni\~na events without the need for deterministic nonlinearities. Furthermore, CAM noise generates SST dynamics with a higher frequency of WWBs accompanying the largest events. This suggests that extreme warm events are better modelled by CAM noise. To cover the full spectrum of warm events, we propose a conditional noise model in which the wind stress is modelled by additive Gaussian noise for sufficiently small SSTs and by additive CAM noise once the SST exceeds a certain threshold. We show that this conditional noise model captures observed bulk statistical properties of ENSO equally well as the commonly used multiplicative Gaussian red noise model, but additionally better reproduces dynamical signatures such as the increased number of WWBs preceding large El Ni\~no events.

physics.ao-ph↗

arXiv2025

From gas to ice giants: A unified mechanism for equatorial jets

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.

astro-ph.EP↗

arXiv2025

Is ENSO a damped or a self-sustained oscillation?

The recharge oscillator (RO) model has been successfully used to understand different aspects of the El Niño-Southern Oscillation (ENSO). Fitting the RO to observations and climate model simulations consistently suggests that ENSO is a damped oscillator whose variability is sustained and made irregular by external weather noise. We investigate the methods that have been used to estimate the growth rate of ENSO by applying them to simulations of both damped and self-sustained RO regimes. We find that fitting a linear RO leads to parameters that imply a damped oscillator even when the fitted data were produced by a model that is self-sustained. Fitting a nonlinear RO also leads to a significant bias toward a damped regime. As such, it seems challenging to conclude whether ENSO is a damped or a self-sustained oscillation by fitting such models to observations, and the possibility that ENSO is self-sustained cannot be ruled out.

physics.ao-ph↗

arXiv2025

On the attribution of weather events to climate change using a fit to extreme value distributions

Changes in extreme weather events are a potentially important aspect of anthropogenic climate change (ACC), yet, are difficult to attribute to ACC because the record length is often similar to, or shorter than, extreme-event return periods. This study is motivated by the ``World Weather Attribution'' initiative (WWA) and, specifically, their approach of fitting extreme value distribution functions to local observations. They calculate the dependence of distribution parameters on global mean surface temperature (GMST) and use this dependence to attribute extreme events to ACC. Applying this method to preindustrial climate simulations with no time-varying greenhouse gas forcing, we still find a strong dependence of distribution parameters on GMST. This dependence results from internal climate variability (e.g., ENSO) affecting both extreme events and GMST. Therefore, dependence on GMST does not necessarily imply an effect of ACC on extremes. We further consider whether an extreme value, normal, or log-normal distribution better represents the data; if a GMST-dependence of distribution parameters is justified using a likelihood ratio test; and if a meaningful attribution is possible given uncertainties in GMST dependence. We find, for example, that an attribution of Australia's 2020--2021 Bushfires to ACC is difficult due to the effects of internal variability. For the 2019--2021 drought in Madagascar we find that the small number of available data points precludes a meaningful attribution analysis. Overall, we find that the effects of internal climate variability on GMST and the uncertain relationship between GMST and regional extremes may lead to inaccurate attribution conclusions using the part of the WWA approach examined here.

physics.ao-ph↗

arXiv2023

The upwelling source depth distribution and its response to wind stress and stratification

Coastal upwelling, driven by alongshore winds and characterized by cold sea surface temperatures and high upper-ocean nutrient content, is an important physical process sustaining some of the oceans' most productive ecosystems. To fully understand the ocean properties in eastern boundary upwelling systems, it is important to consider the depth of the source waters being upwelled, as it affects both the SST and the transport of nutrients toward the surface. Here, we construct an upwelling source depth distribution for parcels at the surface in the upwelling zone. We do so using passive tracers forced at the domain boundary for every model depth level to quantify their contributions to the upwelled waters. We test the dependence of this distribution on the strength of the wind stress and stratification using high-resolution regional ocean simulations of an idealized coastal upwelling system. We also present an efficient method for estimating the mean upwelling source depth. Furthermore, we show that the standard deviation of the upwelling source depth distribution increases with increasing wind stress and decreases with increasing stratification. These results can be applied to better understand and predict how coastal upwelling sites and their surface properties have and will change in past and future climates.

physics.ao-ph↗

arXiv2023

Non-synchronous rotation on Europa driven by ocean currents

It has been suggested that the ice shell of Jupiter's moon Europa may drift non-synchronously due to tidal torques. Here we argue that torques applied by the underlying ocean are also important and can result in non-synchronous rotation (NSR). The resulting spin rate can be slightly slower than the synchronous angular rate that would have kept the same point of the ice shell facing Jupiter. We develop an ice shell rotation model, driven by ocean stress calculated using a high-resolution state-of-the-art ocean general circulation model, and take into account the viscoelastic deformation of the ice shell. We use the ice shell model results together with observed limits on the ice shell drift speed to constrain ice shell parameters such as effective viscosity, which is currently uncertain by at least four orders of magnitude. Our results suggest, at best, sluggish ice shell convection. Depending on the relaxation time scale of the ice shell and on the ocean currents, the ice shell may exhibit negligible drift, constant drift, or oscillatory drift superimposed on random fluctuations. The expected rotation rate exceeds $\sim$30~m/yr; future spacecraft observations can be used to test these predictions and yield insight into the properties of the ice shell and underlying ocean.

astro-ph.EP↗

arXiv2022

An approach for projecting the timing of abrupt winter Arctic sea ice loss

Abrupt and irreversible winter Arctic sea-ice loss may occur under anthropogenic warming due to the collapse of a sea-ice equilibrium at a threshold value of CO$_2$, commonly referred to as a tipping point. Previous work has been unable to conclusively identify whether a tipping point in Arctic sea ice exists because fully-coupled climate models are too computationally expensive to run to equilibrium for many CO$_2$ values. Here, we explore the deviation of sea ice from its equilibrium state under realistic rates of CO$_2$ increase to demonstrate how a few time-dependent CO$_2$ experiments can be used to predict the existence and timing of sea-ice tipping points without running the model to steady-state. This study highlights the inefficacy of using a single experiment with slow-changing CO$_2$ to discover changes in the sea-ice steady-state, and provides an alternate method that can be developed for the identification of tipping points in realistic climate models.

physics.ao-ph↗

arXiv2022

Exploring subtropical stratocumulus multiple equilibria using a mixed-layer model

Stratocumulus clouds cover about a fifth of Earths surface, and due to their albedo and low-latitude location, they have a strong effect on Earths radiation budget. Previous studies using Large Eddy Simulations have shown that multiple equilibria (both cloud-covered and cloud-free states) exist as a function of fixed-SST, with relevance to equatorward advected air masses. Multiple equilibria have also been found as a function of atmospheric CO2, with a subtropical SST nearly 10 K higher in the cloud-free state, and with suggested relevance to warm-climate dynamics. In this study, we use a mixed-layer model with an added surface energy balance and the ability to simulate both the cloud-covered and cloud-free states to study both types of multiple equilibria and the corresponding hysteresis. The model's simplicity and computational efficiency allow us to explore the mechanisms critical to the stratocumulus cloud instability and hysteresis as well as isolate key processes that allow for multiple equilibria via mechanism denial experiments not possible with a full-complexity model. For the hysteresis in fixed-SST, we find that decoupling can occur due to either enhanced entrainment warming or a reduction in cloud-top longwave cooling. The critical SST at which decoupling occurs is highly sensitive to precipitation and entrainment parameterizations. In the CO2 hysteresis, decoupling occurs in the simple model used even without the inclusion of an active SST and SST-cloud cover feedbacks, and the width of the hysteresis displays the same sensitivities as the fixed-SST case. Overall, the simple model analysis and results motivate further studies using higher complexity models.

physics.ao-ph↗

arXiv2022

Testing the consistency of the resonant wave interaction approximation with simulated dynamics of idealized 2D internal wave fields

Nonlinear interaction and breaking of internal ocean waves are responsible for much of the interior ocean mixing, affecting ocean carbon storage and the global overturning circulation. These interactions may affect the observed Garrett-Munk wave energy spectrum, in addition to the recently explored interaction of waves with ocean eddies. According to the resonant wave interaction approximation, that is commonly used to derive the kinetic equation for the energy spectrum, the dominant interactions are between wave triads whose wavevectors satisfy $\mathbf{k}=\mathbf{p}+\mathbf{q}$, and whose frequencies satisfy $\omega_{\mathbf{k}}=|\omega_{\mathbf{p}}\pm\omega_{\mathbf{q}}|$. In order to test the validity of the resonant wave interaction approximation, we examine several analytical derivations of the theory. The assumptions underlying each derivation are tested using idealized direct 2D numerical simulations, representing near-observed energy levels of the oceanic internal wave field. We show that the slow-amplitude assumptions underlying the derivations are inconsistent with the simulated dynamics in this particular set of simulations. In addition, most of the triads satisfying the resonant conditions do not contribute significantly to nonlinear wave energy transfer in our simulations, while some interactions that are dominant in nonlinear energy transfers do not satisfy the resonance conditions. We also point to possible self-consistency issues with some derivations found in the literature.

nlin.CD↗

arXiv2021

Dynamic Europa ocean shows transient Taylor columns and convection driven by ice melting and salinity

The deep (~100 km) ocean of Europa, Jupiter's moon, covered by a thick icy shell, is one of the most probable places in the solar system to find extraterrestrial life. Yet, its ocean dynamics and its interaction with the ice cover have received little attention. Previous studies suggested that Europa's ocean is turbulent using a global model and taking into account non-hydrostatic effects and the full Coriolis force. Here we add critical elements, including consistent top and bottom heating boundary conditions and the effects of icy shell melting and freezing on ocean salinity. We find weak stratification that is dominated by salinity variations. The ocean exhibits strong transient convection, eddies, and zonal jets. Transient motions organize in Taylor columns parallel to Europa's axis of rotation, are static outside of the tangent cylinder and propagate equatorward within the cylinder. The meridional oceanic heat transport is intense enough to result in a nearly uniform ice thickness, that is expected to be observable in future missions.

astro-ph.EP↗

arXiv2021

Mid-depth Ocean Stratification: Southern Ocean eddies vs interior vertical diffusivity

The mid-depth ocean stratification was fitted by Munk (1966) to an exponential profile and shown to be consistent with a vertical advective-diffusive balance. However, tracer release experiments show that vertical diffusivity in the mid-depth ocean is an order of magnitude too small to explain the observed 1 km exponential scale. Alternative mechanisms suggested that the overturning is mostly adiabatic, that interior diapycnal upwelling is negligible, and that nearly all mid-depth water upwells adiabatically in the Southern Ocean (SO). In this picture, SO eddies and wind set isopycnal slopes in the SO and therefore determine a non-vanishing mid-depth interior stratification even in the adiabatic limit. The effect of SO eddies on SO isopycnal slopes can be understood via either a marginal criticality condition or via a near-vanishing residual overturning conditions in the adiabatic limit. We examine the role of SO eddies vs interior mixing in setting the mid-depth stratification by using eddy-permitting numerical simulations, in which we artificially change the diapycnal mixing only away from the SO. We find that SO isopycnal slopes change in response to changes of the interior diapycnal mixing even when the wind forcing is constant, consistent with previous studies. However, in the limit of small interior mixing, the interior stratification is far from exponential, suggesting that SO processes alone do not lead to the observed stratification. The results suggest that while SO eddies contribute to the non-vanishing mid-depth interior stratification, the exponential shape of the stratification must also involve interior diapycnal mixing. Both SO eddies and interior diapycnal mixing are therefore important in determining the interior mid-depth exponential stratification.

physics.ao-ph↗

arXiv2019

Reconciling the Observed Mid-Depth Exponential Ocean Stratification with Weak Interior Mixing and Southern Ocean Dynamics via Boundary-Intensified Mixing

Munk (1966) showed that the deep (1000-3000 m) vertical temperature profile is consistent with a one-dimensional vertical advection-diffusion balance, with a constant upwelling and an interior diapycnal diffusivity of $\mathcal{O}(10^{-4})$ m$^{2}$ s$^{-1}$. However, typical observed diffusivities in the interior are $\mathcal{O}(10^{-5})$ m$^{2}$ s$^{-1}$. Recent work suggested that the deep stratification is set by Southern Ocean (SO) isopycnal slopes, fixed by SO eddies, that communicate the surface outcrop positions to the deep ocean. It is shown here, using an idealized ocean general circulation model, that SO eddies alone cannot lead to the observed exponential temperature profile, and that interior mixing must contribute. Strong diapycnal mixing concentrated near the ocean boundaries is shown to be balanced locally by upwelling. A one-dimensional Munk-like balance in these boundary mixing areas, although with much larger mixing and upwelling, leads to an exponential deep temperature stratification, which propagates via isopycnal mixing to the ocean interior. The exponential profile is robust to vertical variations in the vertical velocity, and persists despite the observed weak interior diapycnal mixing. Southern Ocean eddies link the surface water mass transformation by air-sea fluxes with the deep stratification, but the eddies do not determine the stratification itself. These results reconcile the observed exponential interior deep temperature stratification, the weak diapycnal diffusivity observed in tracer release experiments, and the role of Southern Ocean dynamics.

physics.ao-ph↗

arXiv2019

Tropical and Extratropical General Circulation with a Meridional Reversed Temperature Gradient as Expected in a High Obliquity Planet

Planets with high obliquity receive more radiation in the polar regions than at low latitudes, and thus, assuming an ocean-covered surface with sufficiently high heat capacity, their meridional temperature gradient was shown to be reversed for the entire year. The objective of this work is to investigate the drastically different general circulation of such planets, with an emphasis on the tropical Hadley circulation and the mid-latitude baroclinic eddy structure. We use a 3D dry dynamic core model, accompanied by an eddy-free configuration and a generalized 2D Eady model. When the meridional temperature gradient $T_y$ is reversed, the Hadley cell remains in the same direction, because the surface wind pattern and hence the associated meridional Ekman transport are not changed, as required by the baroclinic eddy momentum transport. The Hadley cell under reversed $T_y$ also becomes much shallower and weaker, even when the magnitude of the gradient is the same as in the normal case. The shallowness is due to the bottom-heavy structure of the baroclinic eddies in the reverse case, and the weakness is due to the weak wave activity. We propose a new mechanism to explain the mid-latitude eddy structure for both cases, and verify it using the generalized Eady model. With seasonal variations included, the annual mean circulation resembles that under perpetual annual mean setup. Approaching the solstices, a strong cross-equator Hadley cell forms in both cases, and about 2/3 of the Hadley circulation is driven by eddies, as shown by eddy-free simulations and using a decomposition of the Hadley cell.

astro-ph.EP↗

arXiv2017

Exploring the Nonlinear Cloud and Rain Equation

Marine stratocumulus cloud decks are regarded as the reflectors of the climate system, returning back to space a significant part of the income solar radiation, thus cooling the atmosphere. Such clouds can exist in two stable modes, open and closed cells, for a wide range of environmental conditions. This emergent behavior of the system, and its sensitivity to aerosol and environmental properties, is captured by a set of nonlinear equations. Here, using linear stability analysis, we express the transition from steady to a limit-cycle state analytically, showing how it depends on the model parameters. We show that the control of the droplet concentration (N) the environmental carrying-capacity (H0) and the cloud recovery parameter (tau) can be linked by a single nondimensional parameter mu=N/(alfa*tau*H0), suggesting that for deeper clouds the transition from open (oscillating) to closed (stable fixed point) cells will occur for higher droplet concentration (i.e. higher aerosol loading). The analytical calculations of the possible states, and how they are affected by changes in aerosol and the environmental variables, provide an enhanced understanding of the complex interactions of clouds and rain.

physics.ao-ph↗

arXiv2016

A full, self-consistent, treatment of thermal wind balance on oblate fluid planets

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 2016-17. 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 analyzing baroclinic wind structures, but have made simplifying assumptions. Those 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 effects but not 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 analyzed, and the results are compared in the barotropic limit to 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 and none of these terms is dominant, so any approximation attempting to improve over the simplified thermal wind approach needs to include all other terms.

astro-ph.EP↗

arXiv2010

Excitation of slow MJO-like Kelvin waves in the equatorial atmosphere by Yanai wave-group via WISHE-induced convection

The intraseasonal Madden-Julian oscillation (MJO) involves a slow eastward-propagating signal in the tropical atmosphere which significantly influences climate yet is not well understood despite significant theoretical and observational progress. We study the atmosphere's response to nonlinear "Wind Induced Surface Heat Exchange" (WISHE) forcing in the tropics using a simple shallow water atmospheric model. The model produces an interestingly rich interannual behavior including a slow, eastward propagating equatorial westerly multiscale signal, not consistent with any free linear waves, and with MJO-like characteristics. It is shown that the slow signal is due to a Kelvin wave forced by WISHE due to the meridional wind induced by a Yanai wave group. The forced Kelvin wave has a velocity similar to the group velocity of the Yanai waves, allowing the two to interact nonlinearly via the WISHE term while slowly propagating eastward. These results may have implications for observed tropical WISHE-related atmospheric intraseasonal phenomena.

physics.ao-ph↗