The Evolving Role of External Forcing in North Atlantic SST Variability Over the Last Millennium

Atlantic Multidecadal Variability (AMV) impacts temperature, precipitation, and extreme events on both sides of the Atlantic basin. Previous studies with climate models have suggested that when external radiative forcing is held constant, the large-scale ocean and atmosphere circulation are associated with sea-surface temperature anomalies that have similar characteristics to the observed AMV. However, there is an active debate as to whether these internal fluctuations driven by coupled atmosphere-ocean variability remain influential to the AMV on multidecadal timescales in our modern, anthropogenically-forced climate. Here we provide evidence from multiple large ensembles of climate models, paleo reconstructions, and instrumental observations of a growing role for external forcing in the AMV. Prior to 1850, external forcing, primarily from volcanoes, explains about one third of AMV variance. Between 1850 and 1950, there is a transitional period, where external forcing explains half of AMV variance, but volcanic forcing only accounts for about 10% of that. After 1950, external forcing explains three quarters of AMV variance. That is, the role for external forcing in the AMV grows as the variations in external forcing grow, even if the forcing is from different sources. When forcing is relatively stable, as in earlier modeling studies, a higher percentage of AMV variations are internally generated.

Low-Temperature Hydrothermal Systems Response to Rainfall Forcing: An Example From Temperature Time Series of Fumaroles at La Soufrière de Guadeloupe Volcano

Volcanoes with highly-developed and shallow hydrothermal systems may be subject to sudden increases of their surface steam emission at vents in response to either deep forcing (e.g. increase of heat flux coming from the magma chamber) or external forcing (e.g. sudden decrease of atmospheric pressure or variation of meteoric water input). Because the vent plumbing has a limited heat and mass transfer capacity, the rise of steam pressure accompanying the increase of flux may destabilize the system in order to augment its net transfer capacity. This reorganization may, for instance, take the form of an enlargement of existing conduits and vents or to the creation of new ones. In such a case, local and extremely dangerous blast phenomena are likely to occur with devastating consequences several hundreds of meters around. Even volcanoes with a moderate activity and considered safe by the local population are exposed to such abrupt and dangerous events. The detection of early warning signals through temperature monitoring in the vents is of a primary importance and a main difficulty is to correctly interpret temperature jumps in order to reduce false alarms. We analyze time series of the temperature measured in three fumaroles located at the top of La Soufrière volcano in Guadeloupe, which are characterized by their relatively low temperature around 99°C, slightly above the boiling temperature of water at this altitude. Thanks to the long duration of the records from January to August 2017 and to their short 1-s sampling interval, a multiscale analysis can be performed over several orders of magnitude. We show that, despite their complex and sometimes erratic appearance, the temperature variations observed in the vents contain components highly correlated with rain input variations. Some remarkable patterns recurrently appear at different periods and we show that the main temperature variations of more than 10°C are related to the rainfall intensity. Our results illustrate the importance of external forcing on the otherwise complex and possibly chaotic dynamics of the shallow hydrothermal system of La Soufrière. They also reveal that a careful analysis of rainfall forcing must be done to be able to draw any conclusion concerning changes caused by the underlying hydrothermal system.

Phase-reduction analysis of periodic thermoacoustic oscillations in a Rijke tube

Phase-reduction analysis captures the linear phase dynamics with respect to a limit cycle subjected to weak external forcing. We apply this technique to study the phase dynamics of the self-sustained oscillations produced by a Rijke tube undergoing thermoacoustic instability. Through the phase-reduction formulation, we are able to reduce these dynamics to a scalar equation for the phase, which allows us to efficiently determine the synchronisation properties of the system. For the thermoacoustic system, we find the conditions for which $m:n$ frequency locking occurs, which sheds light on the mechanisms behind asynchronous and synchronous quenching. We also reveal the optimal placement of pressure actuators that provide the most efficient route to synchronisation.

The impact of gravity wave drag modifications on stratospheric pathways of Arctic-midlatitude linkages using deep learning

<p>The global warming has been observed to be more severe in the Arctic compared to the rest of the world. This enhanced warming i.e. Arctic Amplification is not just the result of local feedback processes in the Arctic. The stratospheric pathways of Arctic-midlatitude linkages and large-scale dynamical processes can contribute to the Arctic Amplification. The polar stratospheric dynamics crucially depends on the atmospheric waves at all scales. The winter polar vortex can be disturbed by gravity waves in the middle atmosphere. To investigate the sensitivity of the polar vortex dynamics, large-scale dynamical processes, and the stratospheric pathways of the Arctic-midlatitude linkages to the modification of gravity wave drag, we conduct sensitivity experiments using the global atmospheric model ICON-NWP (ICOsahedral Nonhydrostatic Model for Numerical Weather Prediction). These sensitivity experiments are performed by imposing a repeated annual cycle of the year 1986 for sea surface temperatures and sea ice as lower boundary conditions and for greenhouse gas concentrations as external forcing. This year is selected as both El-Nino Southern Oscillation and Pacific decadal oscillation were in their neutral phase and no explosive volcanic eruption has occurred. Hence, lower boundary and external forcing conditions in this year can serve as a useful proxy for the multi-year mean condition and an estimate of its internal variability. We performed simulations where in the control simulation the sub-grid parameterization scheme for both orographic and non-orographic gravity wave drags are switched on. The other two experiments are identical to the control simulation except that either orographic or non-orographic gravity wave drags are switched off.</p> <p>Recently, deep learning has extraordinarily progressed our ability to recognize complex patterns in big datasets. Deep neural networks have shown great capabilities to capture the dynamical process of the atmosphere. Applying deep learning algorithms on experiments’ results, the impact of gravity wave drag modifications on large-scale mechanisms of the Arctic Amplification will be analyzed. Special emphasis will be put on the effects of gravity wave drag modifications on the polar vortex dynamics.</p>

Multi-decadal (1953–2017) rock glacier kinematics analysed by high-resolution topographic data in the upper Kaunertal, Austria

Permafrost is being degraded worldwide due to the change in external forcing caused by climate change. This has also been shown to affect the morphodynamics of active rock glaciers. We studied these changes, depending on the analysis, on nine or eight active rock glaciers, respectively, with different characteristics in multiple epochs between 1953 and 2017 in Kaunertal, Austria. A combination of historical aerial photographs and airborne laser scanning data and their derivatives were used to analyse surface movement and surface elevation change. In general, the studied landforms showed a significant acceleration of varying magnitude in the epoch 1997–2006 and a volume loss to variable degrees throughout the investigation period. Rock glaciers related to glacier forefields showed significantly higher rates of subsidence than talus-connected ones. Besides, we detected two rock glaciers with deviating behaviour and one that showed an inactivation of its terminal part. By analysing meteorological data (temperature, precipitation and snow cover onset and duration), we were able to identify possible links to these external forcing parameters. The catchment-wide survey further revealed that, despite the general trend, timing, magnitude and temporal peaks of morphodynamic changes indicate a slightly different sensitivity, response or response time of individual rock glaciers to fluctuations and changes in external forcing parameters.

A very likely weakening of Pacific Walker Circulation in constrained near-future projections

The observational records have shown a strengthening of the Pacific Walker circulation (PWC) since 1979. However, whether the observed change is forced by external forcing or internal variability remains inconclusive, a solid answer to more societal relevantly question of how the PWC will change in the near future is still a challenge. Here we perform a quantitative estimation on the contributions of external forcing and internal variability to the recent observed PWC strengthening using large ensemble simulations from six state-of-the-art Earth system models. We find the phase transition of the Interdecadal Pacific Oscillation (IPO), which is an internal variability mode related to the Pacific, accounts for approximately 63% (~51–72%) of the observed PWC strengthening. Models with sufficient ensemble members can reasonably capture the observed PWC and IPO changes. We further constrain the projection of PWC change by using climate models’ credit in reproducing the historical phase of IPO. The result shows a high probability of a weakened PWC in the near future.

ESD Ideas: A Global Warming Scaling Law

In this study, we highlight a component of global warming variability, a scaling law that is based purely on fundamental physical properties of the climate system. We suggest that three similarity parameters define the system response to external forcing, and an argument of physical similarity with observed climate responses in the past can be made when all three parameters are identical for the current and historical climates. We determined that the scaling law of global warming is the (𝜆 + 1 + m) – power of time, where 𝜆 is prescribed by external forcing and m is defined by climate system internal dynamics. When the climate system develops in the direction of intensified positive feedbacks, the power m changes from m = −1 (negative feedbacks dominate) to m ≥ 1 (positive feedbacks dominate). We also establish that a “hothouse” climate with dominant positive feedbacks will be preceded by a climate having a property of incomplete similarity in feedbacks similarity parameters. It implies that the same future scenario may be produced by climate feedbacks of different magnitudes as long as their positive-to-negative ratio is the same.

A Simplified Framework for Modelling Viscoelastic Fluids in Discrete Multiphysics

A simplified modelling technique for modelling viscoelastic fluids is proposed from the perspective of Discrete Multiphysics. This approach, based on the concept of linear additive composition of energy potentials, aims to integrate Smooth Particle Hydrodynamics (SPH) with an equivalent elastic potential tailored for fluid flow simulations. The model was implemented using a particle-based software, explored and thoroughly validated with results from numerical experiments on three different flow conditions. The model was able to successfully capture a large extent of viscoelastic responses to external forcing, ranging from pure viscous flows to creep-dominated Bingham type of behaviour. It is concluded that, thanks to the modularity and tunable characteristics of the parameters involved, the proposed modelling approach can be a powerful simulation tool for modelling or mimicking the behaviour of viscoelastic substances.

Lorenz-63 Model as a Metaphor for Transient Complexity in Climate

Dynamical systems like the one described by the three-variable Lorenz-63 model may serve as metaphors for complex natural systems such as climate systems. When these systems are perturbed by external forcing factors, they tend to relax back to their equilibrium conditions after the forcing has shut off. Here we investigate the behavior of such transients in the Lorenz-63 model by studying its trajectories initialized far away from the asymptotic attractor. Counterintuitively, these transient trajectories exhibit complex routes and, in particular, the sensitivity to initial conditions is akin to that of the asymptotic behavior on the attractor. Thus, similar extreme events may lead to widely different variations before the perturbed system returns back to its statistical equilibrium.