Lorenz Atmospheric Energy Cycle in Climatic Projections

The aim of this study is to investigate whether different Representative Concentration Pathways (RCPs), as they are determined in the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), lead to different regimes in the energetics components of the Lorenz energy cycle. The four energy forms on which this investigation is based are the zonal and eddy components of the available potential and kinetic energies. The corresponding transformations between these forms of energy are also studied. RCPs are time-dependent, consistent scenarios of concentrations of radiatively active gases and particles. In the present study, four RCPs are explored, namely, rcp26, rcp45, rcp60, rcp85; these represent projections (for the future period 2006–2100) that result in radiative forcing of approximately 2.6, 4.5, 6.0 and 8.5 Wm−2 at year 2100, respectively, relative to pre-industrial conditions. The results are presented in terms of time projections of the energetics components from 2020 to 2100 and show that the different RCPs yield diverse energetics regimes, consequently impacting the Lorenz energy cycle. In this respect, projections under different RCPs of the Lorenz energy cycle are presented.

Role Of Relative Wind Stress In Generating Eddy Instabilities

<p>Relative wind stress (calculated by including the surface current terms) is known to remove energy from mesoscale eddies, but how they respond to this damping mechanism over their lifetime is poorly understood. A method for predicting eddy energy is made by time stepping forward the energy equation of a linear two-layer model using an analytical relative wind stress damping term. Results of this prediction are then compared with numerical experiments of an idealised two-layer anticyclonic eddy in a high-resolution general circulation model. The energy in both experiments displays a quantitative agreement in relative wind stress damping, though this is not the case when the eddy in the numerical experiment becomes baroclinically unstable. In addition to this well-known relative wind stress damping mechanism, we found that relative wind stress can trigger eddy instabilities sooner, leading to quicker decay. The earlier onset of these instabilities by relative wind stress is observed in a Lorenz energy cycle.</p>

The Lorenz Energy Cycle: Trends and the Impact of Modes of Climate Variability

<p>The atmospheric circulation is driven by heat transport from the tropics to the polar regions, implying energy conversions between available potential and kinetic energy through various mechanisms. The processes of energy transformations can be quantitatively investigated in the global climate system through the Lorenz energy cycle formalism. In this study, we examine these variations and the impacts of modes of climate variability on the Lorenz energy cycle by using reanalysis data from the Japanese Meteorological Agency (JRA-55). We show that the atmospheric circulation is overall becoming more energetic and efficient. For instance, we find a statistically significant trend in the eddy available potential energy, especially in the transient eddy available potential energy in the Southern Hemisphere. We find significant trends in the conversion rates between zonal available potential and kinetic energy, consistent with an expansion of the Hadley cell, and in the conversion rates between eddy available potential and kinetic energy, suggesting an increase in mid-latitudinal baroclinic instability. We also show that planetary-scale waves dominate the stationary eddy energy, while synoptic-scale waves dominate the transient eddy energy with a significant increasing trend. Our results suggest that interannual variability of the Lorenz energy cycle is determined by modes of climate variability. We find that significant global and hemispheric energy fluctuations are caused by the El Nino-Southern Oscillation, the Arctic Oscillation, the Southern Annular Mode, and the meridional temperature gradient over the Southern Hemisphere.</p>

TheDiaTo (v1.0) – a new diagnostic tool for water, energy and entropy budgets in climate models

This work presents the Thermodynamic Diagnostic Tool (TheDiaTo), a novel diagnostic tool for investigating the thermodynamics of climate systems with a wide range of applications, from sensitivity studies to model tuning. It includes a number of modules for assessing the internal energy budget, the hydrological cycle, the Lorenz energy cycle and the material entropy production. The routine takes as inputs energy fluxes at the surface and at the top of the atmosphere (TOA), which allows for the computation of energy budgets at the TOA, the surface and in the atmosphere as a residual. Meridional enthalpy transports are also computed from the divergence of the zonal mean energy budget from which the location and intensity of the maxima in each hemisphere are calculated. Rainfall, snowfall and latent heat fluxes are received as inputs for computation of the water mass and latent energy budgets. If a land–sea mask is provided, the required quantities are separately computed over continents and oceans. The diagnostic tool also computes the annual Lorenz energy cycle (LEC) and its storage and conversion terms by hemisphere and as a global mean. This is computed from three-dimensional daily fields of horizontal wind velocity and temperature in the troposphere. Two methods have been implemented for the computation of the material entropy production: one relying on the convergence of radiative heat fluxes in the atmosphere (indirect method) and the other combining the irreversible processes occurring in the climate system, particularly heat fluxes in the boundary layer, the hydrological cycle and the kinetic energy dissipation as retrieved from the residuals of the LEC (direct method). A version of these diagnostics has been developed as part of the Earth System Model eValuation Tool (ESMValTool) v2.0a1 in order to assess the performances of CMIP6 model simulations, and it will be available in the next release. The aim of this software is to provide a comprehensive picture of the thermodynamics of the climate system, as reproduced in the state-of-the-art coupled general circulation models. This can prove useful for better understanding anthropogenic and natural climate change, paleoclimatic climate variability, and climatic tipping points.

Energetics of the Climate System

The energetics considerations based on Lorenz’s available potential energy A focus on identification and quantification of processes capable of converting external energy sources into the kinetic energy of atmospheric and oceanic general circulations. Generally, these considerations consist of: (a) identifying the relevant energy compartments from which energy can be converted against friction to kinetic energy of motions of interests; (b) formulating for these energy compartments budget equations that describe all possible energy pathways; and (c) identifying the dominant energy pathways using realistic data. In order to obtain a more detailed description of energy pathways, a partitioning of motions, for example, into a “mean” and an “eddy” component, or into a diabatic and an adiabatic component, is used. Since the budget equations do not always suggest the relative importance of all possible pathways, often not even the directions, data that describe the atmospheric and the oceanic state in a sufficiently accurate manner are needed for evaluating the energy pathways. Apart from the complication due to different expressions of A, ranging from the original definition by Lorenz in 1955 to its approximations and to more generally defined forms, one has to balance the complexity of the respective budget equations that allows the evaluation of more possible energy pathways, with the quality of data available that allows sufficiently accurate estimates of energy pathways. With regard to the atmosphere, our knowledge, as inferred from the four-box Lorenz energy cycle, has consolidated in the last two decades, by, among other means, using data assimilation products obtained by combining observations with realistic atmospheric general circulation models (AGCMs). The eddy kinetic energy, amounting to slightly less than 50% of the total kinetic energy, is supported against friction through a baroclinic pathway “fueled” by the latitudinally dependent diabatic heating. The mean kinetic energy is supported against friction by converting eddy kinetic energy via inverse cascades. For the ocean, our knowledge is still emerging. The description through the four-box Lorenz energy cycle is approximative and was only estimated from a simulation of a 0.1° oceanic general circulation models (OGCM) realistically forced at the sea surface, rather than from a data assimilation product. The estimates obtained so far suggest that the oceanic eddy kinetic energy, amounting almost 75% of the total oceanic kinetic energy, is supported against friction through a baroclinic pathway similar to that in the atmosphere. However, the oceanic baroclinic pathway is “fueled” to a considerable extent by converting mean kinetic energy supported by winds into mean available potential energy. Winds are also the direct source of the kinetic energy of the mean circulation, without involving noticeable inverse cascades from transients, at least not for the ocean as a whole. The energetics of oceanic general circulation can also be examined by separating diabatic from adiabatic processes. Such a consideration is thought to be more appropriate for understanding the energetics of the oceanic meridional overturning circulation (MOC), since this circulation is sensitive to density changes induced by diabatic mixing. Further work is needed to quantify the respective energy pathways using realistic data.

TheDiaTo (v1.0) – A new diagnostic tool for water, energy and entropy budgets in climate models

This work presents Thermodynamic Diagnostic Tool (TheDiaTo), a novel diagnostic tool for studying the thermodynamics of the climate systems with a wide range of applications, from sensitivity studies to model tuning. It includes a number of modules for assessing the internal energy budget, the hydrological cycle, the Lorenz Energy Cycle and the material entropy production, respectively. The routine receives as inputs energy fluxes at surface and at the Top-of-Atmosphere (TOA), for the computation of energy budgets at Top-of-Atmosphere (TOA), at the surface, and in the atmosphere as a residual. Meridional enthalpy transports are also computed from the divergence of the zonal mean energy budget fluxes; location and intensity of peaks in the two hemispheres are then provided as outputs. Rainfall, snowfall and latent heat fluxes are received as inputs for computing the water mass and latent energy budgets. If a land-sea mask is provided, the required quantities are separately computed over continents and oceans. The diagnostic tool also computes the Lorenz Energy Cycle (LEC) and its storage/conversion terms as annual mean global and hemispheric values. In order to achieve this, one needs to provide as input three-dimensional daily fields of horizontal wind velocity and temperature in the troposphere. Two methods have been implemented for the computation of the material entropy production, one relying on the convergence of radiative heat fluxes in the atmosphere (indirect method), one combining the irreversible processes occurring in the climate system, particularly heat fluxes in the boundary layer, the hydrological cycle and the kinetic energy dissipation as retrieved from the residuals of the LEC. A version of these diagnostics has been developed as part of the Earth System Model eValuation Tool (ESMValTool) v2.0a1, in order to assess the performances of CMIP6 model simulations, and will be available in the next release of the tool. The aim of this software is to provide a comprehensive picture of the thermodynamics of the climate system as reproduced in the state-of-the-art coupled general circulation models. This can prove useful for better understanding anthropogenic and natural climate change, paleoclimatic climate variability, and climatic tipping points.

Energy Conversion Routes in the Western Mediterranean Sea Estimated from Eddy–Mean Flow Interactions

Energy conversion routes are investigated in the western Mediterranean Sea from the eddy–mean flow interactions. The sources of eddy kinetic energy are analyzed by applying a regional formulation of the Lorenz energy cycle to 18 years of numerical simulation at eddy-resolving resolution (3.5 km), which allows for identifying whether the energy exchange between the mean and eddy flow is local or nonlocal. The patterns of energy conversion between the mean and eddy kinetic and potential energy are estimated in three subregions of the domain: the Alboran Sea, the Algerian Basin, and the northern basin. The spatial characterization of the energy routes hints at the physical mechanisms involved in maintaining the balance, suggesting that flow–topography interaction is strongly linked to eddy growth in most of the domain.

The impact of oceanic heat transport on the atmospheric circulation

A general circulation model of intermediate complexity with an idealized earthlike aquaplanet setup is used to study the impact of changes in the oceanic heat transport on the global atmospheric circulation. Focus is put on the Lorenz energy cycle and the atmospheric mean meridional circulation. The latter is analysed by means of the Kuo–Eliassen equation. The atmospheric heat transport compensates the imposed oceanic heat transport changes to a large extent in conjunction with significant modification of the general circulation. Up to a maximum about 3 PW, an increase of the oceanic heat transport leads to an increase of the global mean near-surface temperature and a decrease of its equator-to-pole gradient. For larger transports, the gradient is reduced further but the global mean remains approximately constant. This is linked to a cooling and a reversal of the temperature gradient in the tropics. A larger oceanic heat transport leads to a reduction of all reservoirs and conversions of the Lorenz energy cycle but of different relative magnitude for the individual components. The available potential energy of the zonal mean flow and its conversion to eddy available potential energy are affected most. Both the Hadley and Ferrel cell show a decline for increasing oceanic heat transport, with the Hadley cell being more sensitive. Both cells exhibit a poleward shift of their maxima, and the Hadley cell broadens for larger oceanic transports. The partitioning, by means of the Kuo–Eliassen equation, reveals that zonal mean diabatic heating and friction are the most important sources for changes of the Hadley cell, while the behaviour of the Ferrell cell is mostly controlled by friction.