Generation Of Available Potential Energy Research Articles

On the Local Available Potential Energy Perspective of Baroclinic Wave Development

Abstract Extratropical cyclones convert available potential energy (APE) to kinetic energy. However, our current understanding of APE conversion on synoptic scales is limited, as the well-established Lorenz APE framework is only applicable in a global, volume-integrated sense. Here, we employ a recently developed local APE framework to investigate APE and its tendencies in a highly idealized, dispersive baroclinic wave, which leads to the formation of a primary and a downstream cyclone. By utilizing a Lagrangian approach, we demonstrate that locally the downstream cyclone not only consumes APE but also generates it. Initially, APE is transported from both poleward and equatorward reservoirs into the baroclinic zone, where it is then consumed by the vertical displacement of air parcels associated with the developing cyclone. To a lesser extent, APE is also created within the cyclone when air parcels overshoot their reference state; i.e., air colder than its reference state is lifted and air warmer than its reference state is lowered. The volume integral of the APE tendency is dominated by slow vertical displacements of large air masses, whereas the dry intrusion (DI) and warm conveyor belt (WCB) of the cyclone are responsible for the largest local APE tendencies. Diabatic effects within the DI and WCB contribute to the generation of APE in regions where it is consumed adiabatically, thereby enhancing baroclinic conversion in situ. Our findings provide a comprehensive and mechanistic understanding of the local APE tendency on synoptic scales within an idealized setting and complement existing frameworks explaining the energetics of cyclone intensification.

The Reexamination of the Moisture–Vortex and Baroclinic Instabilities in the South Asian Monsoon

Observational analyses reveal that a dominant mode in the South Asian Monsoon region in boreal summer is a westward-propagating synoptic-scale disturbance with a typical wavelength of 4000 km that is coupled with moistening and precipitation processes. The disturbances exhibit an eastward tilt during their development before reaching their maximum activity center. A 2.5-layer model that extends a classic 2-level quasi-geostrophic model by including a prognostic lower-tropospheric moisture tendency equation and an interactive planetary boundary layer was constructed. The eigenvalue analysis of this model shows that the most unstable mode has a preferred zonal wavelength of 4000 km, a westward phase speed of 6 m s−1, an eastward tilt vertical structure, and a westward shift of maximum moisture/precipitation center relative to the lower-tropospheric vorticity center, all of which agree with the observations. Sensitivity experiments show that the moisture–vortex instability determines, to a large extent, the growth rate, while the baroclinic instability helps set up the preferred zonal scale. Ekman-pumping-induced vertical moisture advection prompts an in-phase component of perturbation moisture relative to the low-level cyclonic center, allowing the generation of available potential energy and perturbation growth, regardless of whether or not a low-level mean westerly is presented. In contrast to a previous study, the growth rate is reversely proportional to the convective adjustment time. The current work sheds light on understanding the moisture–vortex and the baroclinic instability in a monsoonal environment with a pronounced easterly vertical shear.

Modes of Atmospheric Energetics Based on HadGEM3-GC3.1-LL Simulations in the Framework of CMIP6

In this study, the focus is on investigating how different climate scenarios, as they have been adopted in Phase 6 of the Coupled Model Intercomparison Project (CMIP6), can lead to different regimes in the energetics components in Lorenz’s energy cycle, hence impacting the “working rate” of the climate system, which is considered as a “heat engine.” The four energy forms on which this investigation is based on are the zonal and eddy components of the available potential and kinetic energies. The permissible correspondingly considered transformations between these forms of energy are also studied. Generation of available potential energy and dissipation of kinetic energy complete the Lorenz energy cycle that is adopted here. In the CMIP6 approach, the results of different climate change analyses were collected in a matrix defined by two dimensions: climate exposure as characterized by a radiative forcing or temperature level and socioeconomic development as classified by the pathways, known as Shared Socioeconomic Pathways (SSPs). The basis of the calculations in this study is the climatic projection produced by the HadGEM3-GC3.1-LL climatic model in the period from 2015 to 2100. In this respect, the results are presented in terms of time projections of the energetics components under different SSPs. The results have shown that the different SSPs yield diverse energetics regimes, consequently impacting on Lorenz energy cycle and, hence, a “working rate” of the climate system based on the components of this cycle. In this respect, Lorenz energy cycle projections are presented, under different SSPs. The results are also contrasted to the calculations for the historical period 1929 to 2014 as this is simulated by the same climatic model.

Convective Coupling in Tropical-Depression-Type Waves. Part I: Rainfall Characteristics and Moisture Structure

AbstractTropical-depression (TD)-type waves are synoptic-scale disturbances embedded with deep convection over the western North Pacific. Studies of these disturbances began over six decades ago; however, some properties of these disturbances remain vague, e.g., the coupling mechanism between the deep convection and the waves. This two-part study aims to examine the rainfall progression in TD-type disturbances and associated tropospheric moisture controlling convective rainfall. Part I investigates the rainfall and moisture characteristics of TD-type waves using TRMM-derived rainfall products and the ERA-Interim data during the period of June–October 1998–2013. The rainfall features a north–south asymmetrical pattern with respect to a TD-type disturbance, with enhanced convective and stratiform rainfall occurring in the southern portion. Along with the northwestward propagation, deep convective and stratiform rainfall occur in phase with the TD-type disturbance without significant preceding shallow convective rainfall. Following the deepest convection, shallow convective rainfall increases in the anomalous southerlies. Such a rainfall progression differs from the paradigm from shallow to deep convection, then to stratiform rainfall, which is suggested in other convectively coupled equatorial waves. The rainfall progression and the atmospheric moisture anomaly are phase locked to the TD-type disturbances such that the relative displacements change little when the disturbances propagate northwestward. The latent heat release in deep convection, which is obtained from the TRMM 3G25 dataset, superposes with a broad warm anomaly in the mid- to upper troposphere, suggesting wave growth through the generation of available potential energy from diabatic heating.

Radiative Heating Rates Computed With Clouds Derived From Satellite‐Based Passive and Active Sensors and their Effects on Generation of Available Potential Energy

AbstractRadiative heating rates computed with cloud properties derived from passive and active sensors are investigated. Zonal monthly radiative heating rate anomalies computed using both active and passive sensors show that larger variability in longwave cooling exists near the tropical tropopause and near the top of the boundary layer between ~50°N to ~50°S. Aerosol variability contributes to increases in shortwave heating rate variability. When zonal monthly mean cloud effects on the radiative heating rate computed with both active and passive sensors and those computed with passive sensor only are compared, the latter shows cooling and heating peaks corresponding to cloud top and base height ranges used for separating cloud types. The difference of these two sets of cloud radiative effect on heating rates in the middle to upper troposphere is larger than the radiative heating rate uncertainty estimated based on the difference of two active sensor radiative heating rate profile data products. In addition, radiative heating rate contribution to generation of eddy available potential energy is also investigated. Although radiation contribution to generation of eddy available potential energy averaged over a year and the entire globe is small, radiation increases the eddy available potential energy in the northern hemisphere during summer. Two key elements that longwave radiation contribute to the generation of eddy potential energy are (1) longitudinal temperature gradient in the atmosphere associated with land and ocean surface temperatures contrasts and absorption of longwave radiation emitted by the surface and (2) cooling near the cloud top of stratocumulus clouds.

On scale interactions between the MJO and synoptic scale

DYNAMO was a field experiment conducted over the near‐equatorial Indian Ocean during 2011 and 2012 for a better understanding of the mechanisms of the Madden–Julian Oscillation (MJO). This study uses DYNAMO observations to study the organization of convection and maintenance of the MJO within the framework of synoptic‐scale weather systems. Lorenz box energetics is used over the limited DYNAMO domain to study the in‐scale energetics (i.e. the exchange of energy within a single scale). Formal scale energetics in the frequency domain is used to study the in‐scale as well as out‐of‐scale interactions (i.e. the exchange of energy among diverse scales) of the kinetic energy (KE) and available potential energy (APE) over limited DYNAMO domain and a zonal belt around the Earth extending 20° north and south of the Equator. The synoptic‐scale precipitation distributions, generation of eddy APE (from the covariance of convective heating and temperature) and its disposition to eddy KE (from the covariance of vertical velocity and temperature) confirm the importance of in‐scale energetics for the maintenance of the synoptic scale. The in‐scale energetics in the frequency domain suggest that conversion of APE to KE on the MJO scale is crucial for its maintenance. Out‐of‐scale energetics suggest that over both domains, MJO loses KE to the synoptic time‐scale (2–7 days), but a reverse situation is encountered in the vicinity of the summer subtropical jet. Overall, MJO is found weakly modulating synoptic scales via transferring eddy KE. These results also imply that out‐of‐scale KE interactions between MJO and synoptic scales are not crucial in understanding the maintenance of the MJO over the DYNAMO domain.

Spatial Distribution of Generation of Lorenz’s Available Potential Energy in a Global Climate Model

Abstract Previous studies have estimated global available potential energy (APE) and global APE generation, but no study has focused on the geographic distribution of contributions to global APE and APE generation. To obtain the information needed for this analysis, simulations were performed using the NCAR CESM1.0.4 climate model. Based on these simulation results, maps of the spatial and seasonal distribution of APE contributions and APE generation in the atmosphere were obtained from the analysis. APE is generated by processes that cool relatively cool areas or warm relatively warm areas. It was found that there are two regions of the mid- to upper troposphere that contribute primarily to APE generation: 1) the tropics, especially the western tropical Pacific, owing largely to latent heat released in the intertropical convergence zone, and 2) the polar regions, especially in the relatively cold polar night, where longwave cooling is not offset by shortwave warming. It was also found that these qualitative results are largely insensitive to the assumptions examined regarding the treatment of topography in the atmosphere. Further, the analysis was extended to calculate how APE and APE generation is changed in a 4 × CO2 climate relative to a 1 × CO2 climate. It was found that in the high-CO2 climate, APE decreased by 7.0% and APE generation decreased by 10.1%. This is consistent with expectations based on decreased equator-to-pole temperature gradients in warmer climates. The methods, results, and analysis presented here should prove useful in helping to build a better understanding of controls on atmospheric kinetic energy.

The computation of reference state and APE production by diabatic processes in an idealized tropical cyclone

This study investigates the energetics of tropical cyclone intensification using the Available Potential Energy (APE) theory. While the idea is now well accepted that tropical cyclones (TCs) intensify as the result of the conversion into kinetic energy of the APE generated by the release of latent heat extracted from the warm tropical ocean surface, its rigorous theoretical formalization has remained elusive owing to the complexity of constructing a suitable reference state for defining and quantifying APE in a moist atmosphere. Yet the construction of such a reference state is a key fundamental issue, because the magnitude of the APE reservoir and of its temporal evolution, as well as the values of the thermodynamic efficiencies controlling the rate at which diabatic processes generate or destroy APE, depend on its specification. This issue is illustrated in the idealized context of an axisymmetric TC model by comparing the energetics of TC intensification obtained by using two different sorting‐based approaches to compute the reference state defining APE. It is found that the thermodynamic efficiency controlling the APE generation by surface latent heat fluxes is larger when the reference state is constructed using a ‘top‐down’ sorting method, as the APE thus defined absorbs all the CAPE present in the system. However, because a large fraction of the overall CAPE is never released during the TC's lifetime (e.g. in regions dominated by subsidence), there is a better agreement between the production of APE by surface fluxes and its subsequent conversion into kinetic energy when a ‘bottom‐up’ reference state is used. These results suggest that, contrary to what is usually assumed, the reference state in APE theory should be constructed to minimize, rather than maximize, the total APE, so that the introduction of dynamically inert APE is minimized.

Available Potential Energy and the General Circulation: Partitioning Wind, Buoyancy Forcing, and Diapycnal Mixing

AbstractThe ocean energy cycle is calculated using a new available potential energy (APE) decomposition, which partitions adiabatic buoyancy fluxes from diapycnal mixing, applied to results from the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2), eddy-permitting ocean state estimate and observed surface buoyancy fluxes from the WHOI OAFlux project. Compared with the traditional Lorenz energy cycle, this framework provides a more accurate estimate of the background potential energy (PE) of the global oceans and the surface generation and interior fluxes of APE. Calculations of the global energy budget using 16 yr of ECCO2 output suggest that the adiabatic portion of the general circulation is maintained by a balance between the mean wind-driven upwelling that increases APE (+0.27 TW) and time-fluctuating processes, including mesoscale eddies, which release APE (−0.27 TW). The APE generated by surface buoyancy fluxes (0.46 TW) is comparable to the generation by the mean winds. The global rate of irreversible mixing (0.46 TW), which balances surface APE generation, is consistent with previous estimates of the diapycnal fluxes associated with maintaining deep stratification (see Munk and Wunsch) and a global diapycnal diffusivity of O(1 × 10−4) m2 s−1. However, the net contribution of diapycnal mixing to the total potential energy is negligible, which suggests that mixing, contrary to one current paradigm, does not place a global demand on kinetic energy dissipation. However, there are regions where mixing is significant, for example, between 3000 and 5000 m (in ECCO2), where mixing increases PE by 0.1 TW. The work provides a new framework for separating adiabatic–diabatic fluxes and for monitoring the global rate of diapycnal mixing rate using measurable surface properties such as SST and heat flux.

Energetics of Multidecadal Atlantic Ocean Variability

Abstract Oscillatory behavior of the Atlantic meridional overturning circulation (MOC) is thought to underlie Atlantic multidecadal climate variability. While the energy sources and sinks driving the mean MOC have received intense scrutiny over the last decade, the governing energetics of the modes of variability of the MOC have not been addressed to the same degree. This paper examines the energy conversion processes associated with this variability in an idealized North Atlantic Ocean model. In this model, the multidecadal variability arises through an instability associated with a so-called thermal Rossby mode, which involves westward propagation of temperature anomalies. Applying the available potential energy (APE) framework from stratified turbulence to the idealized ocean model simulations, the authors study the multidecadal variability from an energetics viewpoint. The analysis explains how the propagation of the temperature anomalies leads to changes in APE, which are subsequently converted into the kinetic energy changes associated with variations in the MOC. Thus, changes in the rate of generation of APE by surface buoyancy forcing provide the kinetic energy to sustain the multidecadal mode of variability.

Generation of available potential energy due to the long-wave radiation influx in the atmosphere

Investigated are the features of the distribution of the generation of available potential energy (APE) due to the influx of long-wave radiation in the atmosphere. The reanalysis (ERA-Interim) data of the European Center for Medium-Range Weather Forecast are used as the reference values of fields for computing APE generation. The obtained results indicate the influence of cloudiness and atmospheric stratification on APE generation due to the inflow of long-wave radiation which values are by 4–5 times smaller than the values of APE generation due to the phase transitions of moisture in the atmosphere.

Energetics of super cyclone ‘GONU’ and very severe cyclonic storm ‘SIDR’

This paper discusses the energetics aspects of two tropical cyclones formed over the north Indian Ocean during 2007, viz., the Super Cyclonic Storm (GONU) and the Very Severe Cyclonic Storm (SIDR). From the analysis of various energetics terms such as the Eddy Available Potential Energy (AE), Zonal Available Potential Energy (AZ), Zonal Kinetic Energy (KZ), Eddy Kinetic Energy (KE) and their generation and inter-conversions i.e., G(AE), G(AZ), C(AE, KE), C(AZ, KZ), C(KZ, KE) and C(AZ, AE) have been computed on day to day basis during the periods of their intensifications over the domain 5° N to 25° N, 55° E to 75° E in respect of ‘GONU’ and 5° N to 25° N, 77° E to 97° E for ‘SIDR’. Besides the above, the area averaged value of s (Sigma), the vertically averaged Moist Static Energy (MSE), has also been computed on each day. Day-to-day evolution of these parameters is mapped and described. Some of the distinguishing features in the energetic of these two intense vortices which formed in entirely different climatological settings have been brought out. It is noticed that in the case of ‘GONU’, though both barotropic and baroclinic energy conversions have taken place during the life cycle, the intensification phase is characterized by an enhancement in AE, KE and vertically integrated Moist Static Energy. Enhancement in AE can be attributed to the generation of AE, which may again be attributed to the asymmetric latent heat of condensation associated with the asymmetric rainfall in the cyclone field. Enhancement in KE may be attributed to the enhancement in both barotropic and baroclinic conversion into KE. Though most of these observations made for ‘GONU’ are found to be attributable to ‘SIDR’ as well, the intensification of ‘SIDR’ appears to have more similarity to that of a typical growing mid-latitude baroclinic wave. In this case, the enhancement in AE, could also be attributed to positive C(AZ,AE), which is mainly due to interaction with mid-latitude baroclinic westerly wave. The energetics analysis also indicates that GONU had helped in the enhancement of seasonal mean meridional circulation where as the SIDR had inhibited the enhancement of seasonal mean meridional circulation.

Absorbing aerosol‐induced change in the early monsoon Arabian Sea low‐level jet: Modeled transfer from anomalous heating to nondivergent kinetic energy

This study examines the impact of anomalous differential generation of available potential energy by absorbing aerosols on the transition and early active phases of the South Asian summer monsoon. Aerosol direct and indirect radiative forcings can modify tropospheric temperature profiles through direct absorption, scattering, and extended cloud lifetimes. Recent studies have suggested that over monthly and seasonal time scales, these effects can lead to modified flow and rainfall regimes in the South Asian monsoon region. Of special interest is the covariance of heating and temperature prior to active monsoon onset. It can be shown that anomalous generation of available potential energy due to absorption of shortwave radiation by aerosols can impact the cascade of energy from the local Hadley circulation to the monsoon nondivergent flow. In order to quantify the potential impact of aerosol radiative forcing and the resulting changes in monsoon onset timing and intensity, an ensemble of Weather Research and Forecasting with Chemistry regional weather and chemistry model forecasts are created for the South Asian summer monsoon region during May, June, and July. The forecasts including shortwave absorption by aerosol are compared to control forecasts in which aerosols only scatter shortwave radiation. The evolution of irrotational and nondivergent kinetic energy, generation of available potential energy and rainfall are presented. It is found that the ensemble with aerosol shortwave absorption contains on average a more intense dynamical onset over South India which is statistically significant compared to the ensemble variability. The more intense monsoon onset is related to a more intense Arabian Sea low‐level jet. In the ensemble with shortwave absorption by aerosol, the early season monsoon rainfall is diminished over South India and enhanced over the Northeastern Indian states.

Completing the Mechanical Energy Pathways in Turbulent Rayleigh-Bénard Convection

A new, more complete view of the mechanical energy budget for Rayleigh-Bénard convection is developed and examined using three-dimensional numerical simulations at large Rayleigh numbers and Prandtl number of 1. The driving role of available potential energy is highlighted. The relative magnitudes of different energy conversions or pathways change significantly over the range of Rayleigh numbers Ra ~ 10(7)-10(13). At Ra < 10(7) small-scale turbulent motions are energized directly from available potential energy via turbulent buoyancy flux and kinetic energy is dissipated at comparable rates by both the large- and small-scale motions. In contrast, at Ra ≥ 10(10) most of the available potential energy goes into kinetic energy of the large-scale flow, which undergoes shear instabilities that sustain small-scale turbulence. The irreversible mixing is largely confined to the unstable boundary layer, its rate exactly equal to the generation of available potential energy by the boundary fluxes, and mixing efficiency is 50%.

Contributions of Individual Atmospheric Diabatic Heating Processes to the Generation of Available Potential Energy

Abstract The generation of zonal and eddy available potential energy (Gz and Ge) as formulated by Lorenz are computed on a global-, daily-, and synoptic-scale basis to consider the contribution of each diabatic heating component separately and in combination. Using global, mostly satellite-derived datasets for the diabatic heating components and the temperature enables us to obtain Gz and, especially, Ge from observations for the first time and at higher temporal and spatial resolution than previously possible. The role of clouds in maintaining G is investigated. The global annual mean Gz is 1.52 W m−2. Values reach a minimum of 0.63 W m−2 in the Northern Hemisphere during spring and a maximum of 2.27 W m−2 in the Southern Hemisphere during winter. The largest contributors to Gz are latent heating in the tropical upper troposphere, associated with the intertropical convergence zone in the summer hemisphere and surface sensible heat fluxes in the winter pole. Diabatic cooling by radiative fluxes (mostly longwave) generally destroys Gz. The value of Ge is negative and is about an order of magnitude smaller than Gz, with a global annual mean of −0.29 W m−2. However, the small value of Ge results from the cancellation of the contributions from the individual diabatic heating terms, which are actually roughly similar in magnitude to their Gz contributions. The results presented herein suggest that the large-scale dynamics of the atmosphere organize the spatial and temporal distribution of clouds and precipitation in such a way as to increase the energy available to drive the circulation, a kind of positive feedback.

The Effects of Explicit versus Parameterized Convection on the MJO in a Large-Domain High-Resolution Tropical Case Study. Part I: Characterization of Large-Scale Organization and Propagation*

Abstract High-resolution simulations over a large tropical domain (~20°S–20°N, 42°E–180°) using both explicit and parameterized convection are analyzed and compared to observations during a 10-day case study of an active Madden–Julian oscillation (MJO) event. The parameterized convection model simulations at both 40- and 12-km grid spacing have a very weak MJO signal and little eastward propagation. A 4-km explicit convection simulation using Smagorinsky subgrid mixing in the vertical and horizontal dimensions exhibits the best MJO strength and propagation speed. Explicit convection simulations at 12 km also perform much better than the 12-km parameterized convection run, suggesting that the convection scheme, rather than horizontal resolution, is key for these MJO simulations. Interestingly, a 4-km explicit convection simulation using the conventional boundary layer scheme for vertical subgrid mixing (but still using Smagorinsky horizontal mixing) completely loses the large-scale MJO organization, showing that relatively high resolution with explicit convection does not guarantee a good MJO simulation. Models with a good MJO representation have a more realistic relationship between lower-free-tropospheric moisture and precipitation, supporting the idea that the moisture–convection feedback is a key process for MJO propagation. There is also increased generation of available potential energy and conversion of that energy into kinetic energy in models with a more realistic MJO, which is related to larger zonal variance in convective heating and vertical velocity, larger zonal temperature variance around 200 hPa, and larger correlations between temperature and ascent (and between temperature and diabatic heating) between 500 and 400 hPa.

Energetics of horizontal convection

AbstractThree-dimensional direct numerical simulation of horizontal convection is reported for a large Rayleigh number, $\mathit{Ra}\sim O(1{0}^{12} )$, and boundary conditions that allow comparison with previous laboratory experiments. The convection is forced by heating over half of the horizontal base of a long channel and cooling over the other half of the base. The solutions are consistent with the experiments, including small-scale streamwise roll instability developing into a convectively mixed layer within the bottom thermal boundary layer, and a turbulent endwall plume. The mechanical energy budget is shown to be dominated by conversions of available potential energy to kinetic energy by buoyancy flux in the plume and the reverse in the interior of the circulation. These local conversions are three orders of magnitude greater than the total rate of viscous dissipation. The total irreversible mixing is exactly equal to the generation of available potential energy by buoyancy forcing, and one order of magnitude larger than the viscous dissipation. This confirms that dissipation rate is not an indicator of the strength of the circulation and explains why horizontal convection is more energetic than might be expected.

Available Potential Energy and Exergy in Stratified Fluids

Lorenz's theory of available potential energy (APE) remains the main framework for studying the atmospheric and oceanic energy cycles. Because the APE generation rate is the volume integral of a thermodynamic efficiency times the local diabatic heating/cooling rate, APE theory is often regarded as an extension of the theory of heat engines. Available energetics in classical thermodynamics, however, usually relies on the concept of exergy and is usually measured relative to a reference-state maximizing entropy at constant energy, whereas APE's reference state minimizes potential energy at constant entropy. This review seeks to shed light on the two concepts; it covers local formulations of available energetics, alternative views of the dynamics/thermodynamics coupling, APE theory and the second law of thermodynamics, APE production/dissipation, extensions to binary fluids, mean/eddy decompositions, APE in incompressible fluids, APE and irreversible turbulent mixing, and the role of mechanical forcing on APE production.

Sensitivity of Tropical Cyclone Intensity to Ventilation in an Axisymmetric Model

Abstract The sensitivity of tropical cyclone intensity to ventilation of cooler, drier air into the inner core is examined using an axisymmetric tropical cyclone model with parameterized ventilation. Sufficiently strong ventilation induces cooling of the upper-level warm core, a shift in the secondary circulation radially outward, and a decrease in the simulated intensity. Increasing the strength of the ventilation and placing the ventilation at middle to lower levels results in a greater decrease in the quasi-steady intensity, whereas upper-level ventilation has little effect on the intensity. For strong ventilation, an oscillatory intensity regime materializes and is tied to transient convective bursts and strong downdrafts into the boundary layer. The sensitivity of tropical cyclone intensity to ventilation can be viewed in the context of the mechanical efficiency of the inner core or a modified thermal wind relation. In the former, ventilation decreases the mechanical efficiency, as the generation of available potential energy is wasted by entropy mixing above the boundary layer. In the latter, ventilation weakens the eyewall entropy front, resulting in a decrease in the intensity by thermal wind arguments. The experiments also support the existence of a threshold ventilation beyond which a tropical cyclone cannot be maintained. Downdrafts overwhelm surface fluxes, leading to a precipitous drop in intensity and a severe degradation of structure in such a scenario. For a given amount of ventilation below the threshold, there exists a minimum initial intensity necessary for intensification to the quasi-steady intensity.

Mechanical power input from buoyancy and wind to the circulation in an ocean model

We make a systematic quantitative comparison of the effects that surface buoyancy forcing and wind stress have on the energy balance of an idealized, rotating, pole‐to‐pole ocean model with a zonally re‐entrant channel in the south, forced by realistic heat (buoyancy) fluxes and wind stresses representative of global climatology. Surface buoyancy fluxes and wind stress forcing are varied independently; both have significant effects on the reservoirs of various forms of energy and the rates of transfer between them. Importantly, we show for the first time that in the ocean, each power input has a positive feedback on the other. Changes in the rate of generation of available potential energy by buoyancy fluxes at the surface lead to similar changes in the rate of conversion of potential energy to kinetic energy by buoyancy forces (sinking) in the interior, and to changes in the rate of generation of kinetic energy by wind stress. Conversely, changes in the rate of generation of kinetic energy by wind stress lead to changes in the rate of generation of available potential energy by buoyancy forcing. We discuss how this feedback is mediated by the circumpolar current, and processes involving buoyancy, mixing and geostrophic balances. Our results support the notion that surface buoyancy forcing, along with wind and tidal forcing, plays an active role in the energy balance of the oceans. The overturning circulation in the oceans is not the result of a single driving force. Rather, it is a manifestation of a complex and subtle balance.