Mean Available Potential Energy Research Articles

Moist available potential energy of the mean state of the atmosphere and the thermodynamic potential for warm conveyor belts and convection

Abstract. Much of our understanding of atmospheric circulation comes from relationships between aspects of the circulation and the mean state of the atmosphere. In particular, the concept of mean available potential energy (MAPE) has been used previously to relate the strength of the extratropical storm tracks to the zonal-mean temperature and humidity distributions. Here, we calculate for the first time the MAPE of the zonally varying (i.e., three-dimensional) time-mean state of the atmosphere including the effects of latent heating. We further calculate a local MAPE by restricting the domain to an assumed eddy size, and we partition this local MAPE into convective and nonconvective components. Local convective MAPE maximizes in the subtropics and midlatitudes, in many cases in regions of the world that are known to have intense convection. Local nonconvective MAPE has a spatial pattern similar to the Eady growth rate, although local nonconvective MAPE has the advantage that it takes into account latent heating. Furthermore, the maximum potential ascent associated with local nonconvective MAPE is related to the frequency of warm conveyor belts (WCBs), which are ascending airstreams in extratropical cyclones with large impacts on weather. This maximum potential ascent can be calculated based only on mean temperature and humidity, and WCBs tend to start in regions of high maximum potential ascent on a given day. These advances in the use of MAPE are expected to be helpful to connect changes in the mean state of the atmosphere, such as under global warming, to changes in important aspects of extratropical circulation.

Energy Transfers between Multidecadal and Turbulent Variability

Abstract One of the proposed mechanisms to explain the multidecadal variability observed in sea surface temperature of the North Atlantic Ocean consists of a large-scale low-frequency internal mode spontaneously developing because of the large-scale baroclinic instability of the time-mean circulation. Even though this mode has been extensively studied in terms of the buoyancy variance budget, its energetic properties remain poorly known. Here we perform the full mechanical energy budget including available potential energy (APE) and kinetic energy (KE) of this internal mode and decompose the budget into three frequency bands: mean, low frequency (LF) associated with the large-scale mode, and high frequency (HF) associated with mesoscale eddy turbulence. This decomposition allows us to diagnose the energy fluxes between the different reservoirs and to understand the sources and sinks. Because of the large scale of the mode, most of its energy is contained in the APE. In our configuration, the only source of LF APE is the transfer from mean APE to LF APE that is attributed to the large-scale baroclinic instability. In return the sinks of LF APE are the parameterized diffusion, the flux toward HF APE, and, to a much lesser extent, the flux toward LF KE. The presence of an additional wind stress component weakens multidecadal oscillations and modifies the energy fluxes between the different energy reservoirs. The KE transfer appears to only have a minor influence on the multidecadal mode relative to the other energy sources involving APE, in all experiments. These results highlight the utility of the full APE–KE budget.

Energetics of Eddy-Mean Flow Interactions in the Amery Ice Shelf Cavity

Previous studies demonstrated that eddy processes play an important role in ice shelf basal melting and the water mass properties of ice shelf cavities. However, the eddy energy generation and dissipation mechanisms in ice shelf cavities have not been studied systematically. The dynamic processes of the ocean circulation in the Amery Ice Shelf cavity are studied quantitatively through a Lorenz energy cycle approach for the first time by using the outputs of a high-resolution coupled regional ocean-sea ice-ice shelf model. Over the entire sub-ice-shelf cavity, mean available potential energy (MAPE) is the largest energy reservoir (112 TJ), followed by the mean kinetic energy (MKE, 70 TJ) and eddy available potential energy (EAPE, 10 TJ). The eddy kinetic energy (EKE) is the smallest pool (5.5 TJ), which is roughly 8% of the MKE, indicating significantly suppressed eddy activities by the drag stresses at ice shelf base and bottom topography. The total generation rate of available potential energy is about 1.0 GW, almost all of which is generated by basal melting and seawater refreezing, i.e., the so-called “ice pump.” The energy generated by ice pump is mainly dissipated by the ocean-ice shelf and ocean-bottom drag stresses, amounting to 0.3 GW and 0.2 GW, respectively. The EKE is generated through two pathways: the barotropic pathway MAPE→MKE→EKE (0.03 GW) and the baroclinic pathway MAPE→EAPE→EKE (0.2 GW). In addition to directly supplying the EAPE through baroclinic pathway (0.2 GW), MAPE also provides 0.5 GW of power to MKE to facilitate the barotropic pathway.

Generation mechanism of an observed submesoscale eddy in the Chukchi Sea

A submesoscale eddy was detected by satellite in the summer Chukchi Sea, which was generated by the salinity front slump. Multiple reanalysis data suggested that the front was generated as a result of denser water transport northward. The whole story can be separated into two stages: front genesis and eddy genesis (front slump). The front genesis event started with denser water located in southern of the Chukchi Sea. Wind-driven Ekman pumping lifted the salty water upward and increased the salinity in the upper layers. Later, the current transported the denser water northward until it arrived in the northeastern Chukchi Sea, where the fronts and submesoscale eddy were detected. The denser (lighter) water was located in the south (north) of the front, covering over 100 km and 80 m in the horizontal and vertical directions, respectively. The first stage was completed in 15 days. The front contained a great amount of mean available potential energy (MAPE), which was converted into eddy available potential energy (EAPE) via the EAPE production term when the salinity front slumped. EAPE was in turn converted into eddy kinetic energy (EKE) via the buoyancy flux work, and submesoscale eddies appeared. As the salinity front diminished, the salinity field became more homogenous in both the horizontal and vertical directions. The second stage was completed in 18 days. Numerical simulations suggested that the eddy activity was determined by the total depth of the fronts instead of the mixed layer depth. However, if a strong pycnocline was added between the fronts, the vertical buoyancy gradient largely increased there, which reduced the growth rate of unstable energy and produced weaker eddy activities on the denser side of the front.

Energetics and mixing in buoyancy-driven near-bottom stratified flow

Turbulence and mixing in a near-bottom convectively driven flow are examined by numerical simulations of a model problem: a statically unstable disturbance at a slope with inclination $\unicode[STIX]{x1D6FD}$ in a stable background with buoyancy frequency $N$ . The influence of slope angle and initial disturbance amplitude are quantified in a parametric study. The flow evolution involves energy exchange between four energy reservoirs, namely the mean and turbulent components of kinetic energy (KE) and available potential energy (APE). In contrast to the zero-slope case where the mean flow is negligible, the presence of a slope leads to a current that oscillates with $\unicode[STIX]{x1D714}=N\sin \unicode[STIX]{x1D6FD}$ and qualitatively changes the subsequent evolution of the initial density disturbance. The frequency, $N\sin \unicode[STIX]{x1D6FD}$ , and the initial speed of the current are predicted using linear theory. The energy transfer in the sloping cases is dominated by an oscillatory exchange between mean APE and mean KE with a transfer to turbulence at specific phases. In all simulated cases, the positive buoyancy flux during episodes of convective instability at the zero-velocity phase is the dominant contributor to turbulent kinetic energy (TKE) although the shear production becomes increasingly important with increasing $\unicode[STIX]{x1D6FD}$ . Energy that initially resides wholly in mean available potential energy is lost through conversion to turbulence and the subsequent dissipation of TKE and turbulent available potential energy. A key result is that, in contrast to the explosive loss of energy during the initial convective instability in the non-sloping case, the sloping cases exhibit a more gradual energy loss that is sustained over a long time interval. The slope-parallel oscillation introduces a new flow time scale $T=2\unicode[STIX]{x03C0}/(N\sin \unicode[STIX]{x1D6FD})$ and, consequently, the fraction of initial APE that is converted to turbulence during convective instability progressively decreases with increasing $\unicode[STIX]{x1D6FD}$ . For moderate slopes with $\unicode[STIX]{x1D6FD}<10^{\circ }$ , most of the net energy loss takes place during an initial, short ( $Nt\approx 20$ ) interval with periodic convective overturns. For steeper slopes, most of the energy loss takes place during a later, long ( $Nt>100$ ) interval when both shear and convective instability occur, and the energy loss rate is approximately constant. The mixing efficiency during the initial period dominated by convectively driven turbulence is found to be substantially higher (exceeds 0.5) than the widely used value of 0.2. The mixing efficiency at long time in the present problem of a convective overturn at a boundary varies between 0.24 and 0.3.

Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer

The circulation of the Northern Hemisphere extratropical troposphere has changed over recent decades, with marked decreases in extratropical cyclone activity and eddy kinetic energy (EKE) in summer and increases in the fraction of precipitation that is convective in all seasons. Decreasing EKE in summer is partly explained by a weakening meridional temperature gradient, but changes in vertical temperature gradients and increasing moisture also affect the mean available potential energy (MAPE), which is the energetic reservoir from which extratropical cyclones draw. Furthermore, the relation of changes in mean thermal structure and moisture to changes in convection associated with extratropical cyclones is poorly understood. Here we calculate trends in MAPE for the Northern extratropics in summer over the years 1979-2017, and we decompose MAPE into both convective and nonconvective components. Nonconvective MAPE decreased over this period, consistent with decreases in EKE and extratropical cyclone activity, but convective MAPE increased, implying an increase in the energy available to convection. Calculations with idealized atmospheres indicate that nonconvective and convective MAPE both increase with increasing mean surface temperature and decrease with decreasing meridional surface temperature gradient, but convective MAPE is relatively more sensitive to the increase in mean surface temperature. These results connect changes in the atmospheric mean state with changes in both large-scale and convective circulations, and they suggest that extratropical cyclones can weaken even as their associated convection becomes more energetic.

Available Potential Energy in Density Coordinates

AbstractThe vertically integrated potential energy of an incompressible stratified fluid formulated in density coordinates can be simply written as a weighted vertical sum of the squares of the vertical displacements of density surfaces, a general expression valid for arbitrary displacements. The sum of this form of potential energy and kinetic energy is then a conserved quantity for the multilayer shallow water model. The formulation in density coordinates is a natural one to find the Lorenz reference state of available potential energy (APE). We describe the method to compute the APE of an ocean state and provide two applications. The first is the classical double-gyre, wind-driven circulation simulated by a shallow water model at high resolution. We show that the eddy kinetic and eddy potential energies are localized in regions of large gradients of mean APE. These large gradients surround an APE minimum found between the two gyres. The second is the time-mean World Ocean Circulation reconstructed from hydrography (World Ocean Atlas) and reference velocities at 1000 db from the Argo float program to obtain an absolute circulation. The total available potential energy exceeds the total mean kinetic energy of the World Ocean by three orders of magnitude, pointing out the very small Burger number of the circulation. The Gulf Stream, the Kuroshio, the Agulhas retroflection, and the confluence regions are four examples that confirm the shallow water model results that large gradients of mean available potential energy can be used as predictors for the presence of high eddy kinetic energy (obtained here from satellite altimetry).

Projected Significant Increase in the Number of Extreme Extratropical Cyclones in the Southern Hemisphere

Extratropical cyclones are responsible for much of the extreme weather in the midlatitudes; thus, how these cyclones may change under increasing greenhouse gas forcing is of much general interest. Previous studies have suggested a poleward shift in the location of these cyclones, but how the intensity may change remains uncertain, especially in terms of maximum wind speed. In this study, projected changes in extreme cyclones in the Southern Hemisphere, based on 26 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5), are presented. Multiple definitions of extreme cyclones have been examined, including intensity exceeding constant thresholds of sea level pressure perturbations, 850-hPa vorticity, and 850-hPa winds, as well as variable thresholds corresponding to a top-5 or top-1 cyclone per winter month in these three parameters and the near-surface winds. Results presented show that CMIP5 models project a significant increase in the frequency of extreme cyclones in all four seasons regardless of the definition, with over 88% of the models projecting an increase. Spatial patterns of increase are also consistent, with the largest increase projected between 45° and 60°S, extending from the South Atlantic across the south Indian Ocean into the Pacific. The projected increases in cyclone statistics are consistent with those in Eulerian statistics, such as sea level pressure (SLP) variance. However, while the projected increase in SLP variance can be linked to increase in the mean available potential energy (MAPE), the increases in cyclone statistics are not well correlated with those in MAPE.

On the Decadal Variability of the Eddy Kinetic Energy in the Kuroshio Extension

AbstractPrevious studies have found that the decadal variability of eddy kinetic energy (EKE) in the upstream Kuroshio Extension is negatively correlated with the jet strength, which seems counterintuitive at first glance because linear stability analysis usually suggests that a stronger jet would favor baroclinic instability and thus lead to stronger eddy activities. Using a time-varying energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), and the MS-EVA-based nonlinear instability theory, this study investigates the physical mechanism responsible for such variations with the state estimate from the Estimating the Circulation and Climate of the Ocean (ECCO), Phase II. For the first time, it is found that the decadal modulation of EKE is mainly controlled by the barotropic instability of the background flow. During the high-EKE state, violent meanderings efficiently induce strong barotropic energy transfer from mean kinetic energy (MKE) to EKE despite the rather weak jet strength. The reverse is true in the low-EKE state. Although the enhanced meander in the high-EKE state also transfers a significant portion of energy from mean available potential energy (MAPE) to eddy available potential energy (EAPE) through baroclinic instability, the EAPE is not efficiently converted to EKE as the two processes are not well correlated at low frequencies revealed in the time-varying energetics. The decadal modulation of barotropic instability is found to be in pace with the North Pacific Gyre Oscillation but with a time lag of approximately 2 years.

On the response of the Lorenz energy cycle for the Southern Ocean to intensified westerlies

AbstractThe potential impact of intensified westerlies on the Lorenz energy cycle for the Southern Ocean is examined by employing a global eddy‐permitting ocean‐sea ice model. Two idealized sensitivity experiments are designed for this purpose: one is driven by 1992 forcing with weaker westerlies and the other driven by 1998 forcing with stronger westerlies. The intensified westerlies lead to the most significant increase of about 30% in the eddy kinetic energy (EKE) reservoir, followed by the mean kinetic energy (MKE) reservoir increase (17.9%), eddy available potential energy (EAPE) reservoir increase (8.6%), and mean available potential energy (MAPE) reservoir increase (6.5%). In contrast, the increases in the generations of kinetic energy and available potential energy are quite similar, ranging from 21% for EAPE generation to 26% for MKE generation. There are considerably increased energy transfers from MKE to MAPE (about 75%) and from MAPE to EAPE (about 78%), reflecting greatly enhanced baroclinic instability pathway. The conversion rates are strongly influenced by large topography; in particular, a relatively large energy conversion from EKE to MKE exists in the regions associated with large topography, in contrast to the energy flow from MKE to EKE over the broad Southern Ocean. Under stronger wind forcing, all energy conversions are enhanced, and the increases in the conversion rates from EAPE to EKE and from EKE to MKE are more prominent than the increases from MKE to MAPE and from MAPE to EAPE near large topography.

Atmospheric dynamics and internal processes during organization and intensification of Boreal Summer Intraseasonal Oscillation (BSISO) based on TRMM and reanalyses data

ABSTRACTBoreal summer intraseasonal oscillations (BSISOs) show a meridional propagation from the Indian Ocean (IO) region to the continental India. The BSISOs are the main source of rainfall variability over the Indian Summer Monsoon (ISM) region. The recent research has revealed a basic understanding of the scale selection and northward propagation of the BSISOs. But the complete understanding about the organization and intensification of BSISOs remains elusive. This study uses observation and reanalysis data that covers summers (June–September) of 1998–2013 to investigate the dynamical and atmospheric internal processes during the organization and intensification of BSISOs. Based on rainfall analyses, two types of BSISOs are defined, one with weaker and the other with stronger intensity. Stronger BSISOs show persisting lower level moisture convergence to the north of the convection centre (CC) and a strong vertical velocity collocated with CC. The analysis of mean kinetic energy (MKE) budget suggests that MKE significantly increases during the organization of the strong BSISOs but the strength is relatively weaker during the evolution of weak BSISOs. The enhanced MKE during the strong events is supported by the increase of baroclinic energy conversion from mean available potential energy (MAPE) to MKE but the MAPE to MKE conversion is significantly weak during the evolution of weak BSISOs. There is conversion from MKE to eddy kinetic energy (EKE) during organization of strong BSISOs and the conversion increases as the events approach towards the organized and intense phase. The eddy momentum‐vertical wind shear interaction is the significant contributor for conversion of MKE to EKE during the organization of strong BSISOs. The eddy momentum‐mean flow divergence interaction and eddy momentum–vorticity interaction at the lower level support to enhance EKE as the strong BSISOs approach the organized and intense phase. But the above interactions are weak during evolutions of the weak BSISOs.

Storm-Track Shifts under Climate Change: Toward a Mechanistic Understanding Using Baroclinic Mean Available Potential Energy

Abstract Zonal-mean storm-track shifts in response to perturbations in climate occur even in idealized simulations of dry atmospheres with axisymmetric forcing. Nonetheless, a generally accepted theory of the mechanisms controlling the storm-track shifts is still lacking. Here, mean available potential energy (MAPE), a general measure of baroclinicity that is proportional to the square of the Eady growth rate, is used to understand storm-track shifts. It is demonstrated that, in dry atmospheres, the eddy kinetic energy (EKE) in a storm track is linearly related to the mean available potential energy, relative to a local reference state, and that maxima of the two are generally collocated in latitude. Changes in MAPE with climate are then decomposed into components. It is shown that in simulations of dry atmospheres, changes in the latitude of maximum MAPE are dominated by changes in near-surface meridional temperature gradients. By contrast, changes in the magnitude of MAPE are primarily determined by changes in static stability and in the depth of the troposphere. A theory of storm-track shifts may build upon these findings and primarily needs to explain changes in near-surface meridional temperature gradients. The terminus of the Hadley circulation often shifts in tandem with storm tracks and is hypothesized to play an important role in triggering the storm-track shifts seen in this idealized dry context, especially in simulations where increases only in the convective static stability in the deep tropics suffice to shift storm tracks poleward.

Contribution of atmospheric internal processes to the interannual variability of the South Asian summer monsoon

ABSTRACTApart from the external forcings, the interannual variability of the South Asian summer monsoon can arise from atmospheric internal dynamics, although its mechanism has not been fully understood. Based on a three‐dimensional mean kinetic energy (MKE) budget equation, the contributions of internally generated processes associated with mean flow–transient eddy interaction and circulation–heating feedback to the interannual variability of the South Asian summer monsoon were diagnosed using two reanalysis data sets. In years with enhanced convections over South Asia, a strengthened low (high)‐level MKE related to southwesterly (easterly) jet is observed over the Arabian Sea. The enhancement of MKE at the low level is maintained by both the anomalous barotropic energy conversion between eddy kinetic energy (EKE) and MKE and the baroclinic energy conversion from mean available potential energy (MAPE) to MKE. Among the three‐dimensional barotropic energy conversion processes, the vertical eddy momentum flux interacting with the monsoon vertical wind shear anomaly plays a leading role in supporting the MKE anomaly. Moreover, the contribution of eddy activities related to the intraseasonal oscillation (ISO) of 20–90 days is larger than that of high‐frequency (<20 days) disturbances. This mean flow–ISO interaction dominates the atmospheric energy conversion processes and external boundary factors that may exert the interannual variations of monsoonal MKE in the lower troposphere. Different from the low‐level dynamics, scale interaction between eddies and the mean flow shows an insignificant contribution to the strengthening of the easterly jet in the higher troposphere. The Niño3.4 sea surface temperature anomaly appears to be a key factor modulating the interannual variations of the high‐level monsoon easterly.

CMIP5 Projection of Significant Reduction in Extratropical Cyclone Activity over North America

AbstractProjections of storm-track changes over the continental United States and southern Canada made by 23 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) have been compared to changes projected by 11 models from phase 3 of CMIP (CMIP3). Overall, under representative concentration pathway 8.5 (RCP8.5) forcing, CMIP5 models project much more significant decreases in North American storm-track activity than CMIP3 models under the Special Report on Emission Scenarios (SRES) A2 scenario, with the largest decrease in summer and the smallest decrease in spring. The decrease is found both in temporal variance and cyclone statistics, with the frequency of strong cyclones projected to decrease by 15.9%, 6.6%, 32.6%, and 16.9% for winter, spring, summer, and fall, respectively. There is a strong consensus among the 23 models regarding the sign of the projected change, with less than 20% of the models projecting changes in the opposite sign in any of the storm-track parameters examined. Nevertheless, there are also significant model-to-model differences in the magnitude of the projected changes.Projected changes in mean flow baroclinicity have also been examined. Model-to-model differences in the projected storm-track change are found to correlate significantly with model-to-model differences in the projected change in a locally defined mean available potential energy (MAPE) across the ensemble of 34 CMIP5 and CMIP3 models, suggesting that the differences in the projected change in local MAPE can partly account for not only the model-to-model differences but also the differences between CMIP5 and CMIP3 projections. Examination of projected precipitation change suggests that models projecting larger decrease in North American storm-track activity also project a farther northward intrusion of the decrease in subtropical precipitation.

Warming of Western North Pacific Ocean and Energetics of Transient Eddy Activity

Abstract This study examines the increase in the sea surface temperature (SST) in the western North Pacific Ocean (WNPO) during December–February for the period 1959–2008. The relationship of this SST increase with significant interdecadal changes in the baroclinicity and the energetics of transient eddy activity is also examined. These results show that the interannual variations of the WNPO SST and atmospheric conditions including baroclinicity and turbulent heat flux are responses to the upstream atmospheric forcings associated with the East Asian winter monsoon (EAWM). For the interdecadal variations, the intensified baroclinicity downstream of the Pacific storm-track activity is responsible for an increase in the baroclinic energy conversion (BCEC) from the mean available potential energy (MAPE) to the eddy available potential energy (EAPE) to the east of 180°. This in turn increases the BCEC from the EAPE to the eddy kinetic energy (EKE) over this region. The BCEC and generation of EAPE by diabatic heating are enhanced to the east of 180° as a result of the intensified baroclinicity. This could be responsible for the development of transient eddy activity downstream of the Pacific storm track over the North Pacific.

Eddy Energy along the Tropical Storm Track in Association with ENSO

The interaction between the seasonal mean circulation and the transient eddies over the western North Pacific (WNP) during El Nino-Southern Oscillation (ENSO) warm and cold years was investigated by the three- dimensional eddy kinetic energy (EKE) and eddy available potential energy (EAPE) budget equations for total eddy, high-frequency (< 10 days) and low-frequency (20-70 days) components. Composites of the energy results indicate that low-level anomalous cyclonic circulation, westerly jet and ascending motion associated with the east- ward extension of warm SST during warm ENSO years are favorable for eddy barotropic energy conversion (CK) and eddy baroclinic energy conversions (CE). The enhancement of CK and CE might provide kinetic en- ergy for the growth of high- and low-frequency transient eddies including tropical storms (TSs) from the Philip- pine Sea to the date line over the tropical WNP during warm ENSO years. In contrast, high- and low-frequency eddies convert EKE to seasonal mean circulation over the subtropical and mid-latitude WNP during warm years. Enhanced eddy baroclinic energy conversion plays an important role in the maintenance and enhancement of the subsequent development of transient eddies including TSs as they propagate northward. The loss of EAPE to EKE due to the eddy baroclinic energy conversion is mainly supplemented by the gener- ation of EAPE associated with eddy diabatic heating. However, the energy conversion from mean available po- tential energy (MAPE) to EAPE is also important due to the eddy vertical heat transport which is neglected in the two-dimensional EAPE budget equation. It is suggested that high- and low-frequency eddies including TSs may be self-development and intensify through their enhanced diabatic heating and vertical heat transport.

The generation of available potential energy, according to Lorenz’ exact and approximate equations

The generation of available potential energy (APE) in the atmosphere according to Lorenz′ (1955a, b) exact and approximate equations is investigated in the space-time domain. The generation is determined from the fields of diabatic heating and temperature. The heating is evaluated by the residual method, using 4 × daily ECMWF-(European Centre for Medium-Range Weather Forecasts) initialised analyses for the January months of 1986–1992 and the July months of 1985–1991. The contributions of different parts of the atmosphere to the exact and approximate generation are investigated, and the differences between these contributions are explained. In the derivation of the equation of the approximate generation, Lorenz made several approximations. The most serious one appears to be that the average pressure on an isentropic surface is approximately equal to the pressure of the isobaric surface, for which the average potential temperature is equal to the potential temperature of the isentropic surface. Our results show that the largest differences between the contributions to the exact and approximate generation occur in the extratropical low troposphere. Here the contribution to the exact generation of eddy APE is up to 10 × smaller than in the approximation, and the contribution to the exact generation of zonal- and time-mean APE is weakly positive, whereas for the approximation it is strongly negative. Other main differences occur in the tropical middle troposphere where the contribution to the exact generation of mean APE is about two times as small as the approximate value. Despite these large differences, the globally averaged exact generation of mean APE, which is 2.29 W/m 2 in January and 1.88 W/m 2 in July, differs less than 3% from its approximation. The globally averaged exact generation of stationary eddy APE, which is 0.26 W/m 2 in January and 0.45 W/m 2 in July, is about 25% smaller than its approximation. In most individual months the globally averaged exact generation of transient eddy APE is weakly positive (about 0.01 W/m 2 ), whereas the approximation is weakly negative (about – 0.08 W/m 2 ). The globally averaged generation of mean APE is not sensitive to the period and kind of the circulation data. The sensitivity of the globally averaged generation of stationary and transient eddy APE is about 0.1 W/m 2 . DOI: 10.1034/j.1600-0870.1994.t01-3-00002.x

The Physical Oceanography of Two Rings Observed by the Cyclonic Ring Experiment. Part II: Dynamics

Abstract Data from the 1977 Cyclonic Ring Experiment is used to examine the, density, velocity, vorticity and energy distributions in a Gulf Stream cyclonic ring. A time series on ring BOB provides information on the temporal changes in these parameters. The rings are shown to have an interior in near-solid-body rotation surrounded by a frontal jet exhibiting a maximum in potential vorticity. There is evidence that the baroclinic structure of the rings extends to the bottom and that there is significant energy in vertical modes higher than the first baroclinic mode. Calculations of available potential energy (APE) and kinetic energy are presented along with the time rates of change in the APE. The APE is partitioned into the energy in the mean ring and a set of perturbations about the mean. The temporal variations in the perturbation APE are large enough to be important to the adjustment and decay of mean APE in the ring. Ring spindown is discussed in terms of the observations and several ring models prop...