Increase In Eddy Kinetic Energy Research Articles

Westerly Wind Bursts and Their Relationship with Intraseasonal Variations and ENSO. Part II: Energetics over the Western and Central Pacific

Abstract The mechanism of synoptic-scale eddy development in the generation of westerly wind bursts (WWBs) over the western–central Pacific, and their relationship with the El Niño–Southern Oscillation (ENSO) and the Madden–Julian oscillation (MJO), were examined. In the WWB occurrences, barotropic structures of equatorial eddy westerlies with cyclonic disturbances were found from the surface to the upper troposphere. The dominant contributions to substantial eddy kinetic energy (EKE) were the barotropic energy conversion (KmKe) in the lower and middle tropospheres and the conversion from eddy available potential energy (PeKe) in the upper troposphere. Low-frequency environmental westerlies centered near the equator preceded strong zonal convergence and meridional shear, resulting in the substantial KmKe. The activation of synoptic convection also contributed to an increase in EKE through PeKe. These energies were redistributed to the lower-equatorial troposphere through energy flux convergence (GKe). These results showed that environmental fields contribute to the EKE increase near the equator and are important factors in WWB occurrences. Next, eddy growth was compared under different phases of MJO and ENSO. The MJO westerly phases of strong MJO events were classified into two groups, in terms of ENSO phases. Higher EKE values were found over the equatorial central Pacific in the WWB–ENSO correlated (pre–El Niño) periods. The energetics during these periods comported with those of the WWB generations. In the uncorrelated periods, the enhancement of eddy disturbances occurred far from the equator near the Philippines, where the activities of the easterly wave disturbances are well known. It is noteworthy that the enhanced region of the disturbances in the pre–El Niño periods coincided with the vicinity of large-scale MJO convection. It is suggested that coincidence corresponds with an enhancement of the internal disturbances embedded in the MJO, which is found only when the environmental conditions are favorable in association with ENSO.

The Physical Properties of the Atmosphere in the New Hadley Centre Global Environmental Model (HadGEM1). Part II: Aspects of Variability and Regional Climate

Abstract The performance of the atmospheric component of the new Hadley Centre Global Environmental Model (HadGEM1) is assessed in terms of its ability to represent a selection of key aspects of variability in the Tropics and extratropics. These include midlatitude storm tracks and blocking activity, synoptic variability over Europe, and the North Atlantic Oscillation together with tropical convection, the Madden–Julian oscillation, and the Asian summer monsoon. Comparisons with the previous model, the Third Hadley Centre Coupled Ocean–Atmosphere GCM (HadCM3), demonstrate that there has been a considerable increase in the transient eddy kinetic energy (EKE), bringing HadGEM1 into closer agreement with current reanalyses. This increase in EKE results from the increased horizontal resolution and, in combination with the improved physical parameterizations, leads to improvements in the representation of Northern Hemisphere storm tracks and blocking. The simulation of synoptic weather regimes over Europe is also greatly improved compared to HadCM3, again due to both increased resolution and other model developments. The variability of convection in the equatorial region is generally stronger and closer to observations than in HadCM3. There is, however, still limited convective variance coincident with several of the observed equatorial wave modes. Simulation of the Madden–Julian oscillation is improved in HadGEM1: both the activity and interannual variability are increased and the eastward propagation, although slower than observed, is much better simulated. While some aspects of the climatology of the Asian summer monsoon are improved in HadGEM1, the upper-level winds are too weak and the simulation of precipitation deteriorates. The dominant modes of monsoon interannual variability are similar in the two models, although in HadCM3 this is linked to SST forcing, while in HadGEM1 internal variability dominates. Overall, analysis of the phenomena considered here indicates that HadGEM1 performs well and, in many important respects, improves upon HadCM3. Together with the improved representation of the mean climate, this improvement in the simulation of atmospheric variability suggests that HadGEM1 provides a sound basis for future studies of climate and climate change.

The processes governing horizontal resolution sensitivity in a climate model

One of the questions that climate modellers should address is whether their models have sufficient spatial resolution to resolve the physical processes affecting climate. This study addresses this question using the Hadley Centre climate model, HadAM3 (Hadley Centre Atmospheric climate Model version 3, the climate version of the Met Office's Unified Model). The model is run in AMIP2 (Atmospheric Model Intercomparison Project number 2) mode at four resolutions ranging from N48 (2.5° × 3.75°) to N144 (0.833° × 1.25°). The convergence of the model on increasing resolution is evaluated, and the processes leading to resolution sensitivity are investigated in some detail. A parallel set of four dynamical core integrations give an indication of the sensitivity of the dynamics with simple physical parametrization feedback. Increments from individual parametrization schemes during short 'spin-up' integrations with full physics are used to diagnose the sensitivity of individual schemes. The dependency of the results on particular details of the model are also investigated to see how general the results are. Model biases are reduced when resolution is increased. In particular, the troposphere warms, the jets shift polewards, mean sea level pressure over the poles falls, and transient vertical velocity and eddy kinetic energy increase. Some features of the model converge while a number of significant features do not. Non-linearity in both the hydrological cycle and the dynamics play an important role in the lack of convergence.

Local Spectral Energetics Analysis of the Formation of Pacific Blocking Anticlones using an Orthonormal Wavelet Expansion.

In this study, an orthonormal wavelet expansion along a latitudinal circle is applied to decompose atmospheric data into a series of wavelet components with different spatial scales, and longitudinal locations. The application of the local spectral energetics allows the investigation of the local energy transformations for the local structures in the atmosphere. By applying the local spectral energetics analyses to 10 events of the Pacific blocking, we find that different energy conversion terms contribute to the increase of eddy kinetic energy during the development of the blocking events. For 8 out of 10 blocking events, the gain of energy through the wave-wave interaction of eddy kinetic energy is the most important process for the onset of blocking. The intensified trough of the blocking scale which is located at the western side of the blocking and the synoptic disturbances around the blocking, especially the western side, to convert eddy kinetic energy into the other components through the wave-wave interaction. The local spectral energetics analysis demonstrates that the wave-wave interaction of the eddy kinetic energy of the planetary-scale, and synoptic disturbances, at the western side of the blocking anticyclone plays an important role for the formation, and development of the atmospheric blocking. The behavior of the wave-wave interaction of the eddy kinetic energy in terms of the scale and longitudinal location is essentially consistent with the eddy straining hypothesis at the western side of the blocking.

Northern Arabian Sea variability from TOPEX/Poseidon altimetry data: an extension of the US JGOFS/ONR shipboard ADCP study

Sea-level anomalies (SLA) derived from the TOPEX/Poseidon (T/P) altimetry and inferred geostrophic currents within the northern Arabian Sea were examined for the period from March 1993 through November 1996. The primary objective of this study was to confirm and extend our understanding of the upper-ocean mesoscale variability observed in the shipboard acoustic Doppler current profiler (ADCP) data collected during the US JGOFS/ONR Arabian Sea Expedition (September 1994–January 1996). The accuracy of the T/P altimetry data (∼3 cm rms) results in an uncertainty in the altimeter-derived velocities comparable to the ADCP measurement error. Thus the T/P data provide a reasonable method for extending studies of the mesoscale dynamics for the region. Comparison of the T/P-derived geostrophic velocities with concurrent ADCP data showed good correlation, with an r 2 between 0.7 and 0.9 and rms differences of 10 cm s −1. The T/P data confirm both the overall spatial and seasonal current patterns observed by the ADCP. The monsoonally averaged rms sea-level anomalies indicate a high degree of intraseasonal variation due to the generation of squirts, jets and eddies all along the coast, the variability of which increases in both intensity and areal extent during the Southwest Monsoon. The SLA data indicate a much reduced degree of variability over the shelf. The SLA-derived eddy kinetic energy (EKE) fields are consistent with those derived earlier from the ADCP data in both distribution and magnitude. There is a large increase in EKE to the west and southwest and to a lesser extent to the south and a large area of relatively reduced eddy activity over much of the eastern and northern Arabian Sea. The area of reduced eddy activity coincides with the location of the most intense portions of the oxygen minimum zone found in the northern Arabian Sea. The spatial scales of the eddies responsible for the EKE distribution over the study area range between 200 and 500 km in the nearshore region, decreasing to 100–200 km offshore. While there is significant energy variability in annual and semi-annual time scales, a substantial portion of the energy is found between 50 and 120 days, and the relative importance of this frequency band increases offshore. Spectra indicate a distinct break in the frequency content of the eddy field at about 15°N, with little energy at less than annual periods south of this latitude.

On the effects of the damping mechanisms in an atmospheric general circulation model

A version of ECMWF's first spectral model with a comprehensive physical parameterization package has been used for long-term integrations in perpetual January mode. The model has 9 sigma-levels in the vertical and the expansion of the horizontal fields are truncated at total wavenumber n = 31. In order to simulate the observed January climate both in terms of mean-fields as well as the spectral distribution of the kinetic energy as accurately as possible, a series of experiments has been performed with varying parameterizations of some physical processes, especially the damping mechanisms. The inclusion of a simple gravity wave drag is found to have a substantial impact on the simulations. The position and depth of the mean sea level quasi-stationary low pressures in the northern hemisphere is more in accordance with their observed counterparts with the gravity wave drag included, and the excessive stratospheric polar night jet, which is developed in the model without the drag, is effectively controlled by this parameterization. The sensitivity due to changes in the vertical diffusion of momentum, heat and moisture has been considered by crude variations in the mixing length. It turns out that even large changes in the value of the mixing length have a quite modest influence on the mean fields simulated by the model. The horizontal diffusion is necessary in order to represent the effect of the unresolved scales on the explicitly predicted scales. The parameterization of this effect is incorporated in the model as a linear term utilizing that in a spectral formulation, a linear diffusion can easily be formulated with any scale dependency. It is found that an increase of the diffusion in the smallest scales may result in an increase of eddy kinetic energy of the larger scales and even an increase in the total eddy kinetic energy. Moreover it appears that it is quite possible to formulate the linear diffusion of the model to simulate the observed spectral variations of kinetic energy for the medium and smaller scales. DOI: 10.1111/j.1600-0870.1989.tb00390.x

On the effects of the damping mechanisms in an atmospheric general circulation model

A version of ECMWF's first spectral model with a comprehensive physical parameterization package has been used for long-term integrations in perpetual January mode. The model has 9 sigma-levels in the vertical and the expansion of the horizontal fields are truncated at total wavenumber n = 31. Inorder to simulate the observed January climate both in terms of meanfields as well as the spectral distribution of the kinetic energy as accurately aspossible, a series of experiments has been performed with varying parameterizations of some physical processes, especially the damping mechanisms. Theinclusion of a simple gravity wave drag is found to have a substantial impact on the simulations. The position and depth of the mean sea level quasi-stationary low pressures in the northern hemisphere is more in accordance with their observed counterparts with the gravity wave drag included, and the excessive stratospheric polar night jet, which is developed in the model without the drag, is effectively controlled by this parameterization. The sensitivity due to changes in the vertical diffusion of momentum, heat and moisture has been considered by crude variations in the mixing length. It turns out that even large changes in the value of the mixing length have a quite modest influence on the mean fields simulated by the model. The horizontal diffusion is necessary in order to represent the effect of the unresolved scales on the explicitly predicted scales. The parameterization of this effect is incorporated in the model as a linear term utilizing that in a spectral formulation, a linear diffusion can easily be formulated with any scale dependency. It is found that an increase of the diffusion in the smallest scales may result in an increase of eddy kinetic energy of the larger scales and even an increase in the total eddy kinetic energy. Moreover it appears that it is quite possible to formulate the linear diffusion of the model to simulate the observed spectral variations of kinetic energy for the medium and smaller scales.

The kinetic energy budget over the West Pacific during dominant trade wind and active monsoon regimes

From upper-air network data and NMC tropical wind analyses over the West Pacific . during July 1972 and July 1975, the kinetic energy balance in large-scale disturbances and in the general flow of the tropical circulation was calculated. During July 1972, when an active monsoon and its attendant intertropical convergence zone (ITCZ) dominated much of the tropical West Pacific and five typhoons and a depression formed in the area under study, cross-isobaric generation of the eddy kinetic energy took place in the upper (300 to 100 mb) and lower troposphere (1000 to 700 mb), with weak dissipation at middle levels. The total amount of eddy kinetic energy produced in the area during July 1972 was five times greater than during July 1975, when strong southeast trade winds prevailed over most of the West Pacific and no typhoons formed in that area. In the 1972 case, a significant portion of the generated eddy kinetic energy was exported out of the region, whereas in the 1975 case the area imported a considerable amount of eddy kinetic energy to compensate for dissipation mainly in the upper troposphere. For the month with an active monsoon regime the eddy conversion from available potential to kinetic energy, -[ω*α*], was large, with a maximum in the layer 500 to 200 mb, due to the strong convective warming, whereas for the month with a prevailing trade-wind regime only weak positive eddy conversion (increase of eddy kinetic energy) took place in the lower troposphere, with a considerable portion of the troposphere being dominated by negative eddy conversion.

North Atlantic Subtropical Gyre: SOFAR Floats Tracked by Moored Listening Stations

In 1980, SOFAR (sound fixing and ranging) floats were tracked acoustically in the western North Atlantic entirely by means of moored autonomous listening stations. During a 5-month period 17 float trajectories were obtained in the eastern (45 degrees to 65 degrees W) Gulf Stream and subtropical gyre interior at depths of 700 and 2000 meters. These mid-depth trajectories suggest a time-varying Gulf Stream with instances of both a narrow, swift, westward recirculation south of the stream and a northeastward penetration into the Newfoundland Basin. A hundredfold increase of eddy kinetic energy was observed at 2000 meters from the gyre interior (south of 30 degrees N) to the Gulf Stream.

Energetics of a developing wave in the westerlies

The terms expressing conversion between eddy kinetic and zonal kinetic energy and between eddy available and eddy kinetic energy are studied, in order to establish the contribution of barotropic and baroclinic processes respectively to the increase of eddy kinetic energy in an amplifying disturbance. After specification of initial fields the tendencies at time t = 0 are solved analytically using the quasi-geostrophic equations of motion and the ?-equation. Comparisons between this method, which depends on the specification of initial fields, and the ordinary eigen-value method are made, and it is found that the results are similar. The problem of solving the combined barotropic-baroclinic case is complicated using the eigen-value method, but investigation of the initial tendencies of the energy conversion terms gives an insight into this problem in an easy way. DOI: 10.1111/j.2153-3490.1973.tb01591.x

Numerical Simulation of the Breakdown of a Polar-Night Vortex in the Stratosphere

Abstract With a 9-level general circulation model, all attempt was made to simulate numerically the breakdown of the circumpolar vortex in the winter stratosphere for the case of March 1965. The marching computations were started 2 and 5 days prior to the breakdown. The simulation of the vortex elongation and destruction was, to a certain extent, successful, but the split vortex in the prediction erroneously merged again after 8 days. The sudden warming was not simulated at all. The development of the Aleutian high associated with the vortex breakdown was not well computed. Studies are made on zonally averaged quantities pertaining to the tropospheric and stratospheric circulations and their coupling. The increase of eddy kinetic energy at the time of the amplification of zonal wavenumber 2 is discussed both for the numerical simulation and for the observed fields. It is reconfirmed that the eddy kinetic energy in the stratosphere is primarily supplied from below in the form of vertical flux of geopotenti...

EFFECTS OF THE SCALE OF SPATIAL AVERAGING ON THE KINETIC ENERGIES OF SMALL-SCALE TURBULENT MOTION

Abstract The variations of the mean-motion kinetic energy and the eddy kinetic energy with scale of averaging are studied in a laboratory experiment of small-scale turbulence. Velocities and velocity products are spatially averaged for four different scales, and the energy terms are computed for each scale. It is found that, for increasing scale of averaging, there is a percentage decrease of mean-motion kinetic energy and a percentage increase of eddy kinetic energy.