Shigeo Yoden

An update of observed stratospheric temperature trends

An updated analysis of observed stratospheric temperature variability and trends is presented on the basis of satellite, radiosonde, and lidar observations. Satellite data include measurements from the series of NOAA operational instruments, including the Microwave Sounding Unit covering 1979–2007 and the Stratospheric Sounding Unit (SSU) covering 1979–2005. Radiosonde results are compared for six different data sets, incorporating a variety of homogeneity adjustments to account for changes in instrumentation and observational practices. Temperature changes in the lower stratosphere show cooling of ∼0.5 K/decade over much of the globe for 1979–2007, with some differences in detail among the different radiosonde and satellite data sets. Substantially larger cooling trends are observed in the Antarctic lower stratosphere during spring and summer, in association with development of the Antarctic ozone hole. Trends in the lower stratosphere derived from radiosonde data are also analyzed for a longer record (back to 1958); trends for the presatellite era (1958–1978) have a large range among the different homogenized data sets, implying large trend uncertainties. Trends in the middle and upper stratosphere have been derived from updated SSU data, taking into account changes in the SSU weighting functions due to observed atmospheric CO2 increases. The results show mean cooling of 0.5–1.5 K/decade during 1979–2005, with the greatest cooling in the upper stratosphere near 40–50 km. Temperature anomalies throughout the stratosphere were relatively constant during the decade 1995–2005. Long records of lidar temperature measurements at a few locations show reasonable agreement with SSU trends, although sampling uncertainties are large in the localized lidar measurements. Updated estimates of the solar cycle influence on stratospheric temperatures show a statistically significant signal in the tropics (∼30°N–S), with an amplitude (solar maximum minus solar minimum) of ∼0.5 K (lower stratosphere) to ∼1.0 K (upper stratosphere).

A Numerical Experiment on Two-Dimensional Decaying Turbulence on a Rotating Sphere

Abstract A series of numerical experiments on the decaying two dimensional turbulence is performed for a nondivergent barotropic fluid on a rotating sphere by using a high-resolution spectral model with a triangular truncation of T85. Temporal variations of the total kinetic energy, the total enstrophy, and the enstrophy dissipation rate are found to be influenced by both the spherical geometry and the rotation rate. The energy spectrum is different from that in the β-plane experiments with Cartesian geometry. Morphology of streamfunction and vorticity fields is investigated for several rotation rates. In nonrotational cases, isolated coherent vortices emerge in the course of time development as in the planar 2D turbulence. As the rotation rate increases, however, the temporal evolution of the flow field changes drastically, and an easterly circumpolar vortex appears in high latitudes. The flow field is then anisotropic in all the latitudes and elongated in the longitudinal direction. Temporal evolution o...

Wave–Mean Flow Interaction Associated with a QBO-like Oscillation Simulated in a Simplified GCM

Abstract The interaction between convectively excited waves and the mean zonal wind in the equatorial lower stratosphere is investigated with a simplified general circulation model (GCM). The model has T42 truncation, and the vertical resolution is about 700 m in the stratosphere. Although it is an “aquaplanet” model with uniform sea surface temperature, cumulus convection in low latitudes has realistic hierarchical structures with reasonable space–time spectral distributions. The model produced an oscillation having quite similar features to the equatorial quasi-biennial oscillation (QBO), although the period is 400 days. Waves in the equatorial lower stratosphere of the model are excited mainly by the cumulus convection in low latitudes. The energy of these waves is a little larger than that observed in the real atmosphere. The dominant waves are gravity waves having an equivalent depth of about 200 m and those of 40–100 m. About half of the transport and deposition of zonal momentum contributing to the...

Bifurcation Properties of a Stratospheric Vacillation Model

Abstract Nonlinear properties of a stratospheric vacillation model are investigated numerically in the light of bifurcation theory. The model is exactly the same as that used by Holton and Mass, which describes the wave-zonal flow interaction in a β-channel under a nonconservative constraint with zonal-flow forcing and wave dissipation. A set of 81 nonlinear ordinary differential equations with variables depending on time is obtained by a severe truncation and vertical differencing. All of the external parameters are fixed in time. The amplitude of the wave forcing or the intensity of zonal wind forcing at the bottom boundary is changed as a bifurcation parameter. Three branches of the steady solutions are obtained by use of Powells hybrid method and the pseudo-arclength continuation method. Linear stability of these solution branches is investigated by solving an eigenvalue problem in the linearized system. In some range of the bifurcation parameter, there exists a multiplicity of stable steady solution...

Formation of zonal band structure in forced two-dimensional turbulence on a rotating sphere

A series of numerical experiments on the forced two-dimensional turbulence on a rotating sphere were done to investigate the formation processes of zonal band structures and their sensitivity to two experimental parameters of the rotation rate and the forcing wave number. A high-resolution barotropic full spherical model of T199 truncation is used with a homogeneous and isotropic formulation of the vorticity forcing function. Three different flow regimes are obtained and one of them is a new regime previously unknown. In the cases of no rotation, a very large flow pattern is obtained as a result of the upward energy cascade to the lowest wave number. The pattern irregularly fluctuates with time. A zonal band structure that consists of alternating easterly and westerly jets becomes dominant with an increase in the rotation rate. The alternating jets, which are robust and persistent, are already formed to be discernible in the very early stage of the time integration. The width of the jets decrease and the ...

Finite-Time Lyapunov Stability Analysis and Its Application to Atmospheric Predictability

Abstract Finite-time Lyapunov stability analysis is reviewed and applied to a low-order spectral model of barotropic flow in a midlatitude β channel. The tangent linear equations of the model are used to investigate the growth of small perturbations superposed on a reference solution for a prescribed time interval. Three types of reference solutions of the model, stationary, periodic, and chaotic, are investigated to demonstrate usefulness of the analysis in the study of the atmospheric predictability problem. The finite-time Lyapunov exponents, which give the growth rate of small perturbations, depend upon the reference solution as well as the prescribed time interval. The finite-time Lyapunov vector corresponding to the largest Lyapunov exponent gives the streamfunction field of the fastest growing perturbation for the time interval. In the case of the chaotic reference solution, the streamfunction field has large amplitudes in limited areas for a small time interval. The areas of the large perturbation...

Medium-Range Forecast Skill Variation and Blocking Transition. A Case Study

Abstract Temporal variability in skill of operational medium-range forecasts in the winter of 1988/89 is examined. The primary concern is to examine the performance of the Japan Meteorological Agency (JMA) global spectral model, which began daily 8-day forecasts in 1988. The models forecast skill exhibits considerable low-frequency (∼a week or longer) temporal variability. During the period under study, root-mean-square error (rmse) of 500-Mb height showed a pronounced temporal maximum at the end of January 1989, when the atmosphere over the North Pacific underwent a remarkable transition from zonal to blocked circulation. The consecutive forecasts during this period showed large dispersion among one another. A linear measure of forecast spread, or phase-space divergence of atmospheric trajectories, is also evaluated after Lorenz using a nondivergent barotropic model. It shows a temporal maximum concurrent with rmse. Therefore, it is likely that the zonal-to-blocking transition was associated with higher...

Zonal Flow Vacillation and Bimodality of Baroclinic Eddy Life Cycles in a Simple Global Circulation Model

Abstract A global primitive-equation model of the atmosphere is used to study the relationship between the temporal variations of the zonal mean zonal flow and baroclinic eddies. Nonperiodic low-frequency vacillation of the mean zonal flow is found in longtime integrations of the model under a perpetual condition; the zonal-mean jet in the extratropics changes its position nearly barotropically. A potential vorticity–potential temperature (PV–θ) analysis is performed for two extreme periods of the zonal flow vacillation. Anticyclonic breakings of upper troughs are dominant in the period of a high-latitude jet, while cyclonic breakings are dominant in the period of a low-latitude jet. A statistically significant relationship between the zonal flow vacillation and the morphology of life cycles of baroclinic eddies is obtained for the entire period analyzed. An index of the life cycles, which is introduced in this study, shows clear bimodality in its frequency distribution function. The relationship is also ...

An Illustrative Model of Seasonal and Interannual Variations of the Stratospheric Circulation

Abstract A simple wave-zonal flow interaction model, originally developed by Holton and Mass, is used to illustrate a rudimentary conception of seasonal and interannual variations of the stratospheric circulation. The radiative heating is varied periodically with an annual component to investigate the response of the circulation (i.e., the seasonal variation) to the periodic forcing. The response is qualitatively different depending on the wave forcing from the troposphere. The difference resembles that of the climatological seasonal march between the Northern and the Southern hemispheres. No example of interannual variations (namely, nonperiodic responses) was obtained for the periodic annual forcing. Interannual variation of external conditions is necessary in the present model to obtain interannual variations. A finite range of year-to-year variations of the wave forcing can produce large interannual variability as in the Northern Hemisphere.

Internal Variability of the Troposphere–Stratosphere Coupled System Simulated in a Simple Global Circulation Model

Internal variability of a troposphere‐stratosphere coupled system is investigated in a series of numerical experiments with a simple global circulation model under a perpetual winter condition. In order to examine the relative importance of forced planetary waves in the interaction with the zonal mean zonal flow and baroclinic disturbances, amplitude of a sinusoidal surface topography of zonal wavenumber 1 or 2 is swept as an experimental parameter; 1000-day integrations are performed for 110 combinations of the external parameters. Intraseasonal variability of the extratropical winter stratosphere in the parameter sweep experiment of the zonal wavenumber-1 topography is classified into four regimes, dependent on the topographic amplitude: (I) a nearly radiative equilibrium state; (II) small undulations of the polar vortex; (III) intermittent breakdowns of the polar vortex, or occurrence of stratospheric sudden warming events; and (IV) a usually weak and warm state of the polar vortex. The behavior of planetary waves in the stratosphere is characterized by linear propagation in regime I, and by quasi-linear, or weakly nonlinear, wave‐mean flow interaction in regime II. In regimes III and IV, on the other hand, it is in a highly nonlinear state, reflecting the occurrence of sudden warming events and the usually distorted polar vortex, respectively. The Northern Hemisphere winter stratosphere corresponds to regime III in the intraseasonal variations, while the southern counterpart is close to regime II. A lag-correlation analysis shows that the dynamical linkage between the stratosphere and the troposphere is also dependent on the regimes. The vertical linkage is primarily the upward control in regime I; planetary waves are generated by nonlinear interaction of baroclinic disturbances in the troposphere and propagate into the stratosphere. In regime II, stratospheric variations are largely confined in the stratosphere, although planetary waves are generated in the troposphere. The linkage in regime III, on the other hand, is inevitably two-way. Planetary wave variability has upward influence from the troposphere to the stratosphere as well as the zonal mean zonal wind shows preconditioning in the troposphere before stratospheric sudden warming events. Downward propagation of signals to the upper troposphere are also seen in the zonal mean temperature in high latitudes; it is higher than normal at the final stage of the sequence of sudden warming events. The linkage is also two-way in regime IV.