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Shigeo Yoden

Abstract

A new class of vacillations is obtained in the Holton and Mass model with a different bottom boundary condition. The model is a highly truncated spectral model describing wave-zonal flow interactions in a forced-dissipative system. The mean zonal wind and the wave change their vertical structures periodically with a period of the wave progrezsion (5-10 days for the parameters used in this study). The vacillations are interpreted as an interference between a stationary wave and a topographically modified Rossby wave. The modified Rossby wave is an eigenmode of baroclinic flow in the presence of bottom topography within the framework of the highly truncated system. Time variations of the mean zonal wind are essential for the modification of the Rossby wave.

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Shigeo Yoden

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 Powell's 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 solutions with different vertical structures.

Periodic solutions a series of stratospheric vacillations originally found by Holton and Mass, are obtained by time-integrations. It is found that the periodic solutions branch off from a steady solution by a Hopf bifurcation. For a finite increment of the parameter from the bifurcation point, the time average of the periodic solution is significantly different from the unstable steady solution. The nonlinear transience causes the difference.

The multiplicity of stable solutions (steady and periodic) is a possible explanation for the interannual variability of the stratosphere circulation in the middle and high latitudes during winter.

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Shigeo Yoden

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.

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Shigeo Yoden

Abstract

Dynamics of stratospheric vacillations, which were first obtained by Holton and Mass in a highly truncated spectral model, are investigated with an EP flux diagnosis based on the transformed Eulerian-mean equations. The vacillations, which are purely periodic solutions under constant external conditions, can be divided into a dynamically active period and an inactive period. Dynamics controlling the variations are different between the two periods.

During the active period the mean zonal winds vary vigorously like a stratospheric sudden warming. The variation is mainly due to the wave driving, although a large part of it is canceled by the Coriolis torque of the "residual" circulation. The transience term is important in all of the layers to determine the wave driving, while the dissipation term has a comparable magnitude in the upper layers. The wave transience is directly associated with vertical propagations of the wave-activity pulse, which is generated not at the bottom boundary, but inthe interior.

On the other hand, during the inactive period, easterly winds and negative values of the meridional gradient of the zonal mean potential vorticity appear in the lower layers within 10-30 km. These layers prevent the wave from propagating upward. The mean zonal winds above the layers are accelerated gradually by the Coriolis torque of the "residual" circulation due to the diabatic heating.

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Yasuko Hio and Shigeo Yoden

Abstract

Dynamical features of the interannual variations of the seasonal march in the Southern Hemisphere stratosphere are investigated with the NCEP–NCAR reanalysis dataset from 1979 to 2002, and the unprecedented year 2002, in which a major stratosphere sudden warming occurred, is characterized by comparing it with the other 23 yr.

A multiple empirical orthogonal function analysis of the stratospheric mean zonal wind and a composite analysis based on the principal component of the leading mode show that the interannual variations are characterized by early or late deceleration of the polar-night jet, which is well correlated with the variation of a time-averaged upward Eliassen–Palm (EP) flux in the lower stratosphere. The stronger wave activity in the lower stratosphere is associated with the earlier “shift down” of the jet. The composite difference of the stratospheric mean zonal wind can be traced down to the lower troposphere in September and October. These features are consistent with the variations of the Southern Hemisphere annular mode, although the main disturbance to maintain the variations is different between the stratosphere and troposphere.

Some scatter diagrams show the extreme situation of the year 2002. It is far from the cluster of the other 23 yr, but the large deviation in 2002 is consistent with the tendency of the fluctuations in the other years except for its extreme nature.

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Shigenori Otsuka and Shigeo Yoden

Abstract

The temporal–spatial distribution of thin moist layers in the midtroposphere over the tropical eastern Pacific is studied by data analyses of radiosonde soundings and downscaling numerical experiments with a regional model. Radiosonde soundings at San Cristóbal, Galápagos, show frequent existence of thin moist layers between 2 and 10 km in altitude, with a local minimum at 7–8 km. The downscaling experiments with global objective analyses are completed for 2005–06, September and December of 1999–2004, and March of 2000–04. The vertical distribution of thin moist layers has three local maxima at 5, 10, and 16 km, where bimodality of the frequency distribution of water vapor is evident. Between 4 and 7 km, an annual variation is dominant in the occurrence ratio of thin moist layers, which tend to appear in nonconvective regions. In boreal winter, the layers appear to the north of the intertropical convergence zone (ITCZ), whereas in boreal summer the layers appear in the equator-side of the ITCZ. Interannual variations of the appearance of thin moist layers are also studied in 1999–2006, based on the experiments for particular months (March, September, and December). The occurrence ratio is generally high in December and March and low in September. In La Niña years, the annual variation is smaller than that in El Niño years; the occurrence ratio is higher in boreal summer to the south of the ITCZ.

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Koji Akahori and Shigeo Yoden

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 confirmed by two experimental runs with a different intensity of the surface drag. For the low-drag run, the zonal-mean jet is located in high latitudes through the integration period and life cycles of baroclinic eddies are basically characterized by the anticyclonic breaking. For the high-drag run, on the other hand, the zonal-mean jet is located in low latitudes and life cycles of baroclinic eddies are characterized by the cyclonic breaking.

Although these two types of breaking pattern are similar to the two paradigms of baroclinic wave life cycles obtained in some idealized one-shot experiments, there are some differences from the one-shot experiments in the deformation field on an isentropic surface and in the relative location between the zonal-mean jet and the latitude of maximum eddy kinetic energy.

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Takeshi Horinouchi and Shigeo Yoden

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 oscillation is accounted for by the gravest symmetric gravity modes: eastward momentum by Kelvin waves and westward momentum by n = 1 gravity waves. The momentum deposition is done over a wide range of zonal wavenumber (2–30), while about half of it is done over a period of 1–3 days. The deposition has rather continuous phase speed distributions and a considerable portion of it is provided by waves having critical levels. Since gravity waves with small intrinsic phase speeds have small vertical wavelengths, vertical grid spacings of 700 m or less appear to be required in the lower stratosphere for GCMs in order to simulate the QBO.

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Eriko Nishimoto and Shigeo Yoden

Abstract

Influence of the stratospheric quasi-biennial oscillation (QBO) on the Madden–Julian oscillation (MJO) and its statistical significance are examined for austral summer (DJF) in neutral ENSO events during 1979–2013. The amplitude of the OLR-based MJO index (OMI) is typically larger in the easterly phase of the QBO at 50 hPa (E-QBO phase) than in the westerly (W-QBO) phase. Daily composite analyses are performed by focusing on phase 4 of the OMI, when the active convective system is located over the eastern Indian Ocean through the Maritime Continent. The composite OLR anomaly shows a larger negative value and slower eastward propagation with a prolonged period of active convection in the E-QBO phase than in the W-QBO phase. Statistically significant differences of the MJO activities between the QBO phases also exist with dynamical consistency in the divergence of horizontal wind, the vertical wind, the moisture, the precipitation, and the 100-hPa temperature. A conditional sampling analysis is also performed by focusing on the most active convective region for each day, irrespective of the MJO amplitude and phase. Composite vertical profiles of the conditionally sampled data over the most active convective region reveal lower temperature and static stability around the tropopause in the E-QBO phase than in the W-QBO phase, which indicates more favorable conditions for developing deep convection. This feature is more prominent and extends into lower levels in the upper troposphere over the most active convective region than other tropical regions. Composite longitude–height sections show similar features of the large-scale convective system associated with the MJO, including a vertically propagating Kelvin response.

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Shigeo Yoden and Michio Yamada

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 of the flow field is characterized by Rossby wave motions.

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