Low-frequency oscillations appearing in three GCM seasonal cycle integrations are compared with the analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF). All three models have the same resolution: 4 degrees latitude by 5 degrees longitude, with 9 levels. The GLAS GCM simulates a realistic eastward propagation of the 30-60 day oscillation in the tropical upperlevel divergent flow. The eastward travelling planetary scale structure becomes more stationary over the Indonesian region and accelerates over the central Pacific, as observed. In the GLA GCM, the oscillation propagates into the higher latitudes of both hemispheres as the waves leave the convective region. The presence of the eastward propagating oscillation is not obvious in the UCLA GCM. The wavenumber-frequency spectra of the 200 mb velocity potential reveal that all the GCMs have a significantly weaker signal for eastward propagation in the 30-60 day range than the analyses. The spectrum for the GLAS GCM is dominated by 20-60 day periods, while the GLA GCM has a spectral peak around 20-30 days. There is a weak eastward propagating peak near 15 days in the UCLA GCM. The dominant phase speeds and the different vertical structures of the heating profiles in the GCMs are in general agreement with current theory involving the positive feedback between latent heating and moist static stability. The composited patterns of the observations indicate that in the tropics a Kelvin wave-type structure is dominant near the center of the oscillation. The simulated winds are fairly realistic, although the meridional component is too strong, especially in the GLA GCM. The vertical structures of the zonal wind component and moisture suggest that a mobile wave-CISK (Lau and Peng, 1987) is an important mechanism in maintaining the intraseasonal oscillation in these GCMs. The vertical distribution of the moisture field further suggests that evaporation-wind feedback (Neelin, et al., 1987) may play a role in maintaining the eastward propagating tropical waves. The differences in the structure of the oscillation in the GLAS GCM and GLA GCM appear to be a consequence of the different numerical schemes used. The GCMs have preferred zones for diabatic heating, with a turn-on heating occurring when the rising branch of the intraseasonal oscillation passes over these convective regions. All three GCMs fail to capture the detailed evolution in the different stages of the development and decay of the oscillation. The results suggest that an improvement in the boundary layer moisture processes may be crucial for a better simulation of the oscillation.
Numerical experiments have been employed to simulate the "extended sea breeze (ESB)" over the Kanto Plain in a three-dimensional mesoscale model. Through some observations in the field, it has been found that there are two characteristics of ESB which distinguish ESB from a typical sea breeze. (1) The thickness of the ESB is 1-1.5 km, which is almost two times the thickness of a typical sea breeze. (2) The wind direction throughout the Kanto Plain shifts northward in the early afternoon. The numerical experiments reproduced these two characteristics rather well in both cases with no general wind and with a SW general wind (5 m/s). Some additional experiments with artificial topography indicated that when there was no general wind an ESB-like wind was generated from the combination of the sea breeze and the thermally and orographically induced flow over the mountainous area of the central part of Japan. However, when there was a SW general wind, another type of ESBlike wind was formed in combination with the general wind and the thermally and orographically induced flow. It was very difficult to distinguish the two types of ESB by observation, because there were southerly winds in the south of the Kanto Plain in both cases.
A two-dimensional numerical experiment with sea, plain, slope and plateau and a three-dimensional numerical experiment with sea, plain, slope, valley and plateau were executed to investigate the interaction between the sea breeze and the thermally and topographically induced flow. Generation of the so-called "extended sea breeze (ESB)" was also investigated. In the two-dimensional experiment with sea, plain, slope and plateau, a weak wind towards inland appeared between the sea breeze front and inland slope over the plain whose length was 113.3km. The topography was changed to a valley in the three-dimensional model. The mouth and end of the valley were located 20km and 106.7km from the coastline, respectively. The addition of this valley caused the surface wind in the area between the sea breeze front and the end of the valley to become stronger, and showed the long-range penetration of the ESB. The depth of the wind system in the valley was equal to or greater than the depth of the mixing layer in the valley. The additional experiments in which the width of the valley was changed showed that the wind in the valley was strongest in the narrowest valley. The mechanism was basically consistent with the discussion by Defant (1951). In the case that the distance between the coastline and inland topography was short, the sea breeze was enhanced in the morning; however, the same topography worked to stop the advance of the sea breeze in the late afternoon. The non-linear effect was important to the development of the combined sea breeze and valley wind and the generation of a unified circulation (i. e. the ESB).
A tilted-trough vacillation observed in a laboratory experiment is simulated by the use of a nonuniform global grid numerical model. The main results are as follows. (1) A stable tilted-trough vacillation with a definite period has been simulated under steady external conditions. The flow fields are highly rotationally symmetric over the whole duration of the tilted- trough vacillation; the amplitudes of the sidebands are of the order of one thousandth of those of the adjacent dominant mode and harmonics. The amplitudes of harmonics and sidebands decrease exponentially with the increase of their wavenumbers. (2) There is a time variation of the inclination of the deviatoric pressure field from the vertical direction in the cylindrical surface as well as the time variation of its tilt from the radial direction in the horizontal plane. Rates of radial transport of heat and angular momentum vary periodically according to these variations of inclination and tilt, respectively. (3) Dynamical energies and their conversion rates also show distinct time variations. The conversion rate from the zonal mean to the deviatoric kinetic energy CKZKE is positive during the vacillation cycle, although it is one order of magnitude smaller than the conversion rate from the potential to the deviatoric kinetic energy; the rate CKZKE for steady baroclinic waves developing for considerably lower rotation speeds is negative. Therefore, the tilted-trough vacillation is interpreted as the behavior of baroclinic waves affected by weak barotropic instability.
This paper presents an observational study of the low-frequency variation in the Southern Hemisphere troposphere, using the global analyses for 1980-85 provided by the European Center for Medium Range Weather Forecasts. An empirical orthogonal function (EOF) analysis is made for the zonal mean geopotential height at 1000 mb to capture the variation. Based on time series of the second EOF coefficients, which represent the dominant non-seasonal low-frequency variation in the Southern Hemisphere, we define four typical events: negative extreme (D-), positive extreme (D+), negative to positive transition (T+), and positive to negative transition (T-) events. The variations on a hemispheric scale show a barotropic seesaw pattern with an almost axisymmetric node around 60°S and wavenumber 3 anomalies superimposed on it. Maximum westerlies at 500 mb are located at higher latitudes in D- event (50-60°S) than in D+ event (30-40°S). In association with the location of maximum westerlies, storm activity (defined as temporal variances of the high-pass (≤6 day) height field data) shows that large variation around 50°S occupies the entire latitude circle for a D- event but it is weaker in the western hemisphere for a D+ event. Composite analyses on a daily basis are made of several physical quantities at 500 mb for the four events. It is found that the latitudinal movement of maximum westerlies is quicker in T+ events than in T- events, in addition to the higher-latitude maximum westerlies in D- events than in D+ events. The zonal mean temperature is colder at high latitudes and warmer at middle latitudes in D- events than in D+ events, corresponding to the stronger polar vortex resulting from the steeper temperature gradient and more vigorous storm activity in D- events than in D+ events. The eddy momentum flux plays an important role especially during transition events; there is large equatorward transport of momentum at high latitudes around the key day of a T+ event and larger poleward transport at middle latitudes around the key day of a T- event. The heat flux seems to play a less important role. The acceleration of mean zonal winds is mainly determined as a residual between the momentum flux convergence and the Coriolis force.
The behavior of baroclinic waves in a skewed basic field due to a simple shaped orography was investigated through laboratory experiments using a rotating annulus of fluid. Investigation was made of the transition from an upper symmetric to a wave regime. Examination was conducted of the heat flux, the structure of stationary vortices and baroclinic waves along with the time-dependent behavior of the baroclinic waves. For a given zonal wavenumber the transition to a wave regime occurred at a lower thermal Rossby number in the skewed field than in the axisymmetric field. This exhibits the stabilizing effect of the orography, which is consistent with the results of Jonas (1981). The structure of baroclinic waves showed a dependence on the zonal direction, that is, amplification occurred on the lee side of the obstacle, confirming the results of linear theory by Niehaus (1980).
The quasi-biennial oscillation (QBO) in the equatorial lower stratosphere appears to be influenced by the seasonal cycle, as phase transitions at 50mb occur primarily in the northern spring/summer season (April-August). Descent of east wind regimes varies widely from one QBO cycle to another. Most of this variation occurs because easterly shears slow down or "stall" in their descent sometime between July and February. Minimum mean flow accelerations at 50mb occur in the northern winter season, slightly before the annual minimum in equatorial tropopause temperature. Although a weak effect of the semiannual oscillation can be detected near 10mb, the seasonal effect over most of the QBO region is annual. The seasonal cycle apparently modulates the onset of QBO phases, and slightly enhances our ability to predict the QBO, but is of insufficient strength or consistency to exactly synchronize the quasi-biennial oscillation with the seasonal cycle.