Simple air-sea coupled models have been proposed to explain the El Nino phenomena. The stability in an air-sea coupled model with an oceanic surface boundary layer introduced by Zebiak and Cane (1987) was investigated by solving the eigenvalue problem. This model is also associated with the eastward propagating unstable modes similar to the mode found by Hirst (1986). The stability was found to depend on the depth of mixed layer. The unstable mode does not appear in the western Pacific which has a deep mixed layer, even if the air-sea coupling coefficient (the heating parameter) is increased. However, the unstable mode readily appears in the central Pacific. It was found that, in addition to the Rossby mode, the Kelvin mode also becomes unstable at the limit of the advection in the model.
An inertial gravity wave related to a synoptic-scale pressure trough was revealed by observations made with the MU radar. Twelve beams were used to scan zonally to detect the spatial variation of the wind. According to hodograph analysis and theoretical considerations, the gravity wave propagated downward from the tropopause height of 11km, had a horizontal wavelength of 300km and a horizontal phase velocity of 20m/s. The estimated parameters are consistent with all information obtained by the measurements. The height-time section of the echo power shows that a severe turbulent layer was located near the tropopause height while the wave was appearing. A large vertical shear in the horizontal wind was coincidently observed at the same altitude. This suggests that the inertial gravity wave originated from the atmosphere having low stability near the tropopause. During the measurements with the MU radar, a synoptic-scale pressure trough passed near the radar site. The gravity wave was observed just before the northward wind migrated. These results indicate that the synoptic-scale pressure trough is one of the possible sources of gravity waves.
On the basis of results of slope and valley drainage flow models (Kondo and Sato, 1988; Sato and Kondo, 1988), a simple method is presented for the simultaneous evaluation of nocturnal cooling and drainage flow over slopes and in valleys under weak ambient wind conditions. Evaluations are made of the sensitivities of nocturnal cooling, drainage flow characteristics (velocity and thickness), and sensible heat transport by the drainage flow to the following conditions: meteorological conditions, thermal constant of the ground, surface aerodynamic properties and topographic features. Due to the enhancement of sensible heat transport by the drainage flow, surface cooling of an inclined topography is smaller than that of a horizontal plain. The velocity of the drainage flow is sensitive to the conditions mentioned above, but its thickness is dependent only on the surface aerodynamic properties and the topographic features. The side-slope flow plays an essential role in the energy exchange between the atmosphere and a valley. Available observations of nocturnal cooling and drainage flow support the present method.
A sphere probe with sixteen holes has been developed to measure turbulent fluctuations of static pressure in the atmospheric boundary layer. Wind tunnel tests show that the static pressure error of the probe is limited within ±2 per cent for variations of wind direction from 0°to 35°with the probe axis. The results of field test are summarized as follows. (1) The power spectral shape of static pressure fluctuations shows a-1 power slope at high frequency ranges for 2≥n≥0.05Hz. (2) The wp-cospectrum shows negative values for all frequencies analyzed, while the up-cospectrum shows positive values. (3) Measured coherence and phase between the static pressure, p, and the velocities, u and ω, show typically that p is in phase with u for frequencies between 0.005 and 0.1Hz with a root-coherence over 0.8, and p not in phase (ca. 135°) with w for whole frequencies.
Qualitative and quantitative comparisons are made between laboratory measurements and numericalsolutions of steady rotating annulus flows. The governing equations of the fluid with the Boussinesq approximation are solved numerically by the finite difference method on nonuniform grids. The comparisons as to some characteristics of baroclinic waves such as rates of radial heat transport and drift periods of waves, which are measured by reducing probe effects to a negligible extent, demonstrate the reliability of the numerical model. Numerical results of internal structures of steady baroclinic waves are presented on the basis of this model verification and compared with experimental results. Comparisons reveal that not only probe drag but also heat leakage modifies flow fields to a considerable extent. Analyses made by using data obtained by simulations indicate that (1) the inward heat transport by the deviatorid motion overcomes the outward heat transport by the indirect circulation and plays anssential role in the heat transport to the inner cylinder; (2) the deviatoric motion as well as the mean meridional circulation transports angular momentum to the axis of the relative angular momentum jet; and (3) the conversion rate from deviatoric kinetic energy to zonal kinetic energy is positive at lower rotation rates but it becomes negative at higher rotation rates.
Analyses of the global sea surface temperature (SST) were performed to examine year to decadescale variations of SST. It is found that the tropical SST, especially in the central and eastern Pacific and in the Indian Ocean, has being increasing since the late 1970's. SST averaged in the whole tropics between 20°N and 20°S was warmer by about 0.3°C-0.4°C in the 1980's than in the 1970's. Convective activity in the tropics detected by the satellite-measured outgoing longwave radiation becomes more enhanced corresponding to the recent warming of the tropical SST. The Southern Oscillation Index, which represents a strength of the tropical Walker circulation, accordingly tends to become negative after the late 1970's. The Pacific/North American teleconnection pattern with significant lowering of the geopotential height at 500 mb in the North Pacific has become dominant and persistent during winter seasons in the 1980's. It is suggested that these large atmospheric anomalies may be associated with tropical heat sources enhanced by the recent warming of the tropical SST.
Long-term wind stress vector (WSV) fields in winter over the North Pacific, which were calculated by Kutsuwada and Teramoto (1987), are analyzed by means of the composite method with respect to SST anomalies in the mid-latitudes of western North Pacific (SST composite). According to SST anomalies, two categorized winters, i.e., warm and cold winters are selected during 24 years from 1961 to 1984. The numbers of warm and cold winters are six and five, respectively. In order to examine whether or not the composite WSV fields are well-ordered and/or rigid ones, maps of stability of WSV anomalies are constructed, and the new parameter, Degree of similarity of WSV anomaly field of each winter to the composited WSV anomaly field is introduced and discussed. Both parameters show that the extracted patterns for two categorized winters are well-ordered. In warm (cold) winter, mid-latitudes westerly weakens (strengthens) and shifts northward (southward). As a result, the East Asian Winter Monsoon (Kisetsuhu) over Japan weakens (strengthens). It is also seen that in the equatorial region, in warm winter the region with SST higher than 28°Cextends to the central to eastern part and its anomaly fields are very similar to those in ENSO year winter. Actually, winters selected as warm (cold) winter include the ENSO (ENSO+1) year winters, i.e., winters during ENSO events, but not the ENSO+I (ENSO) year winters. Time series of Degree of similarity for westerly region to warm-winter composited WSV anomaly fields are well in agreement with those of "Far East zonal index", which is used by the Japan Meteorological Agency. From the composite map for sea level pressures, it is shown that shifts of westerly axis correspond to north (warm winter)-south (cold winter) shifts of the Aleutian Low.
A simple parameterization scheme, which can estimate heat flux over a complex land-use surface, is presented. In this method, the ground surface in a grid area is classified into several categories. Surface temperature and sensible/latent heat fluxes are estimated on each land-use category. The total heat fluxes on the grid point is given by the average of heat fluxes on each land-use surface weighted by its area. The results of the one-dimensional model with this parameterization is compared with a two-dimensional fine grid model, which estimates heat fluxes on the periodic land-use surface. The heat fluxes estimated by the parameterization method agree well with those given by the twodimensional model. This parameterization allows one to estimate not only accurate heat fluxes over a complex land-use surface, but also the surface temperature on each land-use category in a grid area.
On bay coasts, a characteristic local wind circulation is formed, because bay breezes and ocean breezes are caused by bay-land and ocean-land contrasts, respectively, and they interact with each other. For the purpose of examining basic characteristics of the circulation, several numerical experiments are performed. The three-dimensional model used in this study has a square bay of various sizes with the ocean on the south side, taking a model bay of 50km in width and 100km in length as our standard size. Results for the standard bay model show noticeable differences between the east and west bay coasts. Near surfaces, hodographs rotate anti-clockwise on the east coast and clockwise on the west coast. Along the west coast, the depth of the ocean breeze is almost the same as that of ordinary sea breezes, and strong oceanward return currents blow aloft. On the other hand, along the east coast, the depth of the ocean breeze is much greater than that of ordinary sea breezes, and only a weak oceanward return flow can be seen. The north-south circulation along the east or west coast is the composition of the real ocean breeze and the influence in the opposite direction of the northward bay breeze. Therefore, the scale of the total circulation is much larger than that of ordinary sea breezes, and can be extended to even more than 200km. Through the influence of the eastward or westward bay breeze and its return current, the stratification surrounding the ocean breeze is stable. Since internal gravity waves are generated in the stably stratified layer, signals propagate at the group velocity of the waves. The propagation speed, although it somewhat slows down due to advection, is therefore faster than that of an ordinary sea breeze front. The results from the standard width model show that circulations along the east and west coasts are independent of each other. On the other hand, the 20km width causes the interac- tion of the circulation on both coasts, so that the ocean breeze is no longer deep even on the east coast. The model with the length of 40km does not show a propagating phenomenon but a steady circulation.
Observations are given for the foehn event which occurred in the Abashiri-Ohmu area (N44.3, E143.5), Hokkaido, northern Japan, on 12 May 1975. A numerical investigation is made of the foehn, using a 2-dimensional nonhydrostatic model. The foehn is well simulated, even without any diabatic heating in the presence of an inversion layer below 900hPa. This foehn is confirmed to be dynamically induced, and is an unsteady, highly nonlinear phenomenon. It is accompanied by an atmospheric bore (a shock-like disturbance propagating upstream) and an internal hydraulic jump (a shock-like disturbance propagating downstream), but no wave breaking (wave induced critical layer). The foehn of a homogeneous fluid (with constant mean velocity U and constant Brunt-Vaisala frequency N) is also simulated. It is accompanied by wave breaking and hydraulic jump, but no bore. A foehn index (a non-dimensionalized potential temperature rise on the lee slope) is introduced, and its dependence on the inverse Froude number (Nh/U) is examined for the 4-layered fluid of the Abashiri-Ohmu case and the homogeneous fluid. The critical mountain height for the occurrence of the foehn is found to be lower in the presence of an inversion layer below 900hPa than in the absence of an inversion layer. In addition, the foehn event in the Abashiri-Ohmu area is compared with that of the European Alps (Hoinka, 1985).
A possible cause of the recent decrease of planetary wave activity and Antarctic ozone depletion in spring is proposed. The tropospheric stationary wave structure in the southern hemisphere shows substantial variation during 1979 to 1988. Before the 1980's, especially in 1979, a well-developed tropospheric ridge was situated in the South Pacific and the stationary wave propagated into the stratosphere, while after 1985 the tropospheric ridge became weaker and split into two ridges, and no vertical propagation was clearly seen. In 1988, however, the vertically propagating wave number 1 structure was recovered. This change of planetary wave structure in the South Pacific seems to be related to the sea surface temperature anomaly in the low latitude South Pacific. A possible role of the sea surface temperature variation in the recent development of the ozone hole is also discussed.
Some evidence is presented on the interaction of the QBOs between the stratospheric and the tropospheric zonal wind in the tropics. Zonal winds over some stations (Singapore, Koror, Ponape) in the equatorial Pacific clearly show the coherent vertical phase structure between the downward propagation of zonal wind anomalies in the stratosphere and the zonal wind anomalies in the lower and the upper troposphere. The QBOs in the tropospheric zonal wind are also proved to be coupled with that in the SST anomalies in the equatorial Pacific. These results suggest a possible dynamical link of the QBO in the large-scale coupled atmosphere/ocean system over the Asian monsoon region through the equatorial Pacific with that in the equatorial stratosphere.