Matsuda (1983) has shown that critical points appearing in steady problems of fluid systems are classified mainly into two types (i.e. bifurcation point and snap point) in connection with the symmetry in the velocity field. According to the results of this study, a bifurcation point can appear only for the system whose external conditions have some elements of symmetry in the strict sense. For the system whose external conditions have no symmetry, only a snap point can appear as a critical point. In an actually existing fluid system, even when the external conditions appear to have the symmetry, its symmetric conditions are, more or less, perturbed by unexpected or uncontrollable disturbances. In this study we wish to examine the structure of critical points appearing in such real fluid systems. For this purpose, we consider a system having the symmetric external conditions slightly disturbed; this slight deviation from the symmetric conditions is assumed to be characterized by a single parameter μ. With the spectral equations derived in Matsuda (1983), we examine how a bifurcation point appearing for the system with complete symmetric external conditions (i.e. for μ=0) is modified in the system without them (i.e. μ≠0). It is found that instead of a bifuracation point a snap point appears for, μ≠0. However, in the phase space the snap point and a steady solution existing separately from the snap point constitute a global structure analogous to a bifurcation point. This global structure is continuously modified to a bifurcation point as μ tends to zero. In connection with the present problem we discuss the validity of idealized models as a tool of examining a real fluid system with disturbance.
Stability properties of topographically forced baroclinic Rossby waves and zonal flows are investigated by the use of a two-layer, quasi-geostrophicβ-channel model. Two kinds of instabilities are found when the vertical shear of the zonal flow exceeds the minimum critical shear for the conventional baroclinic instability of the zonal flow: One is the topographic instability which is identical with that examined by Charney and DeVore (1979) and Mukougawa and Hirota (1986a) in the barotropic model. This instability appears in the near-resonant flow. The other is the baroclinic instability composed of synoptic disturbances with a horizontal modulation by effects of the forced wave. This is found to correspond to the storm-track type instability of free baroclinic Rossby waves investigated by Frederiksen (1978, 1982). Examination of various effects of these unstable modes on the basic flow reveals that the roleof the synoptic disturbances on the transition of the weather regime is not so important as suggested by Reinhold and Pierrehumbert (1982). Alternatively, other unstable modes, such as those due to the topographic instability, are expected to cause the regime transition because their effects on the basic flow are completely different from those of the baroclinic instability.
Using a vertical one-dimensional model, we discuss the equilibrium of the stratospheric mean zonal flow and planetary wave forced by the surface corrugation. Their stabilities are also examined. It is shown that in late autumn and early spring, there exist two stable equilibrium solutions and one unstable equilibrium solution, while one stable equilibrium solution in mid-winter. It is noted that we have no vacillating solution as obtained by Holton and Mass (1976) and Chao (1985). Solving the time-dependent problems with changing the vertical shear of the basic zonal wind with one year period as an indicator of seasonal variation, we can obtain the growth of planetary wave twice in winter and the intensification of westerly wind between these two amplitude peaks. In addition, it is shown that a drastic transition of the mean zonal winds from westerlies to easterlies resembling the final warming occurs in later winter or early spring. These are very similar to the results of data analysis by Smith (1983) and those of the numerical simulation by Holton and Wehrbein (1980). Thus, the stratospheric seasonal variation can be interpreted as a hysteresis phenomenon caused by the change in the solar differential heating in the presence of multiple equilibria. The breakdown of polar, night vortex with the final warming is a kind of nonlinear instability. That is, if the winter westerly wind in the upper stratosphere is close to the Charney-Drazin's critical velocity and if it is perturbed by a planetary wave, then the refractive index is changed and this change causes the amplification of the wave. The amplified wave, in turn, changes the refractive index is changed and this change causes the amplification of the wave. The amplified wave, in turn, changes the refractive index by accelerating easterlies. This process provides a feedback, and the polar night vortex breaks down in the end. The present instability is not the resonance breakdown as in Plumb (1981). Finally, we make some comments on Chao's (1985) results.
The intraseasonal oscillation is investigated using the wave-CISK mechanism. It is shown that the vertical distribution of the non-dimensional heat parameter η(z) in the wave-CISK is very important to the slow phase speed of the intraseasonal oscil lation.
A comparison is made of the stratospheric final warmings which occurred in the two hemispheres. The dataset for this study consists of the NMC 1200 GMT analysis between 0.4 and 1000mb during 1982. The transformed Eulerian mean diagnosis is used for examining the wave, mean-flow interaction. The final warming occurred around March 31 in the Northern Hemisphere (NH) and around October 20 in the Southern Hemisphere (SH). Both warmings were associated with the enhanced planetary scale (wavenumber 1) wave activity. The final warming in the SH is more rapid and intense, which is consistent with the fact that the planetary scale wasve activity in the SH stratosphere is more intense than that in the NH stratosphere during the spring season. In the SH the polar easterly did not descend below 10mb after the final warming and circulation reversal below 10mb occurred about one month later, while the polar easterly kept descending to 50mb in the NH. The equatorial easterly in the lower stratosphere extended and connected to the polar easterly in the upper stratosphere of the SH, while the connection was not observed in the NH. A few days before the final warmings, the double jet structures were observed in the troposphere for both cases: The feature of the final warming in the SH of 1982 is very similar to that of 1979 studied by Yamazaki and Mechoso (1985).
The three dimensional structures of sea surface temperature (SST) and atmospheric circulation anomalies associated with ENSO are investigated over the entire globe, by using the objectively analyzed data sets of the years 1964 to 1981. The global data sets of U, V, Z and T are produced by applying the least-square method to fit the time-filtered station data (of more than 320 stations) and NMC tropical wind field data with truncated spherical harmonics of wavenumber 0 to 5. Global SST and sea level pressure data are also utilized. In this paper (Part I) the composite anomaly patterns during El Nino episodes are discussed. Warm SST anomalies and warm upper troposphere are found to be remarkable not only over the equatorial central Pacific but also over the entire tropics, while the mid-latitudes seem to be characterized as cold SST and cold troposphere as a whole. The increasings of zonal winds in the subtropics and the lower mid-latitudes of the two hemispheres are also prominent. These evidences apparently suggest that the intensified Hadley-type circulation in the low latitudes and the equatorward extent of the circumpolar vortex in the high latitudes are fundamental features during El Nino episodes. A nearly symmetric response pattern of the upper tropospheric flows with respect to the equator is also noted over the northern and the southern Pacific. It is also suggested that the North Atlantic Oscillation (NAO) is synchronized with the SO during the present analysis period.
Time evolutions of anomalies of SST and atmospheric parameters over the global domain were investigated for the several ENSO cycles during the period 1964-79, using the data set described in Part I. The principal results are summarized as follows: 1) the eastward propagation of SLP and zonal wind anomalies from the Indian Ocean toward the eastern Pacific is a fundamental nature of ENSO along the tropics and the southern subtropics. 2) the significant anomalies of SLP and circulation field which first appear over the Indian Ocean seem to originate from central Asia or Eurasia. 3) the vertical structures of these anomalies over central Asia at the intermediate stages of the cycle (i.e., about one year before the SST maximum (minimum) over the equatorial Pacific) are suggested to represent large-scale cold (warm) air outbreak associated with more (less) than normal snow cover over there. 4) a time-lag teleconnection between the north Pacific and central Asia is found in the circulation field, which seems to have a key role on the mechanism of ENSO cycle. The first two results have substantially confirmed the evidences of ENSO signals from Eurasia and the Indian Ocean toward the equatorial Pacific, which were noted in the surface fields by Barnett (1984, 1985a) and Krishnamurti et al. (1986). The third result suggests the important role of Asian monsoon as a connector of the process in the extra-tropics and that in the equatorial Pacific. The fourth result seems to be associated with the interaction between the PNA and the EU (Eurasian) pattern with a time lag of half a year or more. The links between the tropics and the extra-tropics and between the ocean and the continent described above strongly suggest that the ENSO should be understood as a global scale land (cryosphere)-atmosphere-ocean coupled system rather than an atmosphere-ocean coupled system over the equatorial Pacific.
Characteristics of the SST anomaly field in the North Pacific and their relationship with the 500mb geopotential height fluctuation in the Northern Hemisphere, were investigated, by using the 5°×5°grid SST data set for the North Pacific. This data set was newly calculated from the SST data file provided by the Japan Oceanographic Data Center. The typical zonal and meridional scales of the SST anomaly pattern are several and one to two thousands km, respectively. Typical SST anomaly patterns can be traced for several months to a year. Fluctuations with periods of several years are also found in spectral analyses for the SST anomaly field. The SST anomaly tends to appear almost simultaneously over a large area. Clear regularity is not found in the speed and the direction of the migration and/or the expansion of the SST anomaly pattern. However, for restricted periods, the SST anomaly pattern seems to move by advection. In an EOF analysis of the SST anomaly, the first EOF shows an elliptic monopole spatial pattern centered on the central North Pacific. The time series of the coefficient of the first EOF shows a variation with periods of several months and of two to three years, but has no peak corresponding to the 1972/73 El Nino event. Our results of the EOF analysis are similar to those of Davis (1976), but somewhat different from Weare et al. (1976) and Kawamura (1984) because of differences in the domain of the analyses. The SST fluctuations are highly correlated with 500mb height variations in the Northern Hemisphere. We could conclude based on the lag correlation analysis that the SST anomalies represented by the first EOF are caused by the 500mb height fluctuation corresponding to the PNA pattern.
Variations of precipitation in June and July over Japan (Baiu precipitation) are studied for a 30-year period of 1951-1980 with special emphasize on the occurrence of wet (large precipitation) and dry (small precipitation) Baiu season. EOF analysis is aplied to monthly total precipitation (R), monthly total sunshine-duration (S), monthly mean surface temperature (T) in June and July. In spite of sporadic and localized nature of Baiu precipitation, ∼50% of the R-variance is accounted by the 1st EOF component. The 1st spatial function (B1(x)) of R shows localized maximum value over southwestern Japan while B1(x) of T shows localized maximum value over northeastern Japan. Any significant trend and periodic long-term variations are not seen in the 1st EOF's time variation function (A1(t)) of R. The relations among A1(t) of R, S, T and large-scale conditions in the East Asia are examined. A1(t) of R is positively correlated with anomaly of U and V-component of 700mb wind in the northern periphery of the Pacific anticyclone (ΔU700 and ΔV700) and negatively with A1(t) of T. While ΔU700 is negatively correlated with A1(t) of T, the correlation between ΔV700 and ΔU700 and that between ΔU700 and A1(t) of T are very small. From the analysis, following two independent large-scale conditions favorable for wet Baiu are pointed out; 1) positive ΔV700, and 2) negative A1(t) of T (in many cases concurrent with positive ΔU700, northeasterly flow from polar airmass to the north side of Baiu front, development of Baiu ridge and trough).
A mesoscale cloud cluster was observed around a cyclone from the 7th to the 9th of October in 1983. Its synoptic situation, behavior and structure were studied using the data of satellites, radars, upper air sounding and rainfall amount. The cloud cluster formed in association with a cyclone which was found in the zone of large moisture gradient. It was initiated in the warm sector of the cyclone and then it approached to the warm front. The area of upward motion and convective instability, which were favorable for the development of convection, were localized in the limited region in the warm sector of the cyclone and the cluster was observed near this region. In spite of the east-north-eastward movement of the cyclone, the cloud cluster stayed in the south of central part of Japan for more than 6 hours. Satellite IR data show that the evolution process of the cloud cluster was divided into two stages. In stage I the minimum equivalent blackbody brightness temperature (TBB) of this cloud cluster was less than -70°C. Its shape was oval. The maximum radii of cloud areas whose TBB were below -40°C and -60°C were about 170km and 120km, respectively. In this stage the cloud cluster existed mainly in the warm sector. In stage II the area of TBB lower than -60°C remained small. In contrast the area of TBB lower than -40°C increased again and it was elongated northeastward. Its width and length were 100km and several-hundred kilometers, respectively. The cloud cluster consisted of the southwestern convective clouds, which were aligned northeastward, and the northeastern layer clouds which were composed of upper-level generating cells and middle-level clouds. In this stage the cloud cluster existed across the warm frontal region. Its northeastern part existed in the region which is characterized by warm moist and stable air above the frontal layer and cold moist air under the frontal layer. There was localized area of convective instability above the frontal layer and this area would have been related to the formation of upper-level generating cells.
An extraordinarily deep local depression formed on the lee side of a mountain range is investigated under a nonhydrostatic condition and is compared with the results of a hydrostatic model (Tomine, 1984). It is found that nonhydrostatic effects on the large amplitude mountain wave seem insignificant within a framework of the present model.