As an extension of M. Hayes' argument (Hayes, 1977), a general relationship is derived between group velocity and fluxes quadratic in wave amplitude for single, weakly-modulated small amplitude plane waves propagating in an inhomogeneous, non-conservative, dispersive system. If a quadratic (4-) flux constructed from wave quantities with a common single harmonic phase is inserted into its linear governing equation, the resultant equation consists of a wave part with a hi-harmonic phase and a mean part, but both parts must vanish separately to satisfy the equation. The amplitude of the former gives the dispersion relation, H(kμ)=0; this in turn gives the complex (4-)group velocity vector νμg=∂H/∂κμ by varying the complex (4-)wavenumber vector κμ(μ=0, 1, 2 and 3). On the other hand, the latter gives a governing equation for the mean of the quadratic flux, and also gives, by taking the same variation, another mean quadratic flux. The latter flux is proportional to the group velocity νμg. The general relationship is illustrated by application to Rossby wave motions. The group velocity and energy flux relation obtained by Longuet-Higgins (1964) is rederived as a special case.
The formation of fronts and the process of occlusion of an extratropical cyclone developing in the midlatitude baroclinic westerly jet are studied numerically with use of a dry primitive equation model. The most unstable normal mode of the linear perturbation equations is adopted as the initial disturbance. This mode has the structure of a baroclinically unstable wave. As the disturbance develops, there appear a strong, long cold front and a weak, short warm front. The intensification of horizontal potential temperature gradient and relative vorticity around these fronts are investigated. The generation term of the potential temperature gradient has positive value in both the regions. However, the generation term at the warm front is cancelled by the negative advection due to the strong northward flow to the east of the low. Thus the warm front does not become sharp and it also accompanies no positive relative vorticity line with as the result of strong upward flow at the warmer side of the front. On the other hand, there exists two-dimensional frontogenesis at the middle part of the cold front. At the southwest end, the cold front is elongated by the advection. At this stage, there exists another strong front to the northeast of the low centre. It is also shown that the advection is important for the formation of the front. We can perform the experiment until the cyclone occludes. The vortex-like pattern is found in the vorticity field around the centre of the low. It is also explained with a combined effect of the generation term and the advection term of the horizontal temperature gradient and of the relative vorticity. These structures found in the experiment correspond well to the cloud pattern formed in some of the midlatitude cyclones.
Atmospheric responses to the sea surface temperature anomalies (SSTAs) composited by Rasmusson and Carpenter (1982) for the mature phase of El Nino are studied with a global general circulation model. The model is a five layer tropospheric model with a horizontal resolution of 5° by 4°in the longitudinal and latitudinal directions, respectively. Experiments are performed under the perpetual January condition; one without the SSTAs (case CNTRL), other with the SSTAs (case SSTA1) and the other with the doubled SSTAs (case SSTA2). Time integration is made for 210 days in each case. In the tropics, precipitation increases over warm SSTAs, and the forced equatorial anomalies are formed in the vicinity of the equator, although they are not so simple as the ones obtained by Gill (1980), etc.. Anomalies in the east-west circulation closely reflect warm SSTA distributions. Atmospheric responses increase almost linearly with the increase of SSTAs. In the extratropics of the Northern Hemisphere, response patterns in the time averaged stream function field are elongated in the zonal direction and the so-called Pacific/North-American (PNA) pattern is missing. The amplitude of responses does not increase with the increase of the equatorial SSTAs. In case SSTA2, wavy anomaly patterns appear in the Northern Hemisphere. However, they are shifted both eastwards and equatorwards from the PNA positions. These results differ from those presented so far (Shukla and Wallace, 1983; Blackmon et al., 1983; Geisler et al., 1985). An additional run (case SSTA1 ∗) is made to interpret those differences. The SSTAs westward of 140°E are eliminated in this case. Elsewhere, the adopted SSTAs are the same as those of SSTA1. This run reveals that the warm SSTAs in the vicinity of the maritime continent and the South China Sea have large impacts not only on the extratropical atmosphere (especially of the Northern Hemisphere) as suggested by Simmons et al. (1983) and Branstator (1985) but also on the equatorial atmosphere. In SSTA1 and SSTA2, equatorial forced Rossby type response has narrower extents in both the longitudinal and latitudinal directions than those in SSTA1∗, especially in the Northern Hemisphere, due to the suppression of upward motions in SSTA1 and SSTA2 over the equatorial central Pacific Ocean by the warm SSTAs westward of 140°E. This change is considered to be an important factor in causing large differences in the extratropics by changing a propagation path of a Rossby wave-train. Wavy response similar to the PNA pattern appears in SSTA1∗ in the extratropics of the Northern Hemisphere. Characteristics of long-term variations in each case are studied with the use of the empirical orthogonal function (EOF) analysis. Dominant patterns of long-term variations change with the changes of equatorial SSTAs. In case CNTRL, the first EOF mode describes basically the north-south seesaw and resembles the one obtained from the observed data by Lau (1981) and Tokioka and Chiba (1986). On the other hand, the PNA-like pattern is obtained as the first and the second modesin case SSTA1 and SSTA2, respectively. This mode oscillates slowly with a period of about 100 days. This type of mode is also found in cases CNTRL and SSTA1*. However, it is only the fourth mode in both cases. The warm SSTAs westward of 140°E seem to enhance the PNA-like standing oscillations in the mature phase of E1 Nino.
The seasonal interactions between mid-latitude circulation patterns and the first two dominant sea surface temperature (SST) anomaly patterns over the North Pacific were statistically analyzed. The first SST mode contributes to the formation and evolution of the Pacific/North American (PNA) pattern during wintertime. In contrast, the second SST mode is partly formed by the cold surge from the East Asia during wintertime closely associated with the dominance of low index circulation. When this mode is dominant, the Western Pacific (WP) pattern is prominent at the 500mb height field. Further this mode is also influenced by the outbreak of polar cold air which is accumulated in early summer and released toward the East Asia in mid-summer. In the middle latitudes the negative SST anomalies of the first mode at about 40°N are mainly formed by mechanical forcing of the atmosphere and the negative SST anomalies of the second mode over the western Pacific are formed by thermal forcing (cold surge) of the atmosphere.
Long-term variations of heat sources and their spatial distributions in the western Pacific region are examined by using satellite-observed high-cloud amount data from 1978 to 1983. There exist large seasonal variations with maximum cloud amount in the summer hemispheric tropical regions. Spatial distributions of cloud amount are largely affected by summer and winter monsoons over Southeast Asia and Australia. Large interannual variability of cloud amount is confined in the equatorial region during northern winter, but large variations are found not only in the equatorial region, but in the northern subtropics around 20°N and in the middle latitudes near 35°N during summer. Two dominant cloud anomaly patterns are found by correlation computations and Empirical Orthogonal Function (EOF) analysis. The first anomaly pattern (El Nino pattern) is the east-west oscillation between the equatorial central Pacific and the western Pacific. This pattern was significantly amplified during 1982-83 years corresponding to the El Nibo event. The second anomaly pattern (South Japan pattern) is the north-south oscillation between northern subtropics and middle latitudes extending from East China to the dateline. This pattern becomes most evident during northern summer. Cloud anomalies along the equator during 1982-83 show that positive anomalies near 150°E started to move eastward in May - June 1982, reached the central and eastern Pacific in late '82 and continued to stay in the eastern Pacific till next summer. The large portion of the western Pacific regions was occupied by intense negative cloud anomalies. During this period cloud amounts along the mid-latitude storm tracks from East China Sea to Northwest Pacific increased probably due to enhancement of extratropical cyclone activities. During the summers of 1978 and 1981 convection centers in the tropical western Pacific are abnormally shifted northeastward from the normal position near Philippines to western Pacific subtropics around 20°N, 140°-150°E. Cloud amounts both in mid-latitude regions to the north of the convection centers and in the equatorial regions to the south are greatly reduced and Japan Islands have experienced hot summers during these years. Relationships between variations of cloud amount and sea surface temperature (SST) in the tropical western Pacific are examined. EOF analysis of SST shows that SST variations are dominated by the first eigen mode with large amplitudes in the tropical western Pacific east of 140°E and from the equator to about 20°N. the amplitude of this mode significantly increases in 1982-83 indicating that large negative SST anomalies appear in the vast tropical western Pacific regions during the El Nino period. The South Japan cloud anomaly pattern develops during summers when large positive SST anomalies in the tropical weatern Pacific appear. Correlation computations also indicate close relationships between cloud amount and SST.
It is pointed out in a critical review of works on tropical data analyses that strong discrepancies between the statistics from the observed fields and those obtained from the standard grid-point data sets over the tropics lead to notable ambiguities in the results obtained from the analyses. It appears that these weaknesses are due principally to the model dependency of these grid-point data sets, which are produced by using objective analysis schemes designed without special attention to the tropics. The necessity of fundamental statistical analyses is stressed in the outline of the new analysis approach devised in order to avoid or reduce these shortcomings. Using a 5-year (1979-1983) tropical upper-air data set, we performed statistical analyses for the wind components (u, ν), the geopotential height (Z) and the temperature (T) over the tropical belt between 25N and 25S of latitude, and 20E and 140W of longitude. Spectral and cross-spectral analyses show the dominance of four large-scale and long-period oscillations: the 2-4 years oscillation, the annual, the semi-annual and the intra-seasonal (40-60 days) ones. Short-period oscillations are shown to be of small scale in space and do not contribute significantly to the horizontal transport processes of heat and momentum. Low-pass filtering is shown to improve the estimates of the spatial correlation of the four variables and to reduce their observational errors by a factor 2-3 as compared to the estimates from the non-filtered data. It is then expected that low-pass filtered data would describe reliably the tropical large-scale phenomena. The correlation field is shown to be homogeneous for all the fields, and anisotropic not only for the wind components, but for Z and T as well. Most of these findings should be taken into account in statistical objective analysis schemes, such as the Optimum Interpolation Method, for better grid-point estimates over sparse observational network areas in the tropics.
Large-scale situation over East Asia during the Baiu period of 1979 was studied on the basis of 10-day averaged fields. The development of warm ridge over the Mongol-Sibirsk Plateaus (Baiu ridge) and cold trough elongated from the Bering Sea cyclone to China (Baiu trough) were the major systems in Baiu season in middle-high latitudes. With the rapid temperature rise over the Continent and the slow temperature rise over the Pacific north of -35°N, the large temperature difference between the Continent and the Pacific was maintained. The circulations in the lower troposphere in the Baiu season were characterized by the northerly wind in the west side of the Baiu trough (Baiu-trough-northerly), the southerly wind over the South China Sea, low-level-jet like strong WSW wind along the northern periphery of the Pacific anticyclone, and the Indian monsoon westerly wind. While the Meiyu front was related with a shear line between the South China Sea southerly and the Baiu-trough-northerly wind (Baiu shear line), the Baiu front was located -1500km south of the Baiu trough and associated with convergence/confluence of SW wind to the strong WSW wind zone. Another feature of interest is the difference between the structure of the Meiyu and the Baiu fronts. The northward advance of the Pacific anticyclone and the Baiu front is the basic seasonal change. The quasi-periodic (-40-day period) intraseasonal variations of the Pacific anticyclone and the Baiu front were superposed upon this seasonal change. The intraseasonal variations of the Baiu front were related to the activity of ITCZ in the South China Sea as well as the variations of the monsoon precipitation. From the analysis, the role of the Tibetan Plateau, the Baiu-trough-northerly wind, cold advection associated with the Bering Sea cyclone over the cold northwestern Pacific in maintaining the Baiu/Meiyu front was inferred. This inference was supported by the results of prediction experiment by Nakamura, Hasegawa and Ninomiya (1985).
A series of aircraft measurements of the three-dimensional distribution of air pollutants and meteorological elements was conducted each summer of 1980, 1981 and 1982 in the Inland Sea area of Japan in order to examine the medium-range transport processes due to local circulations. The data show that polluted air is horizontally distributed over almost all of the Sea and is confined to levels below 1000m in the early morning. The polluted air is transported to upper levels up to 3000m in the evening by upslope winds and/or the sea breeze on the Shikoku side of the Sea. Developed cumulus clouds over the mountains seem to contribute to the vertical transport of ozone and the scavenging of aerosols. The structure of the internal boundary layers formed over the complex terrain is also analyzed. The results agree quite well with a simple model simulation.
Using the upper tropospheric and stratospheric temperatures and total ozone amount data obtained from the TIROS-N satellite over the Indian region during the period May to July 1979 subdivided into six different phases of the monsoon, possible interactions between the 400-1mb atmospheric region and the summer monsoon have been examined in this paper. This investigation shows a noticeable decrease of the stratospheric temperatures and total ozone amount at the onset phase of the southwest monsoon with an increasing trend during its active phase. Variations in the stratospheric temperatures and total ozone amount are found to be consistent with each other parcularly in the 100-70mb layer.