The relationship between the development of the monsoon circulation and the equatorial heat sources is studied. He et al. (1987) showed that two transitions during the early stage of the monsoon of 1979, which correspond to the onset of monsoons over South-East Asia and India respectively, are characterized by the movements of the upper anticyclone over South Asia. The main purpose of our paper is to investigate what determines the behavior of the anticyclone. Gill (1980) with his linear shallow water equation model suggests that the monsoon circulation, including the upper anticyclone over South Asia, could be regarded as the Rossby wave response to the heating over the equatorial region. The intensified equatorial heating can generate the Rossby mode and intensify the anticyclone over South Asia. The eastward-moving mode with a 40 day period is clearly seen in the early summer of 1986 as well as in the season of 1979. Due to this mode, the moist convection over the Western Pacific and the maritime continents changes its intensity periodically. When the moist convection becomes active, the upper anticyclone over the South Asia is rapidly intensified. The behavior of the upper anticyclone in 1986 is similar to that in 1979. In order to confirm the relationship between the upper anticyclone and the equatorial heating, we perform prediction experiments, in which we can control the convective heating over the equatorial region, for two transition periods in 1986 using our nonlinear forecast model with full physics. The heat source, which is given over the domain from 30°S to 30°N and 90°E to 170°W, is estimated from the cloud top temperature measured by the GMS (Japanese Geostationary Meteorological Satellite). In these experiments, we can make sure that the equatorial heat source, away from the monsoon region, plays an important role in generating the anticyclone over South Asia at 200mb.
An analysis is made of dynamical features of planetary waves and zonal mean winds in the troposphere during the Northern winter from January 1963 to December 1982, by paying a special attention to the differences between the years of negative and positive extremes of the Southern Oscillation Index (SOI). The difference is more evident in Late winter (January-February-March) than in Early winter (November-December). Mean zonal winds in the Late winter are closely related to the SOI; they are in positive correlation at extratropical latitudes and in negative correlation at subtropical latitudes. Consequently, the difference of mean zonal wind speeds between the negative and positive extremes of the SOI shows a barotropic seesaw pattern in meridional cross section with a node around 40°N. This pattern is associated with the strong (weak) subtropical jet and weak (strong) mid-latitude westerlies in the negative (positive) extremes. Wave quantities averaged over the Late winter show clear differences between the two categories. In the mid-latitude troposphere, wave driving (DF) has larger negative values in the negative extremes than in the positive extremes; this is due mainly to the vertical derivative of heat flux (F(z)). On the other hand, around the subtropical jet region, DF has smaller negative values in the negative extremes than in the positive extremes; this is due to horizontal derivative of the momentum flux F(y). Thus the weaker DF due to both F(y) and F(z) is related to the stronger mean zonal winds. The height fields over the Northern Hemisphere are also classified into two patterns: Wave 2 in the negative extremes with two troughs over the western Pacific and western Atlantic, and Wave 3 in the positive extremes with another major trough lying over the Black Sea. The differences in the wind field between the two categories are mainly attributed to the development of Atlantic anticyclones. Notable wave-like and teleconnective patterns are found especially in the positive extremes. It is speculated that the differential heating at low latitudes could modulate mid-latitudes circulations.
In order to understand the characteristics of large-scale SST anomaly fluctuations, a cluster analysis with respect to similarities in their temporal variation is made of the SST anomaly field in the North Pacific. This analysis can extract a subdomain of coherent SST anomaly fluctuations in the middle latitude western part of the North Pacific (REGION A), although previous EOF studies of the SST anomaly in the North Pacific did not extract such a subdomain. In addition, the North Pacific Ocean is divided into four major subdomains, that is, the northwestern subdomain (REGION A), the western tropical subdomain (REGION B), the central North Pacific (REGION C), and the eastern boundary region (REGION D). Oceanic and atmospheric conditions characterizing such subdomains are discussed.
In order to clarify the surface thermal conditions during the ENSO events, composite analyses of the SST anomaly fields were performed over the western North Pacific, and for the mixed layer in the Kuroshio current region at sections along the 137°E line and a line over the Izu Ridge. Each winter during the 25 years from 1961 to 1985 was classified into one of four categorized winters, i.e., ENSO-1 year, ENSO year, ENSO+1 year and the other year winters. For example, for the 1982/83 ENSO event, the 1983 winter (January through March) was regarded as the ENSO year winter. It was found that in the ENSO year winter, a well-ordered positive SST anomaly appears in a wide zonal band along the 30°N line, extending from the Asian coast to near 170°E. On the other hand, the distribution of SST anomalies in the ENSO+1 year winter is quite similar to that in the ENSO year winter with its sign reversed. During the ENSO year winter, the mixed layer in the Kuroshio current region south of Japan was thinner and warmer than those in the other three categorized winters. One of possible causes for these differences was attributed to the strength of the east Asian winter monsoon. It is relatively weaker in the ENSO year winter, compared with the other three categorized winters.
A number of duststorms and/or sandstorms occurred in the deserts and loesslands of the Asian Continent in early March of 1986. After a few days the dust laden air was transported over the Yellow Sea to Japan by westerly winds. On 12-13 March, a number of Japanese meteorological observatories reported a "Kosa (Asian dust)"phenomenon. The lidar observation of the Kosa was made at Tsukuba, Japan from 15 JST to 21 JST on 13 March 1986. The vertical structure and time change of the Kosa layer observed by the lidar are presented. At 15 JST, two Kosa layers existed at 4km and 2km, respectively. The upper layer had a thickness of about 1km and a scattering ratio of 3.2. The lower layer had a scattering ratio of 2.6 and appeared to be mixed with background aerosols. Subsequently, the Kosa layer at 4km increased in thickness and scattering intensity, with a thickness of 1.5km and a scattering ratio of 5.7. At 18 JST the Kosa layer at 4km separated into two sublayers at 4.5km and 3.5km. The total thickness of the upper and lower sublayers was 2.3km. The lidar derived optical thickness was 0.086 (wavelength 694.3nm). From 18 to 20 JST, the Kosa layer gradually lowered 0.5km. At 20 JST the Kosa layer separated into three sublayers at 4.0km, 3.2km and 2.7km. Analysis of concurrent radiosonde data showed that the upper and lower sublayers were dry, while the middle sublayer was humid. A numerical simulation was carried out to investigate the long range transport of the Kosa particles. Simulated horizontal and vertical distributions of the tracers were in good agreement with the lidar observation at Tsukuba and the routine meteorological observations in Japan and China. In particular, the observed structure of the two Kosa layers was well simulated. The two Kosa layers were found to originate from different altitudes over the source regions. The numerical simulation reveals the Loess Plateau and its neighboring deserts as important sources for the Kosa. Another possibility includes the Takla Makan Desert. Travel time of the Kosa particles to reach Japan was two to three days from the Loess Plateau and its neighboring deserts, and five to six days from the Takla Makan Desert.
In order to examine the vertical structures of the atmospheric electrical potential gradient and the behavior of charges on precipitation particles during snowfalls in the lower atmospheric layer, tethered balloon observations were carried out with several observation stages secured to its tether. Simultaneously, data observed on the ground and on the roof of the nearby building were analyzed. The main results obtained are as follows: i) The mirror image relation between the polarities of the electric potential gradient and precipitation charges was always observed near the ground, however, this relation was not always observed aloft. ii) Charges on snow particles were weaker aloft and stronger near the ground surface. Thus it is suggested that snow particles were electrified rapidly near the ground surface. iii) When the height of the cloud base was low and the highest observation stage of the tethered balloon was in the cloud or near the cloud base, snow particles were electrified weakly but positively at the highest stage. They were not directly correlated to the polarity of the local electric potential gradient. On the other hand, snow particles were electrified negatively near the ground and the electric potential gradient was positive on the ground. Therefore it is surmised that the snow particles acquired negative charges during their fall and the mirror image relation was present near the ground. iv) The atmospheric electrical potential gradient on the ground was modified by the ions which were emitted by the corona discharge. v) The most suitable electrification mechanism seems to be the Wilson's selective ion capture process below the cloud base. The behavior of atmospheric electrical elements was in good agreement with those of simple numerical experiments reported by Asuma and Kikuchi (1987).
In April 1986, large amount of radioactive pollutant was emitted into the atmosphere by the nuclear accident at Chernobyl, and dispersed around the world. Numerical simulation of the global scale dispersion of the radioactive pollutant was carried out by using the operational meteorological model of the Japan Meteorological Agency. Calculated concentrations in many places over the world agree well with those observed. The numerical model showed that the radioactivity reached Japan about seven days after the accident with the maximum concentration of about 10pCi/m3, in agreement with observations.
The aim of this paper is to investigate the interaction between the Eurasian winter snow cover extent and the location of the April 500hPa ridge along 75°E for the period 1967 to 1986. Snow cover area and mean April ridge position data were from the NOAA/NESDIS Northern Hemisphere snow cover and ice charts and the normal upper-air charts of the India Meteorological Department (IMD) respectively. The relationship between the winter snow cover extent and the ridge location is negative (correlation coefficient =-0.63) and is significant above the 5% level. This preliminary result indicated that an extensive winter snow cover area was likely to maintain the April ridge position South of its 20-year mean location. An inverse relationship is more strong and significant (r=0.68) between the January snow cover and the April ridge at 500hPa position than the winter snow cover and the ridge. The regression equation between the January snow cover (S, in area) and the ridge location (R, in degrees) for the study period is R=33.68-0.65S. This relationship explains about 47% of the total variance. Finally, this interaction study suggested a considerable lingering effect of snow cover on the regional atmospheric circulation over India.