ENERGY BALANCE IN 40－50 DAY PERIODIC OSCILLATION OVER THE ASIAN SUMMER MONSOON REGION DURING THE 1979 SUMMER

Based on calculations of data from FGGE Level III b, a discussion is made of the energy balance in the 40－50 day periodic oscillation over the Asian monsoon region during the 1979 summer. It is found that the main source of 40－50 day periodic perturbation is the monsoon region extending from central South Asia to Southeast Asia. In the upper layer over the North Pacific subtropical area (10－20oN, 150oE－150oW) pres-sure work turns into kinetic energy that maintains 40－50 day periodic perturbation associated with the variation in position and intensity of the mid-Pacific trough. The mean energy budget in the three-dimensional space (0－30oE, 30oE－150oW, 100－1000 hPa) indicates that the 40－50 day periodic perturbation transports kinetic energy to a seasonal mean and a transient perturbation wind field.     

THE TRANSFER OF PHYSICAL QUANTITIES IN QDPO AND ITS RELATION TO THE INTERACTION BETWEEN THE NH AND SH CIRCULATIONS

Based on the 1979 FGGE Level III b data, calculation is made of the transfer of sensible and latent heat and momentum due to a quasi-40-day periodic oscillation (QDPO) on a cross-equatorial meridional vertical cross-section, and analysis is done of the characteristics of the transfer at all phases of QDPO, with the following results obtained:1) During the monsoon's QDPO activation and break phases, a strong transfer of sensible heat to the SH is felt in the upper troposphere over the Asian monsoon region; the conversion of perturbation effective potential into its kinetic energy attains its maximum at 500-300 hPa (15°N), serving as the source of kinetic energy for the quasi-40-day periodic perturbation; an intense transfer of potential energy is found above 200 hPa from the monsoon area to the SH to maintain the QDPO at the tropical latitudes;2) During the QDPO activation-break (and reverse) transitional phase the conversion of perturbation effective potential into kinetic energy reaches its maximum in     

The transfer of physical quantities in QDPO and its relation to the interaction between the NH and SH Circulations

Based on the 1979 FGGE Level III b data, calculation is made of the transfer of sensible and latent heat and momentum due to a quasi-40-day periodic oscillation (QDPO) on a cross-equatorial meridional ver-tical cross-section, and analysis is done of the characteristics of the transfer at all phases of QDPO, with the following results obtained:1) During the monsoon’s QDPO activation and break phases, a strong transfer of sensible heat to the SH is felt in the upper troposphere over the Asian monsoon region; the conversion of perturbation effective potential into its kinetic energy attains its maximum at 500-300 hPa (15oN), serving as the source of kinetic energy for the quasi-40-day periodic perturbation; an intense transfer of potential energy is found above 200 hPa from the monsoon area to the SH to maintain the QDPO at the tropical latitudes;2) During the QDPO activation-break (and reverse) transitional phase the conversion of perturbation effective potential into kinetic energy reaches its maximum in the middle and lower troposphere over the SH middle latitudes and an appreciable lower transfer of potential energy occurs towards the SH tropical latitudes and the NH.3) The upper-tropospheric powerful transfer of westerly momentum caused by QDPO is discovered from the SH tropical latitudes to the NH, and the resulting momentum divergence and convergence are unfavorable for the maintenance of the seasonal mean fields of the NH tropical easterly and SH subtropical westerly winds.Finally possible synoptical processes responsible for QDPO are discussed together with its relation to the interaction between the circulations of both the hemispheres. It is found that QDPO is both the result of and medium for the interaction.     

Barotropic processes associated with the development of the Mei-yu precipitation system

The barotropic processes associated with the development of a precipitation system are investigated through analysis of cloud-resolving model simulations of Mei-yu torrential rainfall events over eastern China in mid-June 2011. During the model integration period, there were three major heavy rainfall events: 9–12, 13–16 and 16–20 June. The kinetic energy is converted from perturbation to mean circulations in the first and second period, whereas it is converted from mean to perturbation circulations in the third period. Further analysis shows that kinetic energy conversion is determined by vertical transport of zonal momentum. Thus, the prognostic equation of vertical transport of zonal momentum is derived, in which its tendency is associated with dynamic, pressure gradient and buoyancy processes. The kinetic energy conversion from perturbation to mean circulations in the first period is mainly associated with the dynamic processes. The kinetic energy conversion from mean to perturbation circulations in the third period is generally related to the pressure gradient processes.     

ON THE PROCESS OF SUMMER MONSOON ONSET OVER EAST ASIA

Using daily observational rainfall data covered 194 stations of China from 1961 to 1995 andNCEP model analyzed pentad precipitation data of global grid point from 1979 to 1997,thedistribution of onset date of rainy season over Asian area from spring to summer is studied in thispaper.The analyzed results show that there exist two stages of rainy season onset over East Asianregion from spring to summer rainy season onset accompanying subtropical monsoon and tropicalmonsoon respectively.The former rain belt is mainly formed by the convergence of cold air and therecurred southwesterly flow from western part of subtropical high and westerly flow from the so-called western trough of subtropical region occurring during winter to spring over South Asia.Thelatter is formed in the process of subtropical monsoon rain belt over inshore regions of South ChinaSea originally coming from south of Changjiang (Yangtze) River Basin advancing with northwardshift of subtropical high after the onset of tropical monsoon over South China Sea.The pre-floodrainy season over South China region then came into mature period and the second peak of rainfallappeared.Meiyu,the rainy season over Changjiang-Huaihe River Basin and North China thenformed consequently.The process of summer tropical monsoon onset over South China Sea in 1998is also discussed in this paper.It indicated that the monsoon during summer tropical monsoononset over South China Sea is the result of the westerly flow over middle part of South China Sea,which is from the new generated cyclone formed in north subtropical high entering into SouthChina Sea,converged with the tropical southwesterly flow recurred by the intensified cross-equatorial flow.     

THE EFFECTS OF THE QINGHAI-XIZANG PLATEAU ON THE MEAN SUMMER CIRCULATION OVER EAST ASIA

Four numerical experiments of simulation have been conducted in this paper by the use of a five-layer primitive equation numerical model with incorporated pressure-sigma vertical coordinate system. The initial fields are taken from the July zonal mean data of many years, while the heat sources and sinks are ideally specified according to the mean heating field over the East Asia calculated from the real data of July, 1979. On the basis of simulated results of temperature and geopctential height patterns we emphatically discuss the effects of the topography and the heating of the Qinghai-Xizang Plateau. From the analyses in this paper, it appears that the heating over the Bengal region makes a larger contribution to the middle and the south branches of the monsoon cell and is also the main cause for the existence of the southerly channel to the east of the Plateau, for the break of the subtropical anticyclcne belt below the 500 hPa level and for the formation of the summer Asian anticyclone at the 300 hP     

EFFECTS OF ANOMALOUS HEATING ON SUMMER MONSOON OVER ASIA

Based on a zonally non-uniform mean circulation in summer simulated by numerical modelling,perturba-tion heatings ever South Asia and a perturbation cooling over Northwest Australia were incorporated in a nu-merical model to discuss their effects on summer monsoon over Asia and the structure of flow disturbance.     

THE EFFECTS OF TOPOGRAPHY ON THE SUMMER ATMOSPHERIC ENERGETICS OF THE NORTHERN HEMISPHERE IN A LOW-RESOLUTION GLOBAL SPECTRAL MODEL

An analysis is made of the effects of topography on the summer atmospheric energetics of the Northern Hemisphere in a low-resolution global spectral model. The numerical model is a global, spectral, primitive equation model with five equally spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. The model includes comparatively full physical processes.Each term of the energy budget equations is calculated in four specific latitudinal belts (81.11°S-11.53°S; 11.53°S-11.53°N; 11.53°N-46.24°N; 46.24癗-81.ll癗) from a five-year simulation with mountains and a one-year simulation without mountains, respectively. Differences between them are compared and statistically tested. The results show that synoptical scale waves transport available potential energy and kinetic energy to long waves and increase conversion from available potential energy of the zonal flow to eddy's and from the eddy kinetic energy to the zonal kinetic energy in region 3 (11.53°N-46.24°N     

THE ATMOSPHERIC HEAT BUDGET IN SUMMER OVER ASIA MONSOON AREA

For better understanding the mechanism of monsoon formation and designing the numerical simulation of the general atmospheric circulation, a new approach of calculating atmospheric radiation is proposed to investigate the distribution of the atmospheric heat source, and the budget of heat component is recalculated. The results show that there is a tremendous atmospheric heat source region over central India, northeast of the Bay of Bengal, east of the South China Sea and about 10 °N at the west Pacific, among which the heating center with a maximum heating rate of 8 ℃/day is located over the Bay of Bengal and the average rate in the Plateau is about 1 ℃/day.     

Quasi-40-Day Oscillation and Its Teleconnection Structure together with the Possible Dependence on Conversion of Barotropic Unstable Energy of Temporal Mean Flow

A study is made of the distribution of the diagnostic quantity vector E and the teleconnection structure of 30-50 (quasi-40) day oscillation, together with the dependence on the conversion of barotropic unstable energy of mean flow in terms of ECWMF daily 500 hPa grid data in winter, indicating that the energy transportation is closely associated with the westerly jet position, with zonal (meridional) propagation in the strong (weak) wind region, that considerable conversion of barotropic energy occurs at the jet exit region where low-frequency oscillation gains energy from the mean flow, leading to maximum kinetic energy for the oscillation observed there, which is marked by evident barotropy in striking contrast to the baroclinicity at low latitudes and that the teleconnection core is related to the center of action in the atmosphere and bound up with the pattern of the west wind.     

THE PHYSICAL EFFECTS OF TOPOGRAPHY AND HEAT SOURCES ON THE FORMATION AND MAINTENANCE OF THE SUMMER MONSOON OVER ASIA

The physical effects of topography and heat bources on the formation and maintenance of the summer monsoon over Asia are discussed ia this paper by using the transformed Eularian-mean equations and a quasi-geostrophic 34-level spherical coordinate model.The computed results of the divergence of the E-P flux, the induced meridional circulation and the perturbation geostrophic wind speed induced by the forcing of topography and heat sources show that the diabatic heating effect over the Tibetan Plateau may play an important role for the formation and maintenance of the summer monsoon over Asia, which is much greater than the dynamical effect of topography.The computed results also show that, of the physical effects of topography and heat sources on the formation and maintenance of the summer monsoon over Asia, the effect of forced meridional circulation is larger than that of the divergence of E-P flux of the induced waves.     

SATELLITE-OBSERVED RADIATIVE ENERGY BUDGET DURING ONSET PHASE OF SUMMER MONSOON

In this paper satellite-derived radiative energy budget such as shortwave radiative heating,longwaveradiative heating and net radiation balance have been studied for the onset phase of summer monsoon1979. Since cloud play an important role in determining diabatic heating field as well as being a reflectionof status of the monsoon itself, the day to day evolution of clouds from TIROS-N satellite has beenmade. Satellite-derived radiative heating rates from surface to 100 hPa were computed for each 100 hPathickness layer. These heating rates were then compared with the observed latitudinal distribution of total radintive heating rates over the domain of the study. From the results of our study it was found thatthe characteristic features such as net radiative heating rates of the order of 0.2℃/day at upper tropospheric level(100-200 hPa)and cooling throughout the lower tropospheric layers with relatively lesscooling between 500-700 hPa layer observed.in a case of satellitexierived radiative energy budget agreewell with the characteristic features of observational radiative energy budget over the domain of thestudy. Therefore, it is suggested that radiative energy budget derived from satellite observati0ns be usedwith great potential and confidence for the evolution of complete life cycle of monsoon over the Indianregion for different years.     

MAINTENANCE AND OSCILLATION MECHANISMS OF SUMMER TROPICAL UPPER-TROPOSPHERIC EASTERLIES

The mechanisms of the maintenance and oscillation of 1982 summer tropical 200-hPa mean easterly flow and extra-long waves are investigated in terms of the energy equations in wavenumber-frequency space. Calculation results show that the difference in heating between land and sea and the boundary effect serve as the main source of energy; frictional dissipation as the sink; the conversion of available potential energy into kinetic takes place dominantly in the waves of number 1-2; such transformation is accomplished in just a small amount in zonal mean flow and therefore can be ignored because of the value.In the interaction between wave and zonal mean flow, the latter loses its available potential and gains kinetic energy. The tropical easterly belt over 20°N-5°S is found barotropically stable and that over 10°N-5°S, unstable. The waves of number 2 and 1 manifest themselves a primary source and sink of kinetic energy, respectively, in the interplay between waves and between zonal mean flow and wave.     

EFFECT OF WINTER MONSOON ON SUMMER MONSOON THROUGH AIR-SEA INTERACTION

In this paper the relationships between the sea surface temperature (SST) of Xisha and that in the northern Indianand northern Pacific Oceans,the geopotential height at 500 hPa level of the Northern Hemisphere,and rainfall in Chinaare studied statistically using data in the period of 1961—1992.Results show that in winter,the interannual variation inSST of Xisha describes that for a large oceanic region off the East Asia coast,and is closely related to the activity of EastAsia winter monsoon.On the other hand,there exist very high values of auto-correlation of Xisha SST anomaly fromDecember through the following July,but the anomalous condition is hardly correlated to that in the preceding autumn.The winter monsoon related anomalous SST condition in Xisha has a strong tendency to persist through the succeedingsummer monsoon season with the same sign.In addition,correlation maps of monthly mean rainfall in China with re-spect to Xisha SST of the same month show positive correlations with confidence level above 95% to the east of 110°Eand to the south of Changjiang (Yangtze) River during the months of October through April;the region becomes smal-ler in May and changes correlation sign in June;the positive correlation region is located in the middle and lower reachesof Changjiang River from July to September.The air-sea interaction plays an important role in these processes.     

THE OPERATIONAL SEASONAL FORECASTING OF THE SUMMER RAINFALL IN CHINA

The paper presents a review of the success and failure of the practical results from summer drought and flood forecasts and seasonal precipitation forecasts in the period from 1976 to 1985. An analysis is made on the anomaly of the general circulation wihch gives rise to summer precipitation and drought-flood occurrences in the country. It is proposed that the subtropical high over the West Pacific, the South Asia high and middle-latitude westerlies are the major synoptical regimes producing summer weather in China. The analysis focuses on the features of low-frequency oscillation and abnormality of the West Pacific subtropical high in the monthly 500 hPa mean charts, and on their interactions with the sea temperature of the North Pacific and the Equatorial Pacific. The result shows that there exist quasi-cycles of 3-4 years, 11 years and 19 years or so in the subtropical high with the feature of strong persistence and seasonal changes. There is a rather good correlation between the behaviour of the sub     

THE SOUTHERN HEMISPHERE MID-LATITUDE QUASI-40-DAY PERIODIC OSCILLATION WITH ITS EFFECT ON THE NORTHERN HEMISPHERE SUMMER MONSOON CIRCULATION

By using the method of power spectrum combined with band-pass filtering with May—September1982 grid data from ECMWF,the spatial structure and propagation characteristics are tentatively examinedof the SH (Southern Hemisphere) mid-latitude quasi-40-day (ranging from 30—60 days) periodic oscilla-tion (QPO) together with the relation to NH (Northern Hemisphere) summer monsoon,with the result thatthere exists similar periodicity in the development of baroclinicity and activities of cold air at the same latitudes,and in response to this the zonal wind shows profound QPO with a nearly vertical axis of disturbance and thatthe air activities can act as periodic external forcing for the monsoon,which intensifies the west wind on thesouth side of the Mascarene or Australian high,and then the system itself,leading to the reinforcement of theSE trade wind on the north side,followed by the strengthening of cross-equatorial flow that,in turn,causesactive monsoon with its northward march over the eastern part of China.The process is responsible for thelow-frequency oscillation propagated in a meridional direction,which confirms the speculation of theauthor.     

INTERACTION BETWEEN TROPICAL CYCLONE AND MEIYU FRONT

Generally speaking,the convection activities are inactive over western Pacific warm pool andtropical cyclone(TC)activity seldom occurs over the offshore of East Asia during the period ofMeiyu rainfall.However,if a TC is active in this area,the Meiyu rainfall will often weaken or endup.Based on a statistical study with the data from 1980 to 1995,it is found that about 91% of 23TC activities affected the intensity of Meiyu rainfall,and 50% of the end-up of Meiyu events wererelated to the active TCs and the change of subtropical high.The present paper simulates the effectof TC on Meiyu circulation by using MM4 model,and the results agree with the observations.From the point of view of vapor and energy transport,the landing of TC cuts not only thetransport of the water vapor to Changjiang-Huaihe River basin from the Bay of Bengal but also theconversion of the mean flow energy to the Meiyu circulation because of the TC forcing to the zonalcirculation.These two effects make the convection and perturbation existing in Meiyu region lackthe supply of the vapor and energy for their maintenance and lead to the end of Meiyu rainfall.     

Weakening of Indian summer monsoon in recent decades

The analysis of 43 years of NCEP-NCAR reanalysis data and station observations reveals the connections between tropospheric temperature variations and the weakening of the Indian summer monsoon circulation. The Indian summer monsoon variation is strongly linked to tropospheric temperature over East Asia, showing significant positive correlations of mean tropospheric temperature with all-Indian summer rainfall and the monsoon circulation intensity. The result shows that Indian summer monsoon circulation underwent two weakening processes in recent decades. The first occurred in circa the mid-1960s, and the other occurred in circa the late 1970s. The finding indicates that the mean tropospheric temperature may play a crucial role in the weakening of the Indian summer monsoon intensity via changing land-sea thermal contrast. The role of the tropospheric temperature contrast between East Asia and the tropical area from the eastern Indian Ocean to the tropical western Pacific is to weaken the Indian summer monsoon circulation.     

THE INDIAN SUMMER MONSOON AND SUMMER RALNFALL IN NORTH CHINA

The analyses have been made of the summer precipitation data over Indian and North China during1891—1983.The statistic results show that the climatic characteristics of the Indian summer monsoon rainfallare similar to summer rainfall in North China,and a steady and significant positive correlation exists be-tween them.The circulation systems associated with the Indian monsoon and the rainfall in North China in summerhave also been discussed.It is found that there are same predictors in April to be used for the forecast ofNorth China rainfall and Indian monsoon.     

Characteristics of Zonal Propagation of Atmospheric Kinetic Energy at Equatorial Region in Asia

Based on the daily NCEP/NCAR reanalysis dataset from 1980 to 1997, the zonal propagations of 850 hPa kinetic energy (KE) and meridional wind (v) at equatorial region are examined respectively. Results show that the strongest center of KE in the tropical Asian monsoon region is located at 75°-90°E, with the secondary over the Somalia low-level jet channel, i.e., about 50°E. East to 90°E, disturbances of both KE and v observed are mainly coming from the western Pacific Ocean and propagating westward to the Bay of Bengal (BOB) passing through the South China Sea. But the propagation directions of both KE and v are rather disorderly between the BOB and the Somalia jet channel. Therefore, the East Asian summer monsoon and the Indian summer monsoon are different in the propagating features of the disturbances of KE and v. Above facts indicate that East Asian monsoon system exists undoubtedly even at the equatorial region, and quite distinct from the Indian monsoon system, it is mainly affected by the disturbances coming from the tropical western Pacific rather than from the Indian monsoon region. The boundary of the two monsoon systems is around 95°-100°E, which is more westward than the counterpart as proposed in earlier studies by 5-10 degrees in longitude.