EFFECT OF THE INDIAN PENINSULA ON THE COURSE OF THE ASIAN TROPICAL SUMMER MONSOON

In terms of the NCAR Community Climate Model (CCM3),the effect of the Indian Peninsula on the course of the Asian tropical summer monsoon is simulated in this paper,and numerical experimental results show that the Indian Peninsula plays a critical role in the establishment process of the Asian tropical summer monsoon.When the CCM3 includes the Indian Peninsula, the model successfully simulates out the course of the Asian tropical summer monsoon,i.e.the South China Sea (SCS) summer monsoon at first bursts in middle May,while the Indian monsoon just establishes until middle June.However when the Indian Peninsula topography is deleted in the model,the Indian and SCS summer monsoons almost simultaneously establish in late May. Numerical results further indicate that in the former experiment the sensible heating of the Indian Peninsula warms the air above and produces evident temperature contrast between the peninsula and its adjacent SCS and Bay of Bengal (BOB).which results in the strengthening and maintenance of the BOB trough in the low-middle layer of the troposphere in the end of spring and early summer and thus the earliest establishment of the Asian tropical summer monsoon in the SCS in middle May.However,the Indian summer monsoon just establishes until middle June when the strong west wind over the Arabian Sea shifts northwards and cancels out the influence of the northwest flow behind the BOB trough.In the latter experiment the effect of Tibetan Plateau only produces a very weak BOB trough,and thus the SCS and Indian summer monsoons almost simultaneously establish.

THE TIMING OF SOUTH-ASIAN HIGH ESTABLISHMENT AND ITS RELATION TO TROPICAL ASIAN SUMMER MONSOON AND PRECIPITATION OVER EAST-CENTRAL CHINA IN SUMMER

The timing of the South Asian High(SAH) establishment over the Indochina Peninsula(IP) from April to May and its relations to the setup of the subsequent tropical Asian summer monsoon and precipitation over eastern-central China in summer are investigated by using NCEP/NCAR daily reanalysis data,outgoing longwave radiation(OLR)data and the daily precipitation data from 753 weather stations in China.It is found that the transitions of the zonal wind vertical shear and convection establishment over tropical Asia are earlier(later) in the years of early(late) establishment of SAH.In the lower troposphere,anti-cyclonic(cyclonic) anomaly circulation dominates the equatorial Indian Ocean.Correspondingly,the tropical Asian summer monsoon establishes earlier(later).Furthermore,the atmospheric circulation and the water vapor transport in the years of advanced SAH establishment are significantly different from the delayed years in Asia in summer.Out-of-phase distribution of precipitation in eastern-central China will appear with a weak(strong) SAH and western Pacific subtropical high,strong(weak) ascending motion in the area south of Yangtze River but weak(strong) ascending motion in the area north of it,and cyclonic(anti-cyclonic) water vapor flux anomaly circulation from the eastern-central China to western Pacific.Accordingly,the timing of the SAH establishment at the upper levels of IP is indicative of the subsequent onset of the tropical Asian summer monsoon and the flood-drought pattern over eastern-central China in summer.

Temperatures and Winds over Tropical Middle Atmosphere during Two Contrasting Summer Monsoons, 1975 and 1979

Using the monthly geopotential heights and winds for 700 and 200 hPa for India during July and August, and the weekly M-100 Soviet rocketsonde temperature and wind data for Thumba (8.5 ° N, 76.9 ° E) during the last week of June and the first week of September for the two contrasting summer monsoon years 1975 (a very strong monsoon year) and 1979 (a very weak monsoon year), a study has been made to examine the mean circulation features of the troposphere over India, and the structures of the temperatures and the winds of the middle atmosphere over Thumba. The study suggested that the axis of the monsoon trough (AMT) at 700 hPa shifted southward in 1975 and northward towards the foothills of the Himalayas in 1979, from its normal position. Superimposed on the low-pressure area (AMT) at 700 hPa, a well-defined divergence was noticed at 200 hPa over the northern India in 1975.The mean temperatures at 25,50 and 60 km (middle atmosphere) over Thumba were cooler in 1975 than in 1979. While a cooling trend in 1975 and warming trend in 1979 were observed at 25 and 50 km, a reversed picture was noticed at 60 km. There was a weak easterly / westerly (weak westerly phase) zonal wind in 1975 and a strong easterly zonal wind in 1979. A phase reversal of the zonal wind was observed at 50 km. A tentative physical mechanism was offered, in terms of upward propagation of the two equatorially trapped planetary waves i.e. the Kelvin and the mixed Rossby-gravity waves, to explain the occurrence of the two spells of strong warmings in the mesosphere in 1975.

The Earliest Onset Areas and Mechanism of the Tropical Asian Summer Monsoon

The multi-yearly averaged pentad meteorological fields at 850 hPa of theNCEP/NCAR reanalysis dada and the TBB fields of the Japan Meteorological Agency during 1980-1994 areanalyzed. It is found that if the pentad is taken as the time unit of the monsoon onset, then thetropical Asian summer monsoon (TASM) onsets earliest, simultaneously and abruptly over the wholearea in the Bay of Bengal (BOB), the Indo-China Peninsula (ICP), and the South China Sea (SCS), eastof 90°E, in the 27th to 28th pentads of a year (Pentads 3 to 4 in May), while it onsets later inthe India Peninsula (IP) and the Arabian Sea (AS), west of 90°E. The TASM bursts first at the southend of the IP in the 30th to 31st pentads near 10°N, and advances gradually northward to the wholearea, by the end of June. Analysis of the possible mechanism depicts that the rapid changes of thesurface sensible heat flux, air temperature, and pressure in spring and early summer in the middleto high latitudes of the East Asian continent between 100°E and 120°E are crucially responsiblefor the earliest onset of the TASM in the BOB to the SCS areas. It is their rapid changes thatinduce a continental depression to form and break through the high system of pressure originallylocated in the above continental areas. The low depression in turn introduces the southwesterly tocome into the BOB to the SCS areas, east of 90° E, and thus makes the SCS summer monsoon (SCSSM)burst out earliest in Asia. In the IP to the AS areas, west of 90° E, the surface sensible heatflux almost does not experience obvious change during April and May, which makes the tropical Indiansummer monsoon (TISM) onset later than the SCSSM by about a month. Therefore, it is concluded thatthe meridian of 90° E is the demarcation line between the South Asian summer monsoon (SASM, i.e.,the TISM) and the East Asian summer monsoon (EASM, including the SCSSM). Besides, the temporalrelations between the TASM onset and the seasonal variation of the South Asian high (SAH) arediscussed, too, and it is found that there are good relations between the monsoon onset time and theSAH center positions. When the SAH center advances to north of 20°N, the SCSSM onsets, and tonorth of 25° N, the TISM onsets at its south end. Comparison between the onset time such determinedand that with other methodologies shows fair consistency in the SCS area and some differences inthe IP area.