| dc.description.abstract | South Asian High(SAH) appears in the upper troposphere in the Northern Hemisphere summer. Based on the analyses of the International Geophysical Year (IGY) data, it has been recognized that the SAH is the strongest and the steadiest circulation system at the 100 hPa level besides the polar vortex. The forming reason of SAH is due to the solar diabetic heating, and the distribution of the SAH center has a great effect on the summer monsoon over East and South Asia.
The purpose of this research is to discuss the relationship between the location of SAH and the early-summer monsoon over Asia. SAH usually appears over Indo-China during April and May, and moves to Tibetan Plateau with time. According to the analyzed result from reanalysis data, we find that owing to the influence of climate change, the time of the SAH staying over different regions is quite different in different time periods for the past few decades.
The distribution and variation of the SAH center during May and June from 1948 to 1994 is investigated by doing pentads analysis of NCEP/NCAR reanalysis data. In the choice of the years, we pick up May and June from 1948 to 1994. The movement of the SAH centers varies a lot after 1978. As a result, we separate the time period into two parts, which are 1948-1978 and 1979-1994.
Based on the result from the reanalysis data, we find that the longitude distribution of SAH center had clear bimodality. In other words, the maximum of the geopotential height of SAH frequently appeared in two different regions. The bimodality structures in different periods are obviously different. According to the longitude distribution of SAH center, we classify it into three types, which are Iranian Mode(IM), Tibetan Mode(TM), Indo-China Mode(ICM). The total appearance percentage of these three modes are both more than 60% in two periods, which means SAH often appeared in these three areas over the past few decades. The variation in the late 1970s mainly comes from TM and ICM.
According to the statistical results, the percentages of TM and ICM are 23.4% and 30.4% respectively from 1948 to 1978. However, the percentage of TM decreased to 20.8% and the percentage of ICM increased to 39.1% from 1979 to 1994, and the main difference is in May. For the average of the climate state in two periods, we find that during 1979-1994, the monsoon circulation over South and East Asia weakened with the increase of temperature compared to 1948-1978. The results of the average of ICM in two periods are quite similar to those of two periods, which also shows weakened monsoon circulation in Northern Indian Ocean and East Asia. The anticyclone of SAH in the upper troposphere also tends to be weak. However, if we just take TM into consideration, then the phenomenon is quite different from ICM. Although we can still notice the weakened monsoon circulation over East Asia, the southwestern wind near the surface becomes stronger over Northern Indian Ocean, including Bay of Bengal and Arabian Sea. In addition, the anticyclone of SAH in the upper troposphere also becomes stronger. The difference of two modes in two periods also show difference on the monsoon index defined by Webster and Yang(1992). The monsoon index of ICM decreases, but that of TM increases.
Because there isn’t any available reanalysis data of precipitation before 1978, we use the data of winds and mixing ratio to calculate the convergence of water vapor flux. According to the results, there are some similar parts and different parts of ICM and TM in two periods. The similar parts are about the divergence of water vapor flux over India and Indo-China to South China. From 1979 to 1994, the divergence over this region of ICM and TM has both extended to West Asia. The difference of two modes are the convergence over western Pacific Ocean, including Maritime Continent, Philippine, Taiwan, Korea, and Japan. From 1979 to 1994, compared to 1948-1978, the convergence here of ICM has become weak, but the convergence here of TM has become strong.
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