南亞高壓中心的位置分布在1970年代末期的變異對於亞洲夏季初期季風的影響

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题名:	南亞高壓中心的位置分布在1970年代末期的變異對於亞洲夏季初期季風的影響
作者:	鄭宇烜;Cheng, Yu-Hsuan
贡献者:	大氣科學學系
关键词:	南亞高壓;South Asia High
日期:	2019-08-21
上传时间:	2019-09-03 12:21:04 (UTC+8)
出版者:	國立中央大學
摘要:	本研究的目的在於探討南亞高壓的位置與亞洲地區初夏季風之間的關聯。透過NCEP/NCAR的重分析資料，藉由五日平均，來分析南亞高壓的中心位置分布，在年份與月份的選擇上，探討1948-1994年這段時期的五、六月。而南亞高壓中心主要活動範圍在1978年前後有較明顯的變化，因此本研究將1948-1994年畫分為兩個時段來分析，也就是1948-1978以及1979-1994兩段時期。根據重分析資料的五日平均分析，發現南亞高壓中心位置在經度分布上呈現顯著的雙峰結構，換言之，其重力位的最大值，也就是南亞高壓的中心，集中出現在兩個不同的區域，而不同時期的雙峰結構也有著明顯的差異。根據南亞高壓中心的經向分布，將其分成伊朗模(Irainian Mode)、青藏模(Tibetan Mode)、中南半島模(Indo-China Mode)，三種模在兩段時期的出現率總和皆大於60%。而在1978年前後的變化主要出現在TM與ICM之間。根據統計結果，由1948-1978年至1979-1994年，TM的比例減少、ICM則增加。就兩段時期的氣候平均狀態，我們發現1979-1994，相較於1948-1978，溫度上升的同時，整個南亞(北印度洋)與東亞區域(中國東部至台灣一帶)的季風環流也減弱了。而兩段時期的ICM平均，與兩段時期的平均相似，1979-1994相較於1948-1978，出現在北印度洋與東亞地區的是減弱的季風環流。若分析兩段時期的TM平均，1979-1994相較於1948-1978，雖然在東亞地區一樣是減弱的季風環流，但在北印度洋的區域，近地表出現的是增強的西南風分量，而在對流層高層，南亞高壓的反氣旋環流也增強。ICM與TM在兩段時期的差異也反映在了Webster and Yang(1992)定義的季風指數上：ICM的季風指數減小，而TM的季風指數則增大。本研究也分析了水氣通量的輻合輻散，由結果發現ICM與TM在兩段時期的差異有相同處，也有著相異處：相似的地方是位於印度洋、中南半島至中國華南一帶的水氣通量輻散場，到了1979-1994年，ICM與TM在此區域的水氣通量輻散皆有往西北延伸至西亞的趨勢。而相異處主要位在西太平洋地區(包含海洋大陸、菲律賓、台灣、韓國與日本)，在1948-1978年，此處的水氣通量為輻合，到了1979-1994年，ICM在此區域的水氣通量輻合有減弱的趨勢，而TM在此區域的水氣通量輻合則有增強的趨勢。 ;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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