氣膠對東亞夏季季風之影響

Abstract

亞洲擁有世界最顯著的季風區，且為人口分布最密集之地區。除了南亞外，東亞是另一個同樣有強季風系統、高人口及高人為汙染的地區。少數針對東亞的研究皆認為，氣膠對季風的影響導致地表溫度降低、降水減少及季風強度減弱，但對其動力回饋機制缺乏深入探討。本研究之目的為瞭解氣膠的效應對東亞季風的影響，利用NCAR/CESM 加入單層海洋模式以提供良好的海洋回饋，模擬東亞在西元1850及2000年不同氣膠排放條件去估計人為氣膠所產生的影響，並從不同的角度，包括動力回饋機制，探討氣膠對夏季氣候之影響。 本研究結果顯示，總氣膠驅動力造成的直接效應反映在氣膠排放源區；氣膠間接效應隨著雲系綜觀系統的移動而擴大範圍，平均造成了東亞地區的地表溫度及降水減少，分別為-1.07°C 和 -0.46 mm/day。氣膠輻射效應造成之動力回饋現象，主要透過間接影響海表面溫度及太平洋高壓脊強度，再經由「風－蒸發－海表面溫度」的正回饋作用(Wind-Evaporation-Sea-Surface-Temperature feedback, WES feedback)使得地表溫度與輻射變化的地理分布有不一致性。主要氣膠排放區地表與低對流層溫度下降，然而上升運動卻加強了，使梅雨鋒面增強及降水增加；這是主要透過激發Pacific-Japan pattern的回饋機制所導致。而整體東亞夏季季風強度則為減弱。 另分析不同性質之氣膠種類的影響，顯示吸收性氣膠（黑碳）及非吸收性氣膠所造成地表溫度變化分別為 0.18 °C 和 -0.53 °C，對降水的影響則分別為-0.35 mm/day和-0.34 mm/day。非吸收性氣膠會散射短波輻射，降低地表溫度與增加大氣穩定度；吸收性氣膠會吸收輻射加熱大氣，而底層較熱的大氣會加熱地表，抵銷其阻擋短波輻射到達地表之作用，但合起來的效果仍增加大氣穩定度。非吸收性氣膠和吸收性氣膠兩者皆有和總氣膠相同的WES feedback及Pacific-Japan pattern回饋機制，惟強度較弱，兩者在東亞夏季季風的強度皆顯示為減弱，而吸收性氣膠影響的減弱程度較小。

Asia has the strongest monsoon system in the world and has the most population continent. In addition to Indian monsoon, East Asia is another area with strong monsoon system, high population and high aerosol pollution. The relevant studies show that aerosol causes decrease in surface temperature, reduction of precipitation and the weakness of monsoon system. However, less research was studied further into the dynamical feedback. This study focuses on the how aerosol will change the East Asian summer monsoon. We apply the NCAR/CESM global model with slab ocean model to get a better ocean feedback, and simulate the year 1850 and 2000 emission scenarios in order to estimate the influence of anthropogenic aerosol forcing and to approach the problem from different perspectives, including dynamical feedback. The result shows that in total aerosol forcing, the direct radiative forcing is main-ly on the high aerosol distribution area. Aerosol indirect effect follow the cloud movement of the synoptic system and influence the whole Asia. The mean regional surface temperature and precipitation decreased by -1.07°C and -0.46 mm/day, respec-tively, over East Asia , but the responses vary significantly from region to region. The dynamical feedback of aerosol forcing, which affect the sea surface temperature and the strength of Pacific high ridge by indirect effect, shows that the wind-evaporation-SST feedback causes the inconsistency distribution between surface temperature and radiation. The main aerosol emission region shows the reduction of temperature, but cause the updraft at the mei-yu front and has more precipitation in the result. This may due to trigger the Pacific-Japan pattern (P-J pattern) feedback, while we see the similar structure in the result. Overall, aerosol particles cause large regional subsidence anomaly, resulting in weakened summer monsoon. When considering only absorbing (black carbon) or non-absorbing (sulfate) aero-sols, the changes in surface temperature are +0.18°C and -0.53°C, respectively; where-as the changes in precipitation are -0.35 mm/day and -0.34 mm/day, respectively. Non-absorbing aerosol can scatter the solar radiation, causing the temperature decrease and stabilize atmosphere. Absorbing aerosol shows heating atmosphere can com-pensate the radiation reduction at surface, while the combined effect shows stabilizing atmosphere. The dynamical feedback of WES feedback and P-J pattern are show in the absorbing and non-absorbing aerosol result, only with a weaker phenomenon.