中南半島生質燃燒對東南亞夏季季風肇始之影響

Abstract

每年夏季季風肇始前的乾季，中南半島因農業活動所造成的生質燃燒，會排放大量的懸浮微粒到大氣中。本研究猜想這些懸浮微粒可能會改變東南亞季風演變的歷程。然而單純以觀測資料，難以去釐清這些懸浮微粒對季風系統的影響、以及可能造成的機制。為此本研究使用GEOS-5全球大氣氣候模式，去模擬2018年夏季季風肇始前後的氣候特徵；藉由開關中南半島上的生質燃燒排放作為實驗組及對照組，來探討懸浮微粒的輻射效應可能對東南亞季風演進所造成的影響。模擬結果顯示懸浮微粒的初步的輻射效應使中南半島當地的低層大氣變得更穩定。中南半島上對流抑制的環境阻礙了對流從赤道蘇門答臘地區北傳到中南半島的季節演變，也因此中南半島的夏季季風肇始時間被往後推移了兩候。季風肇始前的抑制環境也伴隨中南半島上反氣旋增強，進而增加華南、孟加拉灣等周遭區域的水氣輸送。當季節持續推移，夏季降水終究會在中南半島上發展，洗除懸浮微粒並終止其輻射作用。然而懸浮微粒在季風肇始前，已經改變了從孟加拉灣至中南半島上的水氣梯度。這將增加西南季風的水氣平流、強化季風肇始的強度，並加強夏季季風的環流。另外藉由診斷模式中各物理過程加熱率的變化，可發現季風肇始前環流與降水改變所造成的加熱，遠超過懸浮微粒的短波輻射直接影響。這也說明了懸浮微粒輻射的動力回饋效應藉由擾動大氣穩定度、與季風系統的非線性交互作用，可能顯著改變整個季節發展的演變歷程。

Regional emissions through biomass burning in Indo-China Peninsula (ICP) during March and April are considered to influence the monsoon development in ICP and neighboring areas. However, it is difficult to identify the aerosol influence and the relevant mechanism by observational approach alone without modeling experiments. In this study, the Goddard Earth Observing System Model, Version 5 (GEOS-5) Atmospheric Global Climate Model (AGCM) is used to simulate the 2018 monsoon development with and without biomass burning aerosol emission in ICP. Model results show that the aerosol-radiative forcing results in stronger anticyclonic circulation and reduced precipitation in the ICP region during the pre-onset period. The suppressed condition obstructs the convection propagation from Sumatra to ICP and therefore delays the onset of ICP summer monsoon by 2-pentads. The intensified anticyclonic flow increases the moisture and the precipitation in southern China and Bay of Bengal (BOB). The wetter BOB and drier ICP lead to a stronger moist advection by the developing southwesterly flow and consequently an abrupt ICP monsoon onset. As the season goes on, seasonal rainfall starts to wash out the aerosols in ICP that shuts down the radiation effect. By diagnosing the heat budget in the pre-monsoon period, we show that the effect of aerosol forcing in ICP is associated with an enhanced subsidence warming, reduced convective heating, and more longwave cooling, indicating an amplified regional effect of the cumulative radiative heating by circulation and convection in ICP and surrounding areas. Aerosol-cloud-radiative forcing significantly change the seasonal evolution in ICP and neighboring southeast Asia.