人為氣膠排放與灌溉活動對北印度乾季氣候之影響

Abstract

本研究探討人為氣膠和灌溉活動對於北印度地區乾季之乾旱現象是否有顯著的影響。觀測資料顯示北印度地區乾季（十月到十二月）降水在過去十年有顯著下降趨勢，導致乾旱並造成當地經濟衝擊與糧食安全問題。乾季亦是北印度地區人為氣膠排放和灌溉的最主要的季節，而近十年印度地區的人為氣膠排放與灌溉活動皆有明顯增加。過去研究分別指出氣膠及灌溉可造成印度地區濕季季風環流及降水改變，因此本研究希望瞭解北印度乾季降水對人為氣膠與灌溉增加的可能反應及其機制，使用全球模式以給定海溫氣候值的方式進行理想化氣候平衡模擬。結果顯示只提高人為氣膠排放量使北印度乾季平均短波輻射通量減少8.3 W/m2 (-18.6%)，造成平均0.28 ℃的地面冷卻。氣膠吸收短波輻射加熱大氣使北印度地區出現相對的上升運動。且可能受季風風向的影響，在下游的印度西南地區出現下沉區，伴隨印度西側平均降水量減少7.2 mm/month (-29.8%)的。加入灌溉之模擬的結果顯示北印度乾季地表潛熱通量平均減少11.4 W/m2 (-14.1%)，地表平均溫度降低0.42 ℃，且地表降溫範圍較大。灌溉區域的海平面氣壓值上升，在異常高壓南緣平均降水量減少2.4 mm/month (-12.1%)。同時提高人為氣膠排放量以及加入灌溉使北印度乾季平均地表短波輻射通量減少10.1 W/m2 (-20.2%)以及潛熱通量減少14.4 W/m2 (-16.1%)，對地表造成平均1.18 ℃之降溫，降溫與沉降運動幅度增加、範圍變廣，而平均降水量減少9.3 mm/month (-37.2%)。比較氣膠及灌溉分別作用與同時作用的結果，發現在降溫和降水減少上有非線性的反應，未來將針對此現象做進一步探討。未來也將以單層海洋耦合模式進行模擬，探討海洋長期回饋作用對印度乾季氣候反應的影響。

The present study investigates the impacts of the anthropogenic aerosols and irrigation on dry season droughts over northern India. The observation data show a significant declining trend in dry season (Oct-Dec) precipitation over northern India during the last decade. This resulted in severe drought and had a significant impact on local economy and food security. Dry season is also the major season of anthropogenic aerosol emissions and irrigation activities in northern India, which have also intensified significantly over the last decade. Previous studies show that the aerosol and irrigation can alter the Indian summer monsoon circulation and precipitation. Therefore this study aims at understanding the potential responses of dry season precipitation over northern India to increased anthropogenic aerosols and irrigation using idealized equilibrium climate simulations with prescribed sea surface temperature. The result shows that increased anthropogenic aerosol emission reduces mean surface solar radiation flux by 8.3 W/m2 (-18.6%) over northern India during dry season, leading to a 0.28 oC regional surface cooling. The heating from aerosol shortwave absorption induced ascending motion in northern India and descending motion to the downwind over southwestern India. The precipitation was decreased by 7.2 mm/month (-29.8%) in western India. Adding irrigation reduces mean surface latent heat flux by 11.4 W/m2 (-14.1%) and surface temperature by 0.42 oC over northern India, leading to the formation of an anomalous surface high pressure system. Precipitation is decreased by 2.4 mm/month (-12.1%) over the southern edge of the anomalous high. When adding combined forcings of anthropogenic aerosols and irrigation, mean surface short wave radiation is decreased by 10.1 W/m2 (-20.2%), and latent heat flux is decreased by 14.4 W/m2 (-16.1%), with surface cooling of 1.18 oC. Owing to the stronger subsidence and cooling, precipitation is decreased by 9.3 mm/month (-37.2%). The resulted surface cooling and precipitation decline showed nonlinear responses to the separate and combined aerosol and irrigation forcing. Subsequent work will focus on identifying the detailed mechanism responsible for such nonlinearity. In the future the role of long-term ocean feedback in modulating the response of Indian dry season climate can be investigated using simulations with coupled slab ocean model.