海洋大陸森林砍伐及其伴隨火災與氣候之關係

Abstract

熱帶雨林地區之森林砍伐會造成地表能量收支及水循環的改變，進一步影響局地甚至全球之氣候。本研究利用地球系統模式模擬海洋大陸地區森林砍伐對氣候造成的影響。模擬結果顯示森林砍伐會導致該地區地表溫度上升及降水增加。藉由分析垂直積分水氣收支以及濕靜能收支，可以發現地表增溫效應以及低層水氣輻合帶來中層水氣增加的效應共同造成了森林砍伐地區的大氣不穩定，增強上升運動，伴隨更多的水氣輻合，抵銷了森林砍伐後的蒸發散量減少，因而使該地區降水增加。此外，森林砍伐導致海洋大陸地區的上升運動增強，伴隨中太平洋的下沉運動，此環流變化可能會影響沃克環流，進一步影響其他地區的氣候。 海洋大陸地區的森林砍伐除了地表植被的變化之外，通常會伴隨森林火災，而此地區的森林火災強度會受到氣候影響，尤其以聖嬰事件的影響最為顯著。聖嬰事件可以分為東太平洋以及中太平洋聖嬰事件，其中東太平洋聖嬰事件由於海溫異常中心偏東，相較中太平洋聖嬰事件，其造成的沃克環流改變會在婆羅洲南部有更強的下沉運動異常，導致該地區乾季延長至十月，森林火災持續蔓延，造成東太平洋聖嬰事件時婆羅洲南部有較多的火點數量。過去預測此地區的森林火災通常考慮的是聖嬰事件的強度，本研究顯示考慮聖嬰事件的類型可能比考慮其強度重要，此發現將有助於婆羅洲南部森林火災之預報。

Deforestation in tropical regions would lead to changes in local energy and moisture budget, resulting in further impacts on regional and global climate. Previous studies have indicated that the reduction of evapotranspiration dominates the influence of tropical deforestation, which causes a warmer and drier climate locally. Most studies agree that the deforestation leads to an increase in temperature and decline in precipitation over the deforested area. However, unlike Amazon or Africa, Maritime Continent consists of islands surrounded by oceans so the drying effects found in Amazon or Africa may not be the case in Maritime Continent. Thus, our objective is to investigate the local and remote climate responses to deforestation in such unique region. We conduct deforestation experiments using NCAR Community Earth System Model (CESM) and through converting the tropical rainforest into grassland. The results show that deforestation in Maritime Continent leads to an increase in both temperature and precipitation. Moisture budget analysis indicates that the increase in precipitation is associated with the vertically integrated vertical moisture advection, especially the dynamic component (changes in convection). In addition, through moist static energy (MSE) budget analysis, we find the atmosphere among deforested areas become unstable owing to the combined effects of higher specific humidity and temperature in the mid-level. This instability will induce anomalous ascending motion, which could enhance the low-level moisture convergence, providing water vapor from the surrounding warm ocean. Besides the increased precipitation, the enhanced ascending motion over Maritime Continent may participate in the changes in Pacific Walker circulation, producing possible remote climate impacts beyond the tropics. The deforestation in Maritime Continent not only changes the vegetation types and the local climate, but also accompanies the slash-and-burn agricultural fire. Fire activity in Indonesia is strongly linked with El Niño events, whose sea surface temperature (SST) patterns can weaken the Walker circulation leading to a drought condition in the region. Here we show via case analyses and idealized climate model simulations that it is the central location of the SST anomalies associated with El Niño, rather than its intensity, that is mostly linked with the fire occurrence. During our study period of 1997-2015, Eastern Pacific (EP) El Niño events produced the largest fire events in southern Borneo (i.e., in 1997, 2006, and 2015), while Central Pacific (CP) El Niño events consistently produced minor fire events. The EP El Niño is found to be more capable than the CP El Niño of weakening the Walker circulation that acts to prolong Borneo’s drought condition from September to October. The extended dry conditions in October potentially increase the occurrence of fires during EP El Niño years. The 2015 fire event owes its occurrence to the location of the 2015 El Niño but not necessarily its “Godzilla” intensity in affecting the fire episodes over southern Borneo. Projecting the location of El Niño events might be more important than projecting their strength for fire management in southern Borneo.