海洋大陸森林砍伐對當地水文循環與太平洋氣候變異造成之影響

Abstract

過去幾十年來，熱帶地區經歷嚴重的森林砍伐。隨著森林保育的意識逐漸被重視，亞馬遜雨林的森林砍伐已經有大幅的減少。然而由於經濟上的需求，東南亞與海洋大陸的森林砍伐有持續上升的趨勢。 過去已有許多研究在探討海洋大陸森林砍伐對於當地氣候的影響以及其可能的遙相關。然而，海洋大陸之地理位置特殊，受到許多不同時間與空間尺度的氣候變異所影響。其中，聖嬰/反聖嬰現象對海洋大陸的影響尤其重要。因此，本研究著重在聖嬰/反聖嬰現象對於海洋大陸森林砍伐影響的調節，以及海洋大陸森林砍伐如何透過陸氣交互作用回饋影響大氣、海洋甚至是自然氣候變異。本論文將使用美國國家大氣研究中心Community Earth System Model (CESM)氣候模式進行理想化的實驗來評估海洋大陸森林砍伐對於當地、局部氣候水文的影響以及其與聖嬰/反聖嬰現象之間的交互作用。 透過氣候模式進行去森林化的理想實驗，我們發現森林消失後導致的地表潛熱通量降低和可感熱增加等地表通量的改變。同時，森林的減少也對反照率造成影響，從而引起輻射的改變。導致地表溫度上升，使得大氣變得更加不穩定。伴隨著低層水氣的輻合，降水量呈現增加的趨勢。本研究進一步指出，在聖嬰現象期間，由於海洋大陸森林砍伐所引起的地方輻射能量和地表通量的變化，以及相關的大氣反應，會再被加強。主要原因是森林與草地對環境變化的敏感度不同。因此當考慮森林砍伐的效應時，聖嬰年時森林消失導致的潛熱通量減少會比正常年的更多。潛熱通量減少更多伴隨地表溫度增加更多，森林砍伐後的大氣不穩定度會比正常年的森林砍伐還要更不穩定。因此，在聖嬰年的背景場下，海洋大陸森林砍伐造成的降水增加會比正常年砍伐後的降水增加還要更多。 進一步分析森林的潛熱變化特徵時，我們發現海洋大陸森林的潛熱通量(蒸發散量)只有微弱的年際變化，但是海洋大陸的降雨量隨著聖嬰年或反聖嬰年的轉換有很大的年際變化。進一步的分析，我們發現葉表面蒸發與蒸散量有互相補償的效應。在聖嬰年時，海洋大陸的環境偏乾，降水量較少，葉表面能擷取到的降水變少也使得葉面蒸發量降低。然而，降水減少也伴隨著較少的雲量以及較多的太陽輻射入射量，更多的能量使得葉面蒸散量不減反增，也因此抵銷掉減少的蒸發量。而在反聖嬰年時，則是葉面蒸發會隨著降水增加而增加，蒸散量則是會減少。也因此，總蒸發散量的年際變化沒有隨著聖嬰與反聖嬰相位的轉換，而有較劇烈的變化。由於海洋大陸的森林砍伐日益嚴重，本研究進一步評估森林砍伐對於蒸發散量年際變化的影響。結果發現，當海洋大陸上的植被完全被裸土取代後，沒有了蒸發與蒸散的互相補償效應，海洋大陸蒸發散量的年際變化將會變大。同時，進一步分析此結果對於當地水文循環的影響，發現降水的年際變化卻會減少。其原因為海洋大陸森林砍伐造成的降水增加，在偏乾的環境如聖嬰年(偏濕的環境如反聖嬰年)時會被加強(削弱)所導致。 另一方面，我們也發現海洋大陸森林砍伐後會伴隨一個局部的沃克(Walker)環流，海洋大陸的陸地有一個上升環流距平，同時在海洋大陸東側的海洋上有個補償性的下沉環流距平。該下沉環流距平會造成一個非絕熱冷卻的效應，引發羅士比波列(Rossby wave)傳往北太平洋地區並且加強北太平洋的副熱帶高壓系統。同時北太平洋的近地表東北信風也會增強，進一步加強了東北太平洋的海氣交互作用，再影響到海溫變化。海溫變異訊號隨著被增強的東北信風被帶往西南，進而影響到中赤道太平洋海溫的年際變異度，造成更多的中太平洋聖嬰/反聖嬰事件的發生。同時模式結果也顯示，有更多多年性聖嬰/反聖嬰事件發生。本研究的結果指出，海洋大陸的地表變遷可能對於聖嬰/反聖嬰現象的多樣性有所影響。

The Maritime Continent (MC) is strongly influenced by El Niño-Southern Oscillation (ENSO) and has been experiencing severe deforestation in recent decades. This dissertation aims to investigate the local and remote impacts of MC deforestation and their interactions with ENSO using the Community Earth System Model (CESM). Local responses to MC deforestation encompass a reduction in surface latent heat flux and an increase in surface temperature, leading to a potentially unstable atmosphere and an increase in local precipitation. Our investigation further unveiled that these responses are amplified during El Niño, which can be attributed to the different magnitude of deforestation-induced biogeophysical feedbacks during neutral and El Niño condition. During El Niño conditions, these feedbacks were intensified, evolving more surface warming, stronger atmospheric instability, and a more pronounced enhancement of local convection and increased precipitation. A deeper exploration into the biogeophysical characteristics of rainforests in relation to ENSO revealed that latent heat flux (or evapotranspiration) exhibits weak interannual variation, despite substantial fluctuations in precipitation between El Niño and La Niña years. The underlying mechanism involves a self-competing effect among evapotranspiration’s components. Canopy evaporation decrease corresponded to precipitation declines during El Niño, while canopy transpiration increased in response to more incoming solar radiation, offsetting the decrease in evaporation. A converse scenario unfolds during La Niña. Consequently, the interannual variation of evapotranspiration remains weak. However, with idealized deforestation experiments across the MC region, we observed an increased interannual variation in evapotranspiration resulting from the termination of this compensatory effect. Moreover, the interannual variation of precipitation decreased after deforestation. This phenomenon can be attributed to the amplification (or weakening) of deforestation-induced precipitation increase in dry (wet) conditions, such as El Niño (La Niña). The MC land experiences anomalous ascending motion under idealized deforestation, and simultaneously, a compensatory descending anomaly occurs over the sea to the northeast of MC. This diabatic cooling induces a propagation of wave trains toward the North Pacific, strengthening the North Pacific subtropical high, resulting in enhanced northeasterly trade winds at the near-surface and intensifying underlying air-sea coupling processes. This active air-sea coupling further impacts sea surface temperature variability within the tropical central Pacific, referred to as the seasonal footprinting mechanism. Consequently, in deforestation scenarios, there is higher occurrence of central Pacific (CP)-type ENSO events. Additionally, CP-type ENSO could reactivate another seasonal footprinting mechanism after ENSO’s peak, leading to the persistence of the same phase of ENSO events in the subsequent boreal winter. Therefore, MC deforestation can also increase the occurrence of multi-year ENSO events. Our findings revealed that the remote impacts of MC deforestation could potentially influence ENSO complexity.