聖嬰現象在森林砍伐引起海洋大陸降水變異所扮演的角色：對於 NCAR CESM 的模擬分析
Authors: 蔣濬濂 蔣濬濂 , Chun-Lien Chiang 李庭慧 , Ting-Hui Lee 羅敏輝 
Published: May 2022

<p>聖嬰與南方震盪（ENSO）對於海洋大陸的降水量會產生年際性的影響，而過去研究指出森林砍伐造成的地表變遷會使當地氣候長期平均降水量增加。藉由給定觀測海溫的CESM大氣與陸地耦合模式，本研究欲了解森林砍伐造成的降水量的變化在不同 ENSO 相位時之差異，並透過地表能量收支平衡、雲輻射效應、溼靜能與環流場的分析，來探討在不同ENSO相位下森林砍伐的效應不同之原因。在十一月至次年一月期間，當ENSO呈現La Ni&ntilde;a相位時，森林砍伐所造成的降水量增加幅度明顯小於 Neutral 與 El Ni&ntilde;o年。而El Ni&ntilde;o 時，森林砍伐所造成的降水量增加幅度則僅略小於 Neutral。在主要的分析中，我們比較了 Neutral、El Ni&ntilde;o、La Ni&ntilde;a 三種環境條件下的森林砍伐效應。在 La Ni&ntilde;a 時的局部性環流與降水量增加幅度呈現同向的變化趨勢。但其他變數諸如地表輻射能量收支、地表溫度、地表可感熱通量、地表潛熱通量、雲輻射強迫、行星邊界層高度等，與降水量增加幅度的關係不顯著，甚至有反向的關係。由陸地區域與近海區域之間共同構成的局部性環流可以透過近地面的水氣通量輻合將水氣帶入陸地區域，而此局部性環流的強度在 La Ni&ntilde;a 時有明顯的減弱，顯示局部性環流可能為主導森林砍伐後降水量增加幅度的一個重要因素。La Ni&ntilde;a 氣候狀態下低層較乾、中層較溼的水氣環境，以及森林砍伐後較小的上升風速增幅，使得 La Ni&ntilde;a 在森林砍伐後的近地面向上水氣傳輸增幅顯著較小，使得低、中層大氣的水氣量增幅更少，垂直積分總溼靜能較 Neutral 時更低。相對於 Neutral森林砍伐造成的氣候反應，La Ni&ntilde;a 時由森林砍伐造成的氣候反應更不利於降水。相較之下，在 El Ni&ntilde;o 的環境下，森林砍伐所造成的溼靜能、垂直風速以及降水量增幅皆與 Neutral 時無顯著差異。</p>
<p>&nbsp;</p><p>ENSO has an interannual impact on the precipitation of the Maritime Continent. On the other hand, deforestation, as a kind of land-use change, brings to increases in the climatological precipitation. This study analyzed the deforestation-induced precipitation changes over the terrestrial Maritime Continent regions by CESM simulation with prescribed SST, which coupled with atmosphere model and land model. From a perspective of surface energy budget, cloud radiative forcing, moist static energy, and circulation field, we tried to determine the cause of different deforestation effects under different ENSO conditions. Between November to January of the following year, enhanced precipitation from deforestation in La Ni&ntilde;a is significantly less than that in Neutral, while an increase in precipitation in El Ni&ntilde;o is slightly less than that in Neutral. We also compared the deforestation effect under Neutral, El Ni&ntilde;o, and La Ni&ntilde;a events. The local circulation shows a positive correlation with precipitation changes. However, other variables such as surface temperature, surface sensible heat flux, surface latent heat flux, cloud radiative forcing, planetary boundary layer height, etc., have no significant correlation with the increases in precipitation. The local circulation between the land and offshore area can bring more moisture into the land regions through the convergence of moisture flux near the surface; however, the intensity of this local circulation is significantly weakened at La Ni&ntilde;a. This may be a critical factor leading to an increase in precipitation. Upward moisture transport increases less after deforestation due to moisture gradient (drier in lower-atmosphere and wetter in mid-atmosphere) under La Ni&ntilde;a conditions and a smaller increase in upward vertical wind velocity. The effect would lead to less moisture in the mid-atmosphere and smaller vertical integrated moist static energy. Under La Ni&ntilde;a conditions, the climate response after deforestation has a disadvantage of precipitation compared to deforestation under Neutral conditions. On the contrary, moist static energy vertical profile, vertical wind velocity vertical profile, and increases in precipitation in El Ni&ntilde;o have no significant difference from the Neutral condition.</p>
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