自然和人爲因素及其交互作用對海洋大陸水文氣候之影響

Abstract

海洋大陸位於暖池，西臨印度洋東接太平洋。因爲海洋大陸特殊的地理位置，陸地降水有著鮮明的年際變化，海洋大陸不同區域的降水年循環也不盡相同；位於熱帶、充足的水氣供應也使得海洋大陸成爲全球最“濕”的地點之一，使之從能量上有機會通過大尺度波動影響中高緯的氣候。本研究始於探索自然氣候變異對海洋大陸當地的降水影響，接著試圖瞭解人爲地表變遷造成當地降水變化後對中高緯氣候的潛在影響。 印度洋偶極子與聖嬰是對海洋大陸水文氣候產生影響的兩個最主要自然氣候變異。從長期平均來看，我們發現，當正相位的印度洋偶極子與聖嬰同時發生時，海洋大陸產生更强的沉降、降水會急劇減少。兩者共同發生時沉降會比僅有印度洋偶極子及聖嬰來得更强，顯示了印度洋偶極子與聖嬰對各自的發展都有加成作用。 而在年際尺度中，海洋大陸秋季的降水變異度在過去一百多年有一定的傾向性，比如在1965年後變化較爲劇烈。這似乎與聖嬰與印度洋偶極子兩者同時發生的次數在1965年後增加有關。聖嬰與印度洋偶極子的兩者關係主要由聖嬰主導，即較强的聖嬰較容易引發印度洋偶極子：聖嬰變異度較大時，兩者容易共同發生。我們也使用氣候模式瞭解全球暖化對聖嬰與印度洋偶極子兩者關係的影響。然而單一氣候模式並非都能恰當模擬聖嬰與印度洋偶極子的兩者關係，即並非所有模式都能模擬强聖嬰往往伴隨強印度洋偶極子的現象。然而從多模式的比較中可以發現，可以模擬出聖嬰變異度較大的模式往往都可以模擬聖嬰與印度洋偶極子的高度相關現象；同時，模擬聖嬰變異度在暖化下增加較大的模式也可模擬出兩者在暖化後更相關。 除了會大幅改變海洋大陸降水的自然氣候變異外，近年來當地持續且加劇的人爲土地利用，比如森林砍伐也成爲改變局地降水的潛在因素。自然氣候變異導致海洋大陸降水改變並影響中高緯氣候，已在之前的研究中有詳盡敘述，但因人爲活動而改變的局地降水對中高緯氣候產生的影響尚未清楚。我們發現，在海洋大陸發生大規模森林砍伐後，地表溫度升高後造成地表的能量平衡重新分配，同時增加局地的降水。降水增加會釋放大量潛熱、加熱附近大氣，同時在高層產生輻散風。輻散風到達副熱帶噴流時產生渦度距平，即所謂的羅士比波。這些羅士比波會沿著副熱帶噴流傳遞到出區，再向北傳遞到阿留申區域，產生阿留申低壓距平。阿留申低壓距平會將水氣與溫度從中低緯度傳送至中高緯，導致在阿留申低壓距平東北方偏暖；而阿留申低壓距平西北方也偏暖，主要是由一系列正回饋作用所導致。由於副熱帶噴流會影響羅士比波的傳遞路徑，而聖嬰又可調控副熱帶噴流位置及强度，我們進一步分析聖嬰的不同相位時，海洋大陸森林砍伐對中高緯的影響。在反聖嬰年中，副熱帶噴流較短且偏西，因此海洋大陸森林砍伐所產生的羅士比波更容易傳至阿留申區域，產生更强的阿留申低壓距平，同時北美洲西北部的升溫更多。在聖嬰年中，則因爲副熱帶噴流較長，羅士比波不容易傳至阿留申低壓區域，阿留申低壓距平加强較弱。通過線性模式的一系列實驗發現，我們所看到的訊號主要受到海洋大陸東邊的地表變遷所引起。 這份研究發現正相位的印度洋偶極子與聖嬰同時發生的頻率最近有所增加，同時也分析了海洋大陸地表植被改變對中高緯氣候潛在的影響，這些發現說明，未來海洋大陸地區因地表植被改變造成的地表異質性可能更容易引發遙相關的反應，提供了一個未來研究的方向。

The Maritime Continent (MC) located in the warm pool region, bordered by the Indian Ocean to the west and the Pacific Ocean to the east. Due to its unique geographical position, the MC experiences distinct interannual variations in precipitation. As one of the wettest places on Earth, situated in the tropics with an abundant water vapor supply, the MC has the potential to influence mid-to-high latitude climates through strong local convection and induced large-scale atmospheric teleconnections. This study initially explores the impact of natural climate variability on local precipitation in the terrestrial MC regions, subsequently aiming to understand the potential effects on mid-to-high latitude climates following deforestation-induced changes in local precipitation. The Indian Ocean Dipole (IOD) and ENSO are the primary natural variabilities influencing the hydroclimate of the MC. From a long-term average perspective, we found that precipitation over the MC significantly decreases when the positive phases of the IOD (pIOD) and El Niño occur simultaneously (coincident El Niño-pIOD), with enhanced downward motion over the MC regions. The downward motion during coincident El Niño-pIOD years is stronger than the summation of El Niño-only and pIOD-only years, indicating a combined nonlinear effect when El Niño and pIOD occur simultaneously. At the interannual timescale, the variability of precipitation over the terrestrial MC has increased after 1965, possibly associated with an increased number of coincident El Niño-pIOD events. The relationship between ENSO and the IOD is mainly controlled by the variability of ENSO; that is, stronger ENSO events are more likely to trigger the IOD. When there is greater variability in ENSO, more coincident El Niño-pIOD events occur, leading to increased precipitation variability over the MC. Climate models were used to examine the impacts of global warming on the relationship between ENSO and the IOD. However, not all individual climate models accurately simulate this relationship as shown in observed data. However, from multi-model comparisons, those models that simulate larger ENSO variabilities tend to represent the tight relationship between ENSO and the IOD more accurately. Additionally, models showing a greater increase in ENSO variability under global warming also tend to represent a tight relationship. In addition to natural climate variability affecting precipitation over the MC, recent anthropogenic activities such as deforestation may also potentially influence precipitation. However, the impacts of these changed local precipitation patterns due to human activities on the climate of mid-to-high latitudes remain uncertain. This study found that deforestation disrupts the surface energy balance, raising surface temperatures and subsequently increasing local precipitation. The increased local precipitation in the MC is accompanied by vast latent heat release in the upper atmosphere. This diabatic heating can generate divergent winds in the upper levels and anomalous vorticity (known as Rossby waves) near the subtropical jet. These Rossby waves propagate zonally along the subtropical jet and subsequently travel meridionally at the exit of the subtropical jet to the Aleutian region, eventually intensifying the Aleutian Low. Then, the anomalous Aleutian Low transports water vapor and warm temperatures from mid-latitudes to higher latitudes, leading to warmer conditions northeast of the anomalous Aleutian Low, while the northwest also experiences warm, which is primarily due to a series of positive feedback mechanisms. Since the subtropical jet influences the propagation path of Rossby waves, and ENSO can adjust the position and intensity of the subtropical jet, we further analyze the effects of different phases of ENSO on modulating the teleconnection of MC deforestation. During La Niña years, the subtropical jet is relatively shorter and shifts westerly. As a result, the Rossby waves generated by MC deforestation are more likely to propagate to the Aleutian region, intensifying the Aleutian Low and causing robust warming in northwest America. These results contrast during El Niño years. Furthermore, we found that the teleconnection we observed is primarily induced by the eastern part of the MC through a series of experiments using a linear baroclinic model. This study indicates the increased frequency of coincident El Niño-pIOD events and investigates how land-use changes in the MC influence high-latitude climates and the underlying mechanisms. These findings help identify hotspots for future land-use changes and their teleconnections, providing insights into their local, regional, and remote impacts.