季節降水變化及其對陸地水文循環的影響

Abstract

此論文主要探討氣候模式與再分析資料之間季節降水機制差異的原因，進而討論陸地水文循環如何受到全球暖化與極端降水頻率改變的影響，最後討論土壤濕度改變後如何進一步影響大氣環流。 全球暖化下季節降水差異越來越大，過去季節降水變化的機制討論僅止於氣候模式上，在此論文我們發現CMIP5_AMIP與再分析資料的降水變化以及其背後的機制有極大的差異，在CMIP5_AMIP的降水變化主要與熱力機制（水氣增加）有關，然而再分析資料的結果則說明動力上的機制（垂直運動增強）較重要，造成此差異來源主要是因為氣候模式中熱帶地區中高對流層增溫速度大於再分析資料，進而導致氣候模式趨向於較穩定的大氣所致，同時也造成垂直運動減弱，此結果更造成動力機制對降水變化的貢獻在氣候模式中明顯偏弱。 季節降水差距變大隱含著極端降水頻率升高，可能會影響到陸地水文循環過程，因此我們進一步探討熱帶雨林區（亞馬遜、剛果、印尼）在全球暖化下陸地淨流入水通量到土壤的差異，研究發現因為土壤對於降水的反應呈現非線性關係，在印尼熱帶雨林區受到極端降水的影響，河川徑流將大幅增加，更造成淨流入土壤的水有變少的趨勢，同時近地表土壤水則有明顯增加；相反的，另外兩個雨林區則沒有明顯極端降水的改變。進一步使用大氣水氣收支方程，發現此地的降水極端變化的主要原因是全球暖化下大氣水氣增加導致的結果。 此論文最後利用CESM模式設計理想化的土壤濕度實驗，探討其對全球大氣環流的影響，結果顯示當全球土壤變濕時會導致從熱帶到中緯度陸地地區地表蒸散量增加，同時也會降低地表溫度，進而造成經向溫度梯度增強，使得熱帶大氣環流隨之增強。因為陸地溫度降低，也會造成海陸溫度差異減少，南亞季風環流隨之減弱。從大氣能量觀點來探討，也能發現因為環流的改變，南北傳送的潛熱與乾靜能也有相對應的變化。另外，由於土壤濕度變化所造成大氣能量傳送改變的強度與聖嬰-反聖嬰間及全球暖化下所造成的能量改變的強度是一致的。 全球暖化下，氣候變異或土地利用方式的改變皆會造成陸地水循環隨之變化，進一步造成水資源管理困難、水旱災頻率增加、甚至影響糧食作物的生長，使得食物短缺。因此探討大氣如何影響陸地及陸地如何再反饋到大氣的研究在未來的暖化情境下，更顯重要。

This dissertation investigated the mechanisms of seasonal precipitation changes between Coupled Model Intercomparison Project Phase 5 (CMIP5) AMIP-type outputs and reanalysis datasets. We also suggested that the possible impacts of frequent extreme precipitation on the terrestrial hydrological cycle under global warming. Finally, we investigate the responses of tropical and monsoon circulations to the different soil water conditions resulting from different land-atmospheric interactions. Previous studies demonstrated the increased annual range of precipitation as the climate warms. However, these documents only used the model outputs to discuss the mechanism of seasonal precipitation changes and found that the increased water vapor plays a curial role on such precipitation changes. When using the reanalysis datasets, we revealed the changes in the dynamic component in the water budget analysis are more important for the observed precipitation changes. Such discrepancy might be due to the tendency toward stability in CMIP5_AMIP owing to more tropical warming rate in the mid-upper troposphere compared to that of reanalysis datasets. Such a tendency also leads to weakening tropical vertical motion in CMIP5_AMIP. The increased annual range of precipitation is indicative of higher frequency of extreme precipitation, which can affect the terrestrial hydrological cycle. We also investigated net water flux into the soil over the rainforest areas, including Amazon, Congo, and the Maritime Continent. Nonlinear responses to extreme precipitation lead to a reduction of infiltration and a proportionately higher amount of direct runoff, particularly for the Maritime Continent, where both the amount and intensity of precipitation increase under global warming, and such precipitation changes are related to increased water vapor under global warming. In addition, the near-surface soil moisture is obviously increased over the Maritime Continent. Finally, a pair of idealized experiments corresponding to contrast fixed groundwater table depths over the Earth’s continents by AMIP-type simulations in the Community Earth System Model (CESM) was conducted. In the wet (shallow water table) experiment, both land evapotranspiration and soil moisture tend to increase, leading to an increased meridional surface temperature gradient, which also causes the tropical circulation stronger than that of the dry (deep water table) experiment. Relative to the dry experiment, the wet experiment exhibited the enhancement of southward (northward) latent energy (dry static energy) transport coincide with the stronger tropical circulation. Despite larger surface latent heat fluxes to the atmosphere in the wet soil case, the monthly mean of stationary eddy demonstrated the reductions of northward latent energy transports due to compensation by a notably weakening South-Asia monsoon circulation associated with weaker land-sea thermal contrast. This study indicates the importance of groundwater variations and land surface conditions in global energy transport and has further implications for earth system model development. Under global warming, land use and climate changes have profound impacts on the terrestrial hydrological cycle, which further cause the difficulty of water resource management, the higher risk of flooding and drought, and even the food shortage. Consequently, how the atmosphere affects the terrestrial hydrological cycle and its feedbacks to the atmosphere through land-atmospheric interactions will be a critical issue in the warming future.