El Niño diversity and its impacts on East Asian rainfall in the future climate

Ph.D.The East Asian (EA) region, home to more than a billion people, is particularly vulnerable to hydrological extremes, due to its large population as well as rapid economic developments in recent decades. El Niño-Southern Oscillation (ENSO) is widely recognized as the most important driver of EA rainfall variations on the interannual timescale. This study investigates El Niño diversity and its impacts on EA rainfall in the present and future climate, using historical and representative concentration pathway 8.5 (RCP8.5) simulations from state-of-the-art models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Out of the 31 CMIP5 models examined, 17 of them are found to be able to well reproduce El Niño diversity. While all of the 17 models can give realistic eastern-Pacific (EP) El Niño-related Walker circulation anomalies, about one-third of them give erroneous central-Pacific (CP) El Niño-related anomalous Walker cells in the simulated environment. Further inspection reveals that these models tend to have strong cold biases of sea surface temperature (SST) over central-eastern Pacific, leading to a westward displacement of anomalous rising motion during CP El Niño. Overall, models with realistic Walker cell anomalies tend to simulate better El Niño’s impact on EA low-level flow and rainfall, especially for CP El Niño.Comparing the historical and RCP8.5 scenario simulations based on selected models which capture well both El Niño diversity and East Asian Summer Monsoon (EASM), the ENSO-related EA rainfall variability in the future climate is studied. Under global warming, EA rainfall anomalies due to two types of ENSO are projected to be intensified significantly in boreal winter at the peak of El Niño, and the ensuing summer. To better understand the underlying physical processes, moisture budget analyses are carried out. Change of El Niño-induced moisture flux convergence/divergence, which largely coincide with anomalous rainfall changes, are dominated by the dynamic term (i.e. El Niño-induced circulation acting on time-mean moisture). A moister atmosphere under global warming tends to enhance the moisture flux, while weakened or strengthened El Niño-related circulation can play a negative or positive role in modulating moisture flux during different El Niño phases. Importantly, the effect due to mean moisture enhancement can offset (strengthen) the negative (positive) contribution of weaker (stronger) El Niño-related circulation, leading to the intensification of rainfall variations. Inspections of the individual models reveal no consensus on El Niño SSTA and its corresponding circulation changes in the future climate. However, low-level circulation changes are largely determined by the El Niño SSTA changes in terms of their intensity and location. In general, CP El Niño-related rainfall will be enhanced robustly under global warming, regardless of the change of CP-type SSTA for a particular model. For EP El Niño, its related rainfall also tends to be intensified, except for models in which EP-type SSTA is greatly weakened.A series of atmospheric general circulation model (AGCM) experiments, with various states of the climatological SST, CO2 concentration, as well as SSTA patterns, are carried out. Results based on these experiments also point to more variable El Niño-driven rainfall under global warming. However, due to the modulated mean background convection, the anomalous rainfall is displaced slightly eastward over the equatorial Pacific. Moisture budget analyses indicate that the intensification of rainfall anomalies and their pattern change can be mainly attributed to the perturbed anomalous circulation. In comparison, the role of a moister atmosphere background is less important, and it only acts to intensify the original general patterns of El Niño-related rainfall. It is noticed that the warmer background climate can also accelerate anomalous wind flow. The latter effect might arise from stronger circulation response under warmer basic states even with identical El Niño SSST forcing.Finally, atmospheric feedback, thermocline feedback, net heat flux feedback during ENSO, and their changes under global warming, are examined. It is noticed that the strength of atmospheric and thermocline feedbacks from individual models are modulated in the same way for two types of ENSO, but not the case for net heat flux feedback. In fact, models that show stronger (weaker) EP El Niño also tend to give stronger (weaker) CP El Niño in the future climate. The coherency between changes of EP and CP ENSO feedback strength might explain the aforementioned relationship between changes of EP and CP El Niño intensity.伴隨著經濟和人口的快速增長，東亞地區極易受到極端降水的影響。厄爾尼諾事件的發生是東亞降水異常的重要驅動因子。本文利用參與CMIP5評估的31個氣候模式在和RCP8.5情境下的模擬數據，研究了全球變暖背景下兩類厄爾尼諾事件對東亞地區降水的影響。經驗證，具備模擬厄爾尼諾多樣性能力的17個模式可以較好的模擬出東部型厄爾尼諾所對應的Walker環流異常。由於模式氣候態海溫存在異常偏強的東太平洋冷舌型誤差，其中六個模式給出的中部型厄爾尼諾導致的Walker環流異常則表現出顯著的西移偏差。進壹步的研究表明，具備模擬兩類厄爾尼諾引起的異常Walker環流能力的壹組模式，其多模式平均的結果能夠較好的反映出兩類厄爾尼諾與東亞氣候變異的遙相關。對於另壹組模式，它所模擬的中部型厄爾尼諾與東亞異常環流降水的關系與觀測結果存在顯著偏差。通過評估31個模式對東亞夏季風的模擬能力，6個模式能夠較好地模擬出歷史情境下兩類厄爾尼諾異常導致的東亞氣候變異和東亞夏季風的年際變異。基於對比未來和歷史情境下的多模式平均結果，由於兩類厄爾尼諾事件所導致的東亞冬夏降水異常在全球變暖的背景下將會顯著增強。水汽收支分析結果表明，未來厄爾尼諾相關的東亞降水異常主要由水汽動力傳輸項的變化所決定。由於氣候變暖所產生的更多水汽將會增加東亞地區的異常降水，而主要由厄爾尼諾海溫異常變化所驅動的環流異常變化在不同的季節呈現微弱的增強或者減弱的趨勢，但是平均水汽的增強可以抵消環流減弱的影響，並最終導致東亞降水異常的增強。對單個模式的分析表明：盡管兩類厄爾尼諾海溫異常在全球變暖背景下的變化具有較大的模式間誤差，但是其強度和位置的變化與對應東亞/西北太平洋環流異常的強度和位置變化顯著正相關。與此同時，單個模式的結果進一步驗證了：儘管不同模式預估未來中部型厄爾尼諾可能變弱或者變強，其所引發的東亞降水異常都將顯著增強。同樣的結果也適用於東部型厄爾尼諾，除非其強度顯著地減弱。為了進壹步探究全球變暖背景下氣候態背景及厄爾尼諾異常海溫擾動對降水的影響，本文利用CAM4設計了AGCM數值試驗。結果表明：未來兩類厄爾尼諾事件將會顯著引起熱帶降水異常的增強。氣候變暖背景下，環流異常的變化主要導致了厄爾尼諾降水的模態增強及其位置的變化，而平均水氣的增加只能增強厄爾尼諾降水的模態。氣候變暖不僅為背景場提供了更多的水汽，還可能導致了環流異常對兩類厄爾尼諾海溫強迫的非線性響應，進而影響熱帶地區的降水。通過對兩類厄爾尼諾事件未來變化的探究發現，模擬出未來增強（減弱）的東部型厄爾尼諾事件的模式傾向於模擬出增強（減弱）的中部型厄爾尼諾事件。兩類厄爾尼諾異常中心位置的變化也表現出壹定的協同性。與兩類ENSO事件相關的大氣海洋反饋過程在全球變暖背景下的協同變化，可能解釋了這壹現象。Wang, Peng.Thesis Ph.D. Chinese University of Hong Kong 2019.Includes bibliographical references (leaves vii-xxiii).Abstracts also in Chinese.Title from PDF title page (viewed on 1, April 2021)