Eurasian Snow Variability and its Influence on Extratropical Atmospheric Circulation and Climate

雪的變化對地球氣候和人類生活有深遠的影響。 科學家們長久以來都試圖瞭解和預測雪的變化以及研究相應的氣候影響。關於雪的許多問題例如它的年際和年代際變率， 對氣候和大氣環流的影響，雪的水文效應等至今未能被充分研究。本論文旨在更詳細的分析歐亞大陸北部雪的年際變率，它的年代際變率和氣候變率的關係以及它的水文效應。歐亞大陸北部的雪的季节變化和地面气候例如氣溫和降水有密切的關係。 模式的輸出產品基本吻合有關的分析結果。 但是模式仍存在一些困難。進一步的模式模擬表明了海洋和陸地對人為氣候變化的響應引致了該地區春季雪的減少趨勢。通過分析歐亞大陸北部的雪和陸面熱通量以及大氣環流的關係，本論文獲得了更加全面的對於雪和大氣年际耦合的理解。地表大氣溫度和降水對雪的影響是顯而易見的。但是要理解大的過程和獲取更清晰的概念則要顯得困難的多。雪和大氣環流， 地面輻射，水汽輸送以及云量有緊密的聯繫。觀測數據和模式產品的分析均表明雪和大氣的年际耦合很大程度是獨立外部因子的影響。在北歐亞大陸， 80年代末雪的年代際減少，大尺度的地面增暖和北大西洋-歐亞地區大氣環流的異常均同時發生。雪的影響通過調製短波輻射僅僅表現在一些特定的地區。在很多情況下， 大氣環流導致的輻射和平流輸送對於地面氣溫的影響處於主導地位。雪的作用很多時候被抵消或者掩蓋了。模式結果同時表明熱帶海溫增暖和辐射强迫是大氣環流異常的驅動因子。這些環流異常強烈影響到雪的變化而雪的反饋却相當有限。在北歐亞大陸， 融雪對土壤濕度以及地表徑流的季節循環以及年際變化均有重要的影響。 但是相關的土壤濕度異常維持時間過短。大氣的水汽輸送複合而不是局地的蒸發提供主要水汽給异常降水。這在很大程度上削弱了融雪的水文效应對晚春及夏季气候的影响。总体而言， 本論文推進了雪的變化以及它的氣候效應的理解和研究。在全球氣候變化的背景下，雪的響應以及反饋需要進一步研究和理解。Snow is an important part of Earth’s climate system and its variability has far-reaching implications for Earth’s climate and human life. Scientists have long been pursuing the understanding and predictions of the snow variability, and studying the climatic impacts of snow variability. Many aspects of snow including its inter-annual and inter-decadal variations, their influences on climate and atmospheric circulation and researchers’ long-proposed hydrological effects of snow are still not fully explored or addressed. This thesis is to provide a more detailed analysis of snow variability in North Eurasia (NEA), the decadal changes of NEA snow and the associated climate variability, and snow hydrological effects.The climatological seasonal evolution and spatial distribution of NEA snow is tightly associated with those of surface climate such as temperature and precipitation. These observation-based analyses are consistent with analyses based on model output, with several difficulties of model simulation noted. Model output from further model simulations has shown that SST and land responses to anthropogenic forcing are largely responsible for the observed decline trends in spring NEA snow.Analysis of the interannual NEA snow variability, its relationship with surface heat fluxes and atmospheric circulation has provided comprehensive understanding of the snow-atmosphere coupling. The influences of SAT and precipitation on snow variability is most direct and easily understood. The larger picture is much more complicated. Snow anomalies are related to anomalies of atmospheric circulation, surface radiation, water vapor transport and also cloudiness. The snow-atmosphere coupling is basically independent from external forcing such as sea surface temperatures (SSTs), as demonstrated by both observations and model output.Widespread decadal declines in NEA snow are coincident with large-scale surface warming and atmospheric circulation anomalies in Eurasia after late 1980s. Analysis indicates that snow’s contribution to surface warming is significant in some specific areas. The contribution is mainly manifested in surface shortwave radiation changes. However, in many cases, atmospheric circulation-related radiation changes and heat advection are dominant. That means snow influences have been offset or too marginal. Further model experiments suggest that tropical SST warming and radiative forcing are strong driving factors for the decadal atmospheric circulation anomalies. This also indicates that decadal spring snow changes in NEA have been strongly influenced by the anomalous atmospheric circulation and the feedback from snow is limited.Snowmelt does significantly modulate the seasonal cycles of soil moisture (SM) and surface runoff. Year-over-year variations of snowmelt also lead to fluctuations in SM. However, the SM-anomalies associated with snowmelt in most of NEA do not persistent long enough to spread the snowmelt effects. In most areas, precipitation anomalies are more closely controlled by the anomalous atmospheric water vapor divergence rather than by local evaporation. This limits the snowmelt impact.Overall, this thesis has advanced our understanding of NEA snow variability and its climatic influences with emphasis placed on several issues. Within the context of a changing climate, responses of snow and its feedback should be fully studied.Ye, Kunhui.Thesis Ph.D. Chinese University of Hong Kong 2017.Includes bibliographical references (leaves ).Abstracts also in Chinese.Title from PDF title page (viewed on …).Detailed summary in vernacular field only.Detailed summary in vernacular field only.Detailed summary in vernacular field only.Detailed summary in vernacular field only.Detailed summary in vernacular field only.Detailed summary in vernacular field only