北极地区快速升温的驱动机制研究进展

Abstract

受全球持续暖化影响, 北极地区近地表气温正在以超过全球平均两倍以上的速度急剧升高, 称为"北极放大"效应. 北极的快速升温与地气系统的能量收支异常直接相关, 学术界基于模式模拟、卫星观测等多种手段开展了大量研究工作, 并提出了一系列的理论和证据, 尝试从能量收支的角度对"北极放大"效应的驱动机制进行解释. 对驱动"北极放大"的各种潜在机制的理论基础进行了阐释, 并回顾了各种驱动机制的研究进展. 综合现有研究表明, 北极的快速升温既与受北极独特的地理、大气环境影响形成的多种辐射反馈机制(如海冰反照率辐射反馈机制、边界层大气逆温引起的温度梯度正反馈机制、普朗克反馈机制等)有关, 也可能是由地球大气、海洋环流异常引起的极向物质与能量传输增强所致. 通过对现有研究进行综合分析指出, 受研究方法存在一定缺陷、高质量且时空完整的长时序辐射产品的缺乏, 以及探索不同因素之间关联机制的综合性研究的缺少等影响, 目前对于北极地区快速升温的关键驱动机制仍无定论, 不同理论之间存在很大的分歧. 进一步指出, 长时序、高质量的地表与大气辐射产品研发, 研究方法的进一步优化与各种气候因子变化对地气系统能量收支扰动的更准确量化, 以及针对不同气候因子之间内在关联机制的综合分析, 将是未来提升北极地区气候变化研究的重要方向.

Under the influence of recent global warming, the Arctic climate is continuing to experience unprecedented changes. Surface temperatures in the Arctic are increasing at a rate double that of the global average, known as the Arctic amplification. The Arctic amplification has led to a series of consequences locally such as the accelerated sea ice retreat, the continuous permafrost melting and methane release, and the marine and terrestrial ecological dynamics. It also might cause an increased frequency of extreme weather event in mid-latitudes via perturbing the strength of the polar vortex. The Arctic warming is mainly resulted from the imbalance of energy budget at both surface and top of atmosphere. In order to explore the driving mechanism of Arctic amplification, lots of efforts have been made based on model simulations and observations. Several major competing theories and related evidences have been proposed to explain their contributions to the imbalance of energy budget over the Arctic. The sea-ice albedo feedback, caused by the continuously shrinking summer sea ice potentially increasing the absorption of shortwave radiation, is believed to play a critical role in recent Arctic amplification. But certain model simulations have indicated that sea-ice albedo feedback was likely not the dominant factor-robust warming amplification still occurs in the Arctic in the absence of albedo feedback. The positive lapse rate feedback, is also thought to cause greater warming at the Arctic surface through damping the deep convection process at the bottom atmosphere because of the strong temperature inversion in the cold Arctic. However, whether the lapse rate feedback plays a dominant role in the Arctic amplification has not been widely accepted. The Planck feedback-cold region requires larger temperature increase than warm area to balance certain external radiative forcing, is also believed to contribute to the Arctic amplification. But the magnitude of its contribution remains to be further quantified. An enhanced Atlantic meridional overturning circulation (AMOC) transporting extraordinary amounts of heat northward to the Arctic Ocean has been hypothesized to substantially amplify Arctic warming and accelerate summer sea-ice melting in this region. However, evidence has instead shown significant slowing of the AMOC over the last few decades. The enhanced greenhouse effect, resulted from the increased water vapor and cloudiness, has been suggested as an important driver of the Arctic winter warming and summer sea-ice dynamics. However, whether the increased water vapor comes from the strengthened sea surface evaporation or from an enhanced atmospheric northward transportation, is still unclear. This paper elucidates the theoretical basis of all potential mechanisms, and systematically reviews the research progress of these theories. Based on the comprehensive analysis of these studies, we point out that due to the limitation of current study approaches, the data quality problems such as scarce in situ measurements, difficulty in discriminating clouds from snow cover for some remote sensing products, problematic parameterization schemes and model inputs in some reanalsysis and GCM projects, and the lack of systematic analysis for investigating the internal relations between different climate variables, the conclusions from different studies are discrepant, or even contradictory. The causes of the unprecedented climate change in the Arctic remain unclear and are still under heated debate. Therefore, the development of high quality, long-term radiative product, the optimization of the researching approaches and more accurate quantification of these feedback mechanisms, and the systematic analysis of the potential linkage between different climate variables, will help to advance the study of the Arctic amplification and climate change.