前冬南半球热带外海温对春季北半球副热带降水的影响及其机理

Variability of extratropical sea surface temperature (SST) plays an important role in modulating short-term climate anomalies. Cross-hemispheric influence caused by extratropical SST on interannual time scale is one of the rapidly developing research fields in the domestic and overseas during recent years. Until now, researches about the Northern Hemisphere (NH) extratropical SST have got rapid development. However, regarding influences of the large-scale Southern Hemisphere (SH) extratropical SST anomalies on the NH climate, there are still some valuable scientific problems which worth deeply exploring. Focusing on the boreal winter (December-February) and the following boreal spring (March-May), we expect to explore modulation of the boreal winter SH extratropical SST on the following spring NH subtropical precipitation by statistical analysis and dynamic diagnosis methods. In order to reveal the corresponding mechanisms, using theoretical analysis and numerical simulation, influences of the boreal winter SH extratropical SST on the winter and spring SH Annular Mode and the SH meridional circulation are examined, and as a basis, we further investigate physical processes responsible for modulation of the SH extratropical SST on the NH meridional circulation, and conclude the conceptual model for influence of the boreal winter SH extratropical SST on boreal spring NH subtropical precipitation. The implement of these relevant work could provide some scientific references for understanding the coupling processes in the SH extratropical atmosphere-ocean system, and would be helpful to provide new clues for seeking influencing factor of the boreal spring NH subtropical precipitation.

热带外海温变率对调控短期气候异常发挥着显著作用。年际尺度上热带外海温引起的跨半球的气候影响，是近年来在国内外迅速发展的研究领域之一。目前围绕北半球热带外海温的研究已取得诸多重要进展。然而，针对南半球热带外大尺度海温异常对北半球的气候影响，尚有许多科学问题值得深入探索。本项目以冬季（12-2月）和其后的春季（3-5月）为切入季节，采用资料统计和动力诊断的方法，分析冬季南半球热带外海温对春季北半球副热带降水的调控。为了揭示其中的物理机制，采用理论分析和数值模拟的手段，探讨冬季南半球热带外海温对冬、春南半球环状模和南半球经圈环流的影响，在此基础上讨论其对春季北半球经圈环流的调控机制，提出冬季南半球热带外海温影响春季北半球副热带降水的概念模型。相关工作的开展，可为认识南半球热带外海气耦合及与之相关的南北半球相互作用提供相应的理论参考，并为寻找春季北半球副热带降水的前期影响因子提供新的线索。

考察了年际尺度上南半球热带外环流变率主要模态（南半球环状模，SAM）对北半球副热带降水的影响。结果表明：经圈环流和纬向平均降水的部分年际变率可以由SAM解释。前冬（12-2月）SAM和春季（3-5月）经圈环流及纬向平均降水存在跨季节的相关关系。这种相关不仅存在于南半球，也扩展到北半球副热带地区。前冬SAM正位相时，逆时针和顺时针式的春季经圈环流异常依次出现在南半球中纬度至北半球副热带之间的地区，并伴随着增多和减少的降水异常依次由南至北分布。具体到北半球副热带，SAM正（负）位相时，降水偏少（多）。CMIP5的多模式集合平均，也反映了上述前冬SAM和春季北半球副热带降水的负相关关系。SAM可以通过海气界面热量交换影响南半球热带外海温。SAM正（负）位相时，中纬度风速减弱（增强），海温偏暖（偏冷）；高纬度风速增强（减弱），海温偏冷（偏冷）。这种SAM引起的南半球热带外海温偶极子型被称之为南大洋偶极子（SOD）。由于海温的热惯性，12-2月SAM引起的SOD可以持续到次年3-5月，并通过对局地斜压性的调整，进而对经圈环流产生重要影响。由于经圈环流是纬向平均降水的直接影响因子，因此12-2月的SAM通过热带外海温对春季经圈环流的影响，进而影响春季北半球副热带降水。