三维副热带高压强度指数及对中国东部雨带异常表征的改进

The three-dimension intensity index for western Pacific subtropical high and its link to the anomaly of rain belt in eastern China

西太平洋副热带高压(副高)是位于对流层中下层的深厚环流系统,其位置和强度的年际差异直接决定着东亚气候的异常.但现有副高参数定义均囿于500 hPa单层及5880 gpm这一特定阈值,造成副高强度指数不能很好表征我国东部雨型特征尤其是长江流域的旱涝.统计结果表明,东亚副热带夏季风强度与同期各层位势高度的显著负相关性随经度自西向东有明显的自高层向低层下传的特征.在西太平洋暖池区,最显著的负相关区位于对流层中低层,且500 hPa的相关系数明显弱于700和850 hPa,表明东亚副热带夏季风的强度更容易受到低层副高强度异常的影响.因此本文分析了不同季节西太平洋副热带地区该阈值在各格点的代表性,发现基于该阈值定义有很大的局限性和季节差异,由此从副高空间结构特征重新定义了一个新的三维副高强度指数,即(10°~30°N,110°~150°E)区域1000~500 hPa各层位势高度标准化值累积值,并和两种传统的副高强度指数对比,发现新的三维副高强度指数无论是对副热带夏季风还是对长江中下游降水异常的表征均有明显提高.虽然3种副高强度指数都能反映出和东部降水最强正相关位于长江中下游这一基本特征,但仅有本文定义的三维强度指数与之相关性可通过显著性检验,这主要是因为三维副高强度指数比其他两种指数能更好反映东亚热带和副热带地区经、纬向水汽输送的异常.

Western Pacific subtropical high（WPSH） is a deep circulation system located at middle-lower troposphere over the East Asia. The anomalies of its position and intensity determine the shift of climates in eastern China, especially the summer rainfall belt. However, previous definitions of WPSH intensity index were limited mainly by a single threshold of 5880-gpm geopotential height contour at 500-hPa, which cannot denote the rain pattern in eastern China reasonably, in particular the floods and droughts along the Yangtze River valley. In this paper, the link between the intensity of East Asian summer monsoon（EASM） and the geopotential heights at each level were analyzed by correlation analyses. The results indicate that the significant negative correlations spread obviously from the upper to lower levels with longitude changed from west to east. At the upper levels over the Asian continent, the most significant negative correlation was found at the central region of the South Asia High（SAH）. This implies that the EASM would be weaker if the SAH was stronger, and vice versa. Over the western Pacific warm pool, the most significant negative correlation was found in the middle-lower troposphere. The correlation coefficients at 500-hPa were much weaker than those at 700-hPa and 850-hPa, indicating that the EASM was easier influenced by the subtropical high at lower levels. Similar, the relationship between the WPSH and the precipitation along the Yangtze River valley was more significant at lower levels indicated by the correlation coefficient. In this study, the representativeness of the threshold of 5880 gpm at 500-hPa level over the western Pacific subtropical regions was analyzed at different grids and seasons, and the limitations of previous definitions were also discussed. Considering the three-dimension（3D） structures of the subtropical high, a new WPSH intensity index was defined by integrated the standardized geopotential heights from 500-hPa to 1000-hPa levels over the region（10°–30°N, 110°–150°E）. Compared with the two traditional WPSH indices, the new 3D WPSH intensity index can better represent the subtropical summer monsoon and the precipitation along the middle and lower reaches of the Yangtze River. Although all the three indices could reflect the primary correlation patterns between WPSH and the precipitation in eastern China, only the correlation coefficient between the 3D intensity index and precipitation is significant at the 95% confidence level to pass the student t-test. Compared to the other two indices, the 3D WPSH index can better reflect the anomalous of both meridional and zonal water vapor transportation between the tropical and subtropical regions of East Asia. During the transition seasons of boreal spring and autumn, the mechanisms of anomalous precipitation are more complicated, which are obviously different from EASM. Preliminary analysis shows that even in these two transition seasons, the 3 D WPSH index defined in this paper also has better link with the precipitation anomalies in eastern China. As mentioned before, the WPSH is a deep circulation system over the western Pacific. Besides its intensity, the position also impacts on the climates over East Asia, especially its west boundary and meridional axis. In the future study, the definitions of other parameters of WPSH need to be studied from its 3 D structure.