天山地区大气降水氧同位素的影响因素及其对西风环流变化的指示意义

利用全球降水同位素观测网（GNIP）提供的乌鲁木齐大气降水氧同位素（δ¹⁸O_p）观测数据（1986-2003年）,结合和田δ¹⁸O_p数据及天山冰芯δ¹⁸O记录,分析了天山地区δ¹⁸O_p在年内和年际尺度上的变化特征,及其与主要气候要素（温度和降水量）的关系。结果表明,年内尺度上,δ¹⁸O_p与月均温和月降水量均为显著正相关,表现出明显的“温度效应”;年际尺度上,加权平均年δ¹⁸O_p与年均温度却呈现负相关关系,与降水量仍为正相关。近40~50年的天山地区4支冰芯的δ¹⁸O具有整体偏负的变化趋势,与研究区逐步升高的温度呈反向变化,说明在年际至年代际尺度上,这一区域δ¹⁸O_p与温度之间的负相关关系是客观存在的。进一步对研究区水汽来源路径的对比分析发现：δ¹⁸O_p值偏负的高温年份（1997年）的暖季水汽相对更多来源于远源的高纬度区域,而δ¹⁸O_p值偏正的低温年份（1988年）则相对更多来自近源的中、低纬度区域;同时,研究区上风方向的欧亚大陆14个GNIP站点1997年和1988年的暖季加权平均δ¹⁸O_p值存在高纬度区域偏负而中、低纬度区域偏正的特征;这些结果说明年际至年代际尺度上,天山地区δ¹⁸O_p与温度之间的负相关,本质上指示了西风环流南北摆动所引起的水汽来源变化,可以作为西风环流变化的指示器。

Variations in the oxygen isotopic composition of precipitation in the Tianshan Mountains region and their significance for the Westerly circulation

Proxy records of the oxygen isotopic composition of meteorological precipitation (δ¹⁸O_p) preserved in archives such as ice cores, lacustrine carbonates and stalagmite calcite are important for paleoclimatic studies. Therefore, knowledge of the variations and controlling mechanisms of modern δ¹⁸O_p on different time scales is necessary. Here, we investigate the linear correlations between δ¹⁸O_p and corresponding temperature and precipitation on monthly and inter-annual timescales, using data from the Urumqi (1986-2003) and Hotan stations of the Global Network of Isotopes in Precipitation (GNIP), and δ¹⁸O data from 4 ice cores in the adjacent Tianshan Mountains. Consistent with previous reported results, modern δ¹⁸O_p variations on a seasonal time scale in the Tianshan region are mainly controlled by a 'temperature effect' (indicated by a significant positive correlation between δ¹⁸O_p and temperature), with more positive δ¹⁸O_p values occurring in summer. However, on an inter-annual timescale, there is a weak inverse correlation between weighted average annual δ¹⁸O_p and annual average temperature at Urumqi. This finding is supported by the inversely varying trends of δ¹⁸O data from 4 ice cores in the central and eastern Tianshan Mountains compared to annual average temperatures in the same region during the past 40-50 years. The data from Urumqi station and the 4 ice cores demonstrate that the inverse correlation between δ¹⁸O_p and temperature on inter-annual to decadal time scales is genuine. Analysis of water vapor sources and pathways for the warm year of 1997 and the cold year of 1988 reveal that more water vapor for the Tianshan area was derived from long-distance transport from high-latitude sources than during the warm year of 1997; and that more water vapor was transported from more proximal sources from mid- to low-latitude areas during the cold year of 1988. In addition, the δ¹⁸O_p values are more negative in the high latitude areas than those in mid- to low-latitude areas in the Eurasian continent at the upper wind direction of Tianshan Mountains region, according to the weighted averaged warm season (May to September) δ¹⁸O_p values for 14 GNIP stations in the years 1997 and 1988. Due to the distribution of δ¹⁸O_p within the Eurasian continent, the relative shift of water vapor sources between warm and cold years convincingly explains the observed variations of δ¹⁸O_p in the Tianshan area. Therefore, we conclude that variations in δ¹⁸O_p in the Tianshan Mountains region are mainly controlled by changes in water vapor sources which are ultimately caused by northward and southward shifts in the Westerly circulation.