清江流域水热关系与东亚夏季风的联系

降水（P）与气温（T）之间的关系，即水热关系（PTR），在夏季一定程度上可反映干旱—高温热浪（或洪涝多雨—凉夏）极端气候灾害事件的发生情况。三峡库区内的清江流域，存在发育良好的洞穴，其沉积物中碳氧同位素序列长，可一定程度代表降水或气温信息，存在构造长序列PTR指标的可能性。从现代器测与再分析记录出发，研究清江流域PTR的演变特征和成因，可为利用石笋碳氧同位素构造PTR代用指标提供一些物理基础和启发。本文利用清江流域6个站（巴东、建始、利川、恩施、五峰、宜昌）1960～2016年逐日气温与降水观测资料及NCEP耦合预报模式再分析（CFSR）、日本气象厅55年再分析（JRA55）和欧洲中期天气预报中心再分析（ERA-interim）等3套资料，分析了清江流域PTR季节性差异、年际与年代际变化及其与东亚夏季风的联系。结果表明:1）PTR有明显季节性差异，夏季为显著负相关，其他季节不明显；2）夏季PTR存在明显年代际变化，表现出20～25年准周期振荡特征。在过去的几十年间， 1965～1976年和1993～2011年期间水热相关不显著，而1977～1992年为显著负相关，即分别在1974年、1986年、1992年发生了年代际转折；3）夏季PTR年代际变化和东亚夏季风强度有一定的联系。在水热显著负相关时段（1977～1992年），夏季风偏弱；PTR不显著时段（1965～1976年和1993～2011年），夏季风偏强；4）机制上，夏季风减弱，南北方气流交汇位置靠南，清江流域水汽增加，上升运动增强，云量与降水增加，导致地表热通量减少，地面气温下降，PTR为显著负相关。夏季风增强时，南北气流交汇区北移，清江流域水汽减少，下沉运动增强，导致云量减少而地表热通量增加，使得气温上升，进一步使气温垂直递减率增加，降水效率对降水贡献为正，表明气温上升利于降水增加，同时云量/降水对气温的影响减弱，但并未消失，使得PTR不显著。5）太平洋年代际振荡（IPO/PDO）对清江流域夏季PTR起着一定的调制作用。

Correlation of Surface Air Temperature and Precipitation in the Qingjiang River basin and Its Connection with the East Asian Summer Monsoon

The Precipitation?surface air Temperature Relationship (PTR) constitutes an important indicator of the ecological environment and is closely related to the occurrence of extreme drought/heat wave (or flood/coldness) events. Research has been taking place in several caves of the Qingjiang River basin of the Three Gorges Area. The cave sediments are characterized by long carbon and oxygen isotope sequences which may lead to the construction of a long sequence PTR. Based on modern meteorological observation and reanalysis data, the evolution characteristics and forming reason of the PTR in the Qingjiang River basin can be analyzed, which can provide some physical basis for understanding the relationship between the carbon–oxygen coupling coefficient of the stalagmites and PTR. Based on daily surface air temperature and precipitation observations from six stations, namely Badong (31°2'N, 110°22'E), Jianshi (31°36'N, 109°43'E), Lichuan (30°17'N, 108°56'E), Enshi (30°17'N, 109°28'E), Wufeng (30°12'N, 110°40'E), and Yichang (30°42'N, 111°18'E), and three sets of reanalyses, namely the NCEP Climate Forecast System Reanalysis (CFSR), Japan Meteorological Agency 55-year reanalysis (JRA55), and European Centre for Medium-range Weather Forecasts Interim Reanalysis (ERA-interim), this study investigated the seasonal, interannual, and decadal variability of the PTR and its connection with the East Asian summer monsoon. The following results were obtained: 1) PTR exhibits an obvious seasonality while a significant negative correlation is observed only in summer. 2) Summer PTR exhibits a decadal variability with a periodicity of 20–25 years, close to IPO/PDO. Significant shifts occurred in 1974, 1986, and 1992, with PTR altering from “insignificant” to “significant” and back to “insignificant” again. Specifically, PTR between 1977 and 1992 was significantly negative. 3) There is an overall correspondence between the decadal variability of summer PTR and the strength of the East Asian Summer Monsoon (EASM). When PTR was significantly negative (1977–1992), EASM was weak. However, when PTR was insignificant (1965–1976 and 1993–2011), EASM was strong. 4) Physically, when the summer monsoon weakens, the north and south air flow converges to the south. When the water vapor and ascending motion increases in Qingjiang River basin, it causes an increase in the cloud coverage and precipitation but it reduces the surface heat flux and decreases the surface temperature, leading subsequently to a negative PTR. When the summer monsoon strengthens, the north and south air flow converges to the north, which is not beneficial for the increase of water vapor. On the contrary, it is beneficial for the descending motion. In addition, cloud coverage reduces and temperature increases, resulting in the increase of the adiabatic rate and the positive contribution of precipitation efficiency to precipitation. It is important that, on one hand, the increase of temperature can increase precipitation, and on the other hand, the impact of cloud coverage/precipitation on the temperature weakens but does not disappear. Both conditions make PTR insignificant. 5) The decadal variability of PTR may be linked to the IPO/PDO.