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黄土高原地区作为气候敏感区和生态脆弱区地表干 湿状况的年际和年代际变化特征十分明显. 但以往主要是针对夏季进行分析, 而对黄土高原秋季干湿变化规律及大气环流机理的认识非常有限. 本文基于中国589站最近50 a (1961–2010年)月降水和气温月平均资料、NCEP/NCAR提供的再分析资料以及NOAA提供的海表温度(Sea Surface Temperature, SST) 资料, 运用带通/低通滤波、小波分析、EOF/REOF和回归分析等方法, 在对中国秋季干湿时空演化分类的基础上, 通过研究秋季黄土高原中部干湿演变周期、大气环流特征及与海温的多尺度相关关系, 以揭示影响黄土高原中部秋季干湿变化的物理机理, 并确定影响该区域干湿状况的前兆信号. 小波功率谱分析表明, 黄土高原中部秋季干湿指数存在准4 a和准8 a的周期, 1970–1990年准8 a尺度周期振荡尤为明显. 年际(周期≤ 8 a) 尺度上偏湿年的大气环流特征是, 欧亚大陆中高纬呈“双阻型”, 200 hPa西风急流显著北移, 日本海-鄂霍茨克海受反气旋控制, 其底部的偏东水汽输送带将水汽输入研究区. 年代际(周期 > 8 a)尺度上偏湿年的大气环流特征是, 东亚大陆为一致的低值系统; 200 hPa东亚副热带西风急流减弱北移, 研究区主要水汽来源由经孟加拉湾在中南半岛转向的南风水汽输送及中纬度的西风水汽输送组成. 整个序列上, Nino3区SST指数(Nino3I)超前5个月与秋季干湿指数已呈显著的负相关关系, 而孟加拉湾–中国南海SST指数(BayI)则超前3个月与干湿指数呈现显著的负相关关系. 年际尺度上, 秋季Nino3I, BayI均与秋季干湿指数存在显著相关(准4 a, 4–6 a), 而年代际尺度上, 只有BayI与秋季干湿指数存在显著相关性(准10 a). 黄土高原中部秋季干湿的年际和年代际周期的确定、大气环流异常特征的认识及与海温的多尺度相关关系的建立, 不仅揭示了影响该区域干湿变化的物理机理, 也为干旱气候预测提供了重要的前兆信号.The ecological environment is vulnerable and extremely sensitive to climate change in the loess plateau. The interannual and interdecadal variations of dry/wet index are particularly obvious in this region. Up to now, the analisis of the dry/wet variation in the loess plateau has been basically understood during summer, but the autumn dry/wet evolution of the loess plateau and the atmospheric circulation characteristic are still laking in its knowledge. Based on the monthly mean rainfall and temperature of 589 China weather stations, the NCEP/NCAR reanalysis dataset and NOAA extended reconstructed sea surface temperature (SST) data from 1961 to 2010, using the EOF/REOF, bandpass filtering, wavelet technique and regression methods, the autumn dry/wet evolution of the loess plateau and the atmospheric circulation characteristic associated with them, and the multi-scale relationship between dry/wet index and SST were analyzed on the basis of classification of dry/wet over China region in autumn. The continuous wavelet transform shows that the autumn wet/dry index exists has ～4 a and 8 a period over the loess plateau, and the ～8 year oscillation from 1970–1990 is rather obvious. On the interannual scales, the atmospheric circulation anomalies of 500 hPa associated with the wet years exhibit a ’double-blocking’ in the mid-high latitude, and the westerly jet stream turns to be weakened and the jet axis moves to the north significantly. Japan-Okhotsk Sea is featured with anticyclone, and the east wind at the bottom of the anticyclone provides favorable moisture for the middle Loess plateau. On the interdecadal scales, the atmospheric circulation anomalies associated with the wet years show the negative geopotential height anomaly over East Asian, and the East Asian subtropical westerly jet stream of 200 hPa is weakened and moves north too. The vapour of the middle Loess plateau comes from two paths, one is the westerly water vapor transport in the middle latitude, and the other is the southerly water vapor transport from the Bay of Bengal and turns to the north at the Indo-China Peninsula. When the Nino3 index (Nino3I) is in advance of dry/wet index for 5 months, there is a significantly negative correlation between them. When the Bay of Benga and South China Sea SST index (BayI) goes shead of dry/wet index for 3 months, there is a significantly negative correlation between them. On the interannual scales, the cross wavelet transform indicates that both the Nino3I and BayI have significant connections with the dry/wet index (～4 a and 4–6 a cycles). On the interdecadal scales, only BayI has significant connections with dry/wet index (～10 a cycles). By establishing the interannual and interdecadal variations of dry/wet cycle of the loess plateau, recognizing the atmospheric circulation anomalies, and building the relationship between the dry/wet index and SST, this paper reveals the physical mechanism of the dry/wet variation and finds the early waring signals for drought climate change in the region.
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Keywords:
- middle Loess plateau/
- dry and wet in autumn/
- interannual and interdecadal timescales/
- sea surface temperature
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