云南土壤湿度长期变化的初步研究

利用1948—2013年美国NOAA/CPC全球0.5°×0.5 °月平均格点化土壤湿度资料、1951—2013年云南地区125个站月降水和1993—2013年22个站月土壤湿度观测资料对云南土壤湿度及其表征的旱涝长期变化进行时空分布及演变特征分析。结果表明:云南地区旱涝变化的空间结构相对简单且具有大尺度特征，长期变化特征明显。由20世纪50年代的滇中部地区偏旱，其余地区偏涝，逐步发展为相反；20世纪90年代开始滇中部地区偏涝，其余地区偏旱，且旱情日趋加重，范围扩展，2010年、2012年和2013年严重干旱蔓延至云南全省。研究揭示，云南冬半年旱涝与前期海温异常密切相关，冬半年云南地区旱涝不同的EOF模态时间变化对应着不同的前期海温异常变化分布。云南全省旱涝一致的第1模态对应前期孟加拉湾、阿拉伯海、西太平洋以及大西洋的海温异常正相关。云南西北—东南旱涝反向的第2模态对应前期孟加拉湾、南海、西太平洋及东太平洋ENSO区海温异常负相关。

Preliminary Results on Long term Soil Moisture Variation in Yunnan

Long-term spatial-temporal distributions of soil moisture anomalies related drought-flood events and their evolution features in Yunnan, China of recent 66 years are explored using 1948-2013 NOAA CPC 0.5°×0.5° global monthly soil moisture data, along with 125-station based monthly observed precipitation for period of 1951-2013, and monthly soil moisture observation from 22 stations for the period of 1993-2013. Comparing with precipitation, soil moisture has some unique physical properties and statistical features, such as longer memory and closer to normal distribution, making it an important component for drought-flood analyzing and monitoring. The mutual validation reveals that the gridded NOAA CPC monthly soil moisture data can capture those major observed drought-flood events very well. The preliminary results show that spatial structures of soil moisture anomaly related drought-flood events in Yunnan, China have relatively simple and large-scale features. The first four leading EOF modes can explain more than 2/3 of total variance of soil moisture variations. The soil moisture related drought-flood events also have obvious long-term variations or trends, such as from 1950s it is drier in central Yunnan and wetter in most rest parts, and then the condition gradually evolved to the opposite, from 1990s the central part of Yunnan becomes wetter than most rest parts. The drought condition tends to become worse since then, especially for the years of 2010, 2012 and 2013, when extreme drought conditions extends to almost everywhere in Yunnan. Further study on possible causes for this spatial-temporal evolution reveals some good relationships between monthly soil moisture variations and global sea surface temperature anomalies (SSTA) one or two months ago, especially in cool season. The 1st EOF mode of soil moisture anomalies, which represents soil moisture anomalies through the whole Yunnan, has good correlation with previous month's SSTA nearby the Bay of Bengal, Arab Sea, the western tropical Pacific and the remote Atlantic, and the 2nd EOF mode which reflects soil moisture anomalies related drought-flood variations from northwest to southeast Yunnan also well correlates with previous month's SSTA in the Bay of Bengal, the South China Sea, western Pacific, and the remote eastern tropical Pacific ENSO region and the tropical Atlantic. The 3rd EOF mode and 4th EOF mode of soil moisture anomalies, which present south to north and east to west drought-flood variations in Yunnan, also relate to unique SSTA in previous months.