Long-term changes in soil moisture conditions and their relation to atmospheric circulation in the Poyang Lake basin, China

Understanding the reasons for past changes in soil moisture conditions is key to improving forecasts of this extremely important variable coupling land and atmosphere. To contribute to this understanding, we analyzed the variability of soil moisture and its relationship to regional climate features. In this study we focused on the Poyang Lake basin, which is located on the southern bank of the Yangtze River at the junction of its middle and lower reaches in Jiangxi Province. Using in situ soil moisture observations from Nankang and Hukou stations for the period from 1998 to 2012, we tested the regional applicability of the high-resolution long-term Essential Climate Variable (ECV) soil moisture (SM) dataset (1978-2013) by means of trend and correlation analyses. We only used the ECV SM product here that is based on passive microwave technology. Both observed and ECV SM data show the same variations on monthly and inter-annual time scales including identical trend direction and strong correlations. To investigate the relationship between soil moisture conditions in the Poyang Lake basin and the Western Pacific Subtropical High (WPSH), correlation coefficients were calculated between the ECV SM data, the strength index (IS), and the northern limit index (IN) of the WPSH. The IS of the last autumn and winter shows a strong positive correlation with soil moisture during the subsequent rainy season. A strong WPSH in the preceding autumn and winter could result in more humid climate conditions in the Poyang lake basin during the following rainy season. The IN shows a strong negative correlation with concurrent soil moisture from July to September. The further the WPSH extends towards the north, the drier the area.