Seasonal variations of snow cover over the Tibetan plateau and ultimate effects on the east Asian summer monsoon

Seasonal snow cover over the Tibetan Plateau (TP) is an important indicator of the mean conditions of temperature and precipitation in the area and its surroundings. However, the assessment of snow cover variability over the TP has been a challenging problem due to lack of conventional observations. High-resolution snow mapping products acquired from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Terra spacecraft have provided an excellent opportunity to define details in the spatial and temporal snow cover distribution in the area. In this study, the accuracy of MODIS/Terra snow cover data is first evaluated by comparing the data with in situ Chinese station snow observations; results show that the overall accuracy of MODIS snow data is about 90% over the TP. Statistical analysis is then performed to explore the distribution and variability of snow cover based on the available data from 2000-2006. It was found that the most persistent snow cover is located in t h e southern and western edges of the TP, especially large mountain ridges and the western part of Yarlung Zangbo Valley. The higher snow cover fractions (SCFs) are largely concentrated in the regions where the elevation is higher than 6000m. The snow persistence varies in different elevation ranges and generally becomes longer with increases in the terrain elevation. In addition, the spatial distribution of the snow cover not only depends on the elevation, but also varies with the terrain features (e.g., aspects, slopes etc.) in the local areas. With 7-year observational data, seasonal and interannual variability of snow cover has been detected, but the persistence of the seasonal and interannual anomalies in snow cover is generally lacking due to the small sample of data available. There are slight decreasing trends in SFCs over the TP during 2000-2006. With the MODIS satellite snow cover fraction data during 2000-2006, and NCEP/DOE reanalysis 2 dataset, the relationship between snow cover anomalies over the TP and the East Asian summer monsoon (EASM) is examined. It is found that the onset of EASM is well associated with snow cover anomalies in spring. Specifically, positive or negative snow cover anomalies are followed by a later or earlier onset of EASM, respectively. The strength of EASM shows a weak connection with snow cover anomalies with fewer exceptions. Based on a previous conceptual model, physical mechanisms associated with this snow-monsoon connection are discussed.