Variability of temperature in the Tibetan Plateau based on homogenized surface stations and reanalysis data

The Tibetan Plateau (TP) with an average elevation of over 4000 m a.s.l. is the world's highest and most extensive highland. The scarcity of climatic observations limits our understanding of surface air temperature change in the region. Thus, we compare temperatures and their trends from 71 homogenized surface stations (with elevations above 2000 m a.s.l.) with National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis (NCEP/NCAR hereafter) and European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-40 hereafter) in the eastern and central TP during 1961–2004. For current climatology, ERA-40 is more similar to the surface stations than NCEP/NCAR. Compared with surface stations, both NCEP/NCAR and ERA-40 reanalyses have cold biases, which are mainly a result of differences in topographical height, and station aspect and slope. Warming trends at the surface stations are on average stronger than in both reanalyses, but ERA-40 captures the surface warming more clearly than NCEP/NCAR on an annual and seasonal basis. Since ERA-40 more closely represents the surface temperatures and their trends in the central and eastern TP, ERA-40 predictions are selected to examine change in the western TP where there are few surface stations. NCEP/NCAR, on the other hand, is more representative of free air temperature conditions. The ‘observation minus reanalysis’ (OMR) method can be used to estimate the impact of surface changes on climate by computing the difference between surface observations and NCEP/NCAR (which only contains the forcing influencing the assimilated atmospheric trends). The OMR trend is significantly increasing but the extent to which the changes in local environment are responsible needs further study.