Relationship between temperature trend magnitude, elevation and mean temperature in the Tibetan Plateau from homogenized surface stations and reanalysis data

Temperature trend magnitudes at 71 homogenized surface stations with elevations above 2000 m asl in the eastern and central Tibetan Plateau (TP) and 56 grid points from surface NCEP and ERA-40 reanalyses in the TP's vicinity are examined. Both the surface meteorological stations and ERA-40 show general warming trends at the majority of locations, especially in winter. NCEP fails to identify this. Compared with the surface stations, both NCEP and ERA-40 reanalysis data underestimate air temperature trends in the TP, but ERA-40 is better than NCEP. There are no simple linear relationships between elevation and temperature trend magnitudes on an annual or seasonal basis in the surface data or ERA-40, and in NCEP this relationship is inconsistent. Instead there are significant correlations between mean annual and seasonal temperatures and temperature trend magnitudes in the surface dataset and NCEP data (but not ERA-40). We suggest this is due to cryospheric feedback since trends are enhanced when mean annual temperatures are near freezing. The absence of any simple elevation dependency in temperature trends suggests that the rapid warming rate derived from high elevation ice-cores in this region should be interpreted with caution. In addition, more attention should be given to the selection of reanalysis to represent surface climate in the TP, since topographical differences between grid points and stations, and other reanalysis model differences such as surface land schemes, cause differences in trend identification and patterns in this critical region.