Discrepancy of interdecadal changes in the Asian region among the NCEP-NCAR reanalysis, objective analyses, and observations

This study compares decadal means and interdecadal changes of surface and sea level pressures, tropospheric heights, and winds in the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis with objective analyses and observations. It is found that over Asia the NCEP-NCAR reanalysis pressures and heights are systematically lower than objective analyses and observations before the late 1970s. The magnitude of the differences changes from one decade to another and shows obvious seasonal dependence. The nonuniform spatial distribution of pressure and height differences is consistent with the discrepancy in lower-level meridional winds along the east Asian coast. The seasonal dependence of pressure differences affects the strength of the seasonal cycle over Asia. More importantly, large changes in the discrepancies from one decade to another lead to inconsistent interdecadal changes between the reanalysis and objective analyses or observations in the Asian region. While the reanalysis displays a large increase of pressure around 1977 and in the mid-1960s and an obvious decrease in the late 1950s, the changes are small in objective analyses and observations. Inconsistent interdecadal changes are also present in tropospheric heights and winds. The results indicate that the reanalysis may overestimate interdecadal changes over Asia. This calls for caution in utilizing the reanalysis output to study the interdecadal variability or the interannual variability without removal of interdecadal variations in the Asian region.