Quality of reanalyses in the Tropics

Broad vertical layer-averaged temperatures from the microwave sounder unit (MSU) are used as a quasi-independent validation of temperature fields from the U.S. National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses. While the MSU and NCEP-NCAR temperatu res show fairly good agreement overall, large discrepancies with ECMWF temp eratures indicate that changes in the satellite observing system may have a dversely affected the ECMWF reanalyses, especially in the Tropics. Two spur ious discontinuities are present in tropical temperatures with jumps to war mer values throughout the Tropics below 500 mb in late 1986 and early 1989, and further spurious interannual variability is also present. These featur es are also reflected in the specific humidity fields. The temperature disc repancies have a complex vertical structure with height that is not fully u nderstood, although it seems that the problems partly arise from positive r einforcement of biases in satellite radiances with those of the assimilatin g model first guess. Changes in the observing system provide a limit to the usefulness of the reanalyses in some climate studies.