Changes of the boreal winter Hadley circulation in the NCEP-NCAR and ECMWF reanalyses: A comparative study

Both the ECMWF and the NCEP-NCAR reanalyses show a strengthening of the atmospheric Hadley circulation in boreal winter over the last 50 years, but the intensification is much stronger in the ECMWF than in the NCEP-NCAR reanalysis. This study focuses on the difference of these trends in the two reanalyses. It is shown that trends in the Hadley circulation in the two reanalyses differ mainly over the tropical western Pacific. This difference is found to be consistent with respective trends of the atmospheric transport of moist static energy, longwave cloud radiative forcing, and upper-level clouds in the two reanalyses. Two independent datasets of upper-level cloud cover and sea level pressure from ship-based measurements are then used to evaluate the reanalyses over the tropical western Pacific. They are found to be more consistent with the trends in the NCEP-NCAR reanalysis than those in the ECMWF reanalysis. The results suggest a weakening of the vertical motion associated with the Hadley circulation in the tropical western Pacific.