The Changes of the Northern Hadley Cell Strength in Reanalyses and Radiosonde Observations

This study examines mean meridional winds and their trends in the Northern Hadley Cell (NHC) from 1980 to 2022 using reanalysis datasets and radiosonde observations. Compared to radiosonde data, reanalyses underestimate the mean upper-tropospheric poleward flow of the NHC but accurately capture the mean equatorward flow in the lower troposphere. While climate models generally project a weakening of the NHC, our study finds no significant trend in radiosonde observations, adding to the uncertainty in future climate projections. In contrast, reanalyses indicate a strengthening, primarily due to an intensification of the upper-tropospheric poleward flow. Our examination of ERA5 analysis increments confirms that the NHC strengthening trend in ERA5 is not an artefact of data assimilation. Instead, the increments correct the first-guess, which underestimates the strength of the NHC, nudging it toward a stronger circulation.