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

This study investigates the mean meridional wind patterns and their long-term trends within the Northern Hadley Cell (NHC) from 1980 to 2022, using reanalysis datasets and radiosonde observations. The Hadley circulation, a key component of Earth’s atmospheric circulation, is vital in redistributing heat and momentum between the equator and mid-latitudes. Understanding changes in its strength and structure is essential for interpreting current climate dynamics and improving projections of future climate change.

Our analysis reveals a consistent discrepancy between reanalyses and radiosonde measurements in representing the vertical structure of the NHC. Specifically, reanalyses tend to underestimate the mean poleward flow in the upper troposphere compared to radiosonde data while accurately capturing equatorward flow in the lower troposphere. Despite climate model projections generally indicating a weakening trend of the NHC under global warming, our examination of radiosonde data shows no statistically significant long-term trend. This absence of a clear signal in observational data introduces additional uncertainty into future projections of the Hadley circulation's response to climate change.

In contrast, reanalysis datasets, including ERA5, suggest a strengthening of the NHC over recent decades, mainly driven by an intensification of the upper-tropospheric poleward branch. To assess the reliability of this trend in reanalyses, we analysed the analysis increments from ERA5, which represent the corrections applied during data assimilation. Our findings indicate that the observed strengthening trend is not an artefact introduced by the data assimilation process. Instead, the analysis increments tend to nudge the first-guess model state, which underestimates the NHC strength, toward stronger meridional circulation, aligning more closely with assimilated observations.

These results highlight the importance of using multiple data sources when assessing long-term atmospheric trends and caution against relying solely on either reanalyses or climate model projections. The divergence between observational and reanalysis-based trends underscores ongoing uncertainties in the behaviour of large-scale circulation systems under climate change and calls for further investigation into the causes of these discrepancies.