Large uncertainty in observed estimates of tropical width from the meridional stream function

Abstract. Recent Hadley cell expansion rate estimates vary substantially, as a multitude of methods and reanalysis datasets yield conflicting results. Among the many estimating width, meridional-stream-function 500 hPa zero crossing is most widely used, it directly related to poleward edge (HC). Other common metrics use atmospheric phenomena associated with HC proxy, for instance zonal-surface-wind crossing. As each these requires different data, varying error, level data-driven uncertainty differs between metric. While previous work has analyzed statistical dynamical relationships metrics, date no study quantified compared due data error in metrics. In this study, we ERA5 ensemble members, which include small perturbations variables based on quantify six commonly used well range their trend estimates. Northern Hemisphere, tropical calculated by stream function roughly 0.05∘ per decade, while Southern Hemisphere 0.2∘ decade over period from 1979–2022. Of only meridional precipitation minus evaporation have substantial uncertainties. The errors are large underlying meridional-wind presence regions near-neutral circulation at tropics. These decreased recent decades because improvements assimilated observations. Despite improvements, well-observed constrained quantities such lower uncertainty, particularly summer fall Hemisphere.