02 Jan 2023
02 Jan 2023
Status: this preprint is open for discussion and under review for Weather and Climate Dynamics (WCD).

Large Uncertainty in Observed Meridional Stream Function Tropical Expansion

Daniel Baldassare1, Thomas Reichler1, Piret Plink-Björklund2, and Jacob Slawson2 Daniel Baldassare et al.
  • 1Department of Atmospheric Sciences, University of Utah, Salt Lake City, UT 84112, USA
  • 2Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, USA

Abstract. Recent tropical expansion rate estimates vary substantially, as a multitude of methods and reanalysis datasets yield conflicting results. Among the many methods of estimating the tropical width, the meridional stream function 500 hPa zero-crossing is the most widely used, as it is directly related to the poleward edge of the Hadley Cell (HC). Other common metrics use atmospheric phenomena associated with the HC as a proxy, for instance the zonal surface wind zero-crossing. As each of these metrics require different data, each with varying error, the level of data-driven uncertainty differs between each metric. While previous work has analyzed the statistical and dynamical relationships between metrics, to date no study has quantified and compared the uncertainty in different HC metrics. In this study, we use ERA5 ensemble members, which include small perturbations in atmospheric variables based on the data error, to quantify the uncertainty associated with six commonly used HC metrics as well as the range of their trend estimates. In the Northern Hemisphere, the tropical expansion rate calculated by the stream function is roughly 0.05 degrees per decade, while the Southern Hemisphere rate is 0.2 degrees per decade. Of the six metrics, only the meridional stream function and precipitation minus evaporation have substantial uncertainties. The stream function errors are large due to uncertainty in the underlying meridional wind data and the presence of large regions of near-neutral circulation at the poleward edge of the tropics. These errors have decreased in recent decades because of improvements in the assimilated observations. Despite these improvements, we recommended using the zonal surface wind zero crossing to analyze tropical extent trends in reanalyses. This is particularly important in the Northern Hemisphere, before the year 2000, and when studying individual seasons other than winter.

Daniel Baldassare et al.

Status: open (until 25 Feb 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • AC1: 'Comment on egusphere-2022-1438', Daniel Baldassare, 04 Jan 2023 reply
  • RC1: 'Comment on egusphere-2022-1438', Anonymous Referee #1, 16 Jan 2023 reply
  • RC2: 'Comment on egusphere-2022-1438', Anonymous Referee #2, 03 Feb 2023 reply

Daniel Baldassare et al.

Data sets

ERA5 Accessed from Copernicus Hersbach et al., 2020

Model code and software

ERA5 Analysis Python Code Daniel Baldassare

Daniel Baldassare et al.


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Short summary
Using ensemble members from the ERA5 reanalysis, the most widely used method for estimating tropical width trends, the meridional stream function, was found to have large error, particularly in the Northern hemisphere and in the summer, because of weak gradients at the tropical edge and poor data quality. Another method, using the latitude where the surface wind switches from westerly to easterly, was found to have lower error and is recommended for future studies.