29 Jul 2021

29 Jul 2021

Review status: this preprint is currently under review for the journal WCD.

Indices of the Hadley circulation strength and associated circulation trends

Matic Pikovnik1, Žiga Zaplotnik1, Lina Boljka2, and Nedjeljka Žagar3 Matic Pikovnik et al.
  • 1Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, 1000, Slovenia
  • 2Geophysical Institute and Bjerknes Centre for Climate Research, University of Bergen, Bergen, 5020, Norway
  • 3Meteorological Institute, Center for Earth System Research and Sustainability, Universität Hamburg, Hamburg, 20146, Germany

Abstract. This study compares the trends of Hadley cell (HC) strength using different HC measures applied to the ECMWF ERA5 and ERA-Interim reanalyses in the period 1979–2018. The HC strength is commonly evaluated by indices derived from the mass-weighted zonal-mean stream function. Other measures include the velocity potential and the vertical velocity. Six known measures of the HC strength are complemented by a measure of the average HC strength, obtained by averaging the stream function in the latitude-pressure (φ-p) plane, and by the total energy of unbalanced zonal-mean circulation in the normal-mode function decomposition. It is shown that measures of the HC strength, which rely on point values in the φ-p plane, produce unreliable long-term trends of both the northern and southern HCs, especially in ERA-Interim; magnitudes and even the signs of trends depend on the choice of HC strength measure. The two new measures alleviate the vertical and meridional inhomogeneities of the trends in the HC strength. In both reanalyses, there is a positive trend in the total energy of zonal-mean unbalanced circulation. The average HC strength measure also shows a positive trend in ERA5 in both hemispheres, while the trend in ERA-Interim is insignificant.

Matic Pikovnik et al.

Status: open (extended)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on wcd-2021-50', Anonymous Referee #1, 17 Aug 2021 reply
    • AC1: 'Reply on RC1', Žiga Zaplotnik, 09 Sep 2021 reply

Matic Pikovnik et al.

Data sets

Hadley cell strength Žiga Zaplotnik, Matic Pikovnik

Matic Pikovnik et al.


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Short summary
Potential future changes of the Hadley cells (HCs), either to their strength or their meridional extent, will have a profound impact on the global distribution of precipitation. An objective analysis of past and future changes of HCs requires a unified index of the their strength. The measures, which rely on the single point values, produce unreliable long-term trends of the HC strength. Thus, two new measures are proposed, which alleviate the spatial inhomogeneities of HC strength trends.