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.
Highlights
The Hadley circulation is a thermally forced overturning circulation, consisting of two symmetrical cells, which span between the tropics and the subtropics
It is shown that measures of the Hadley cell (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 same applies to the new measure (8), which is by definition a global measure, but in large part explained by the Hadley circulation (Fig. A2) and does not distinguish between the two HCs. 130 we explore the sensitivity of the trends to different measures of the HC strength
Summary
The Hadley circulation is a thermally forced overturning circulation, consisting of two symmetrical cells, which span between the tropics and the subtropics. The reported magnitude and uncertainty of the trends differ (Tanaka et al, 2004; Mitas and Clement, 2005; Stachnik and Schumacher, 2011; Nguyen et al, 2013; Chemke and Polvani, 2019). The Hadley circulation strength can be defined by the minimum pressure velocity ω at some predefined mid-tropospheric level (Wang, 2002). Both measures describe the properties of the ascending branch of the Hadley circulation. While several studies have compared the Hadley circulation in different reanalyses and climate models (e.g. Stachnik and Schumacher, 2011; Chemke and Polvani, 2019), no study (to our knowledge) has yet compared the measures of the HC strength 45 in the same dataset.
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