Metrics of the Hadley circulation strength and associated circulation trends

<p>This study compares trends of the Hadley cell (HC) strength using different metrics applied to the European Centre for Medium-range Weather Forecasts ERA5 and ERA-Interim reanalyses in the period 1979-2018. The HC strength is commonly evaluated by metrics derived from the mass-weighted zonal-mean stream function. Other metrics include the upper-tropospheric velocity potential, the vertical velocity in the mid-troposphere, and the water vapour transport in the lower troposphere. Seven known metrics of the HC strength are here complemented by a metric of the spatially-averaged HC strength, obtained by data-adaptive averaging of the stream function in the latitude-pressure ($\varphi$-$p$) plane, and by the total energy of zonal-mean unbalanced circulation in the normal-mode function decomposition. It is shown that metrics, which rely on single-point values in the $\varphi$-$p$ plane, produce unreliable 40-year trends of both the northern and southern HCs, especially in ERA-Interim; magnitudes and even the signs of the trends depend on the choice of the HC strength metric. The two new metrics 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 spatially-averaged HC strength metric also shows a positive trend in ERA5 in both hemispheres, while the trend in ERA-Interim is insignificant. The study calls for a unified metric of the HC strength, that allows easier comparison of different studies on the Hadley circulation changes. This would also allow a better estimation of the likelihood of the future changes in the global atmospheric circulation.</p>