<p>This study explores the possible drivers of the recent Hadley circulation strengthening in modern reanalyses. Predominantly, two recent generations of reanalyses provided by the European Centre for Medium-range Weather Forecasts (ECMWF) are used: the fifth-generation atmospheric reanalysis (ERA5) and the interim reanalysis (ERA-Interim). Some results are also evaluated against other long-term reanalyses: ERA-20C, CERA-20C, NOAA-20CR and NCAR/NCEP. To assess the origins of the Hadley cell (HC) strength variability we employ the Kuo-Eliassen equation. ERA5 shows that both HCs were strengthening prior to 2000s, but they have been weakening or remained steady afterwards. Most of the long-term variability in the strength of the HCs is explained by the meridional gradient of diabatic latent heating, which is related to precipitation gradients. However, the strengthening of both HCs in ERA5 is larger than the strengthening expected from the observed zonal-mean precipitation gradient (via Global Precipitation Climatology Project, GPCP). This suggests that the HC strength trends in the recent decades in ERA5 can be explained partly as an artifact of the misrepresentation of latent heating and partly through physical long-term variability. To show that the latter is true, we analyze ERA5 preliminary data for the 1950-1978 period, other long-term (e.g. 20<em><sup>th</sup></em> century) reanalyses, and sea surface temperature observational data. This reveals that the changes in the HC strength can be a consequence of the Atlantic multidecadal oscillation (AMO) and related diabatic and frictional processes, which in turn drive the global HC variability. This work has implications for further understanding of the long-term variability of the Hadley circulation.</p>

<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>