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The southward circulation embedded within the conventional northward circulation of the Northern Hemisphere (NH) Hadley circulation (HC) is defined as a minor circulation. Motivated by the vital influence of this minor circulation, we investigate whether the minor circulation in the NH HC is a robust feature during boreal summer (June to September). Our results show that although the uncorrected NH HC includes the minor circulation, the corrected NH HC is characterized as one single northward circulation regardless of the reanalysis data set and method used. This finding implies that the minor circulation concept is introduced by not considering orography (i.e., surface pressure). After correction, the significance and degree of the poleward expansion of the NH HC during boreal summer are obviously reduced. We also note that regional biases in the meridional wind can result in considerable biases throughout the entire NH HC via the process of vertical integration.

The fake below-ground meridional wind (FBGMW) exists in reanalysis products which is not present in the real atmosphere and should be removed before calculating the mass stream function (MSF). In this study, the impacts of FBGMW on Hadley circulation (HC) in terms of climatology, interannual variability, and long-term trends were investigated using five reanalysis data sets based on three different computing methods. Generally, the impacts of FBGMW on the HC are most notable, although the absolute magnitude of the FBGMW is rather small. The key finding of this study is that the FBGMW has vital influences on the Northern Hemisphere (NH) HC during boreal summer. This is because the NH HC during boreal summer is very weak; the errors of the MSF caused by not considering FBGMW have more obvious influences on the NH HC during boreal summer than that in other months. The previous analysis without considering FBGMW led to overestimation of the poleward expansion of the NH HC during boreal summer, and the long-term trends of the HC should be more accurately estimated after considering the FBGMW. This finding suggests that the previous studies related to the NH HC during boreal summer without considering FBGMW should be reconsidered.