Significant Role of Orography in Shaping the Northern Hadley Circulation and Its Poleward Expansion During Boreal Summer

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. Plain Language Summary In this study, based on the mass stream function derived from three methods using four reanalysis data sets during the period of 1979–2016, we investigate the influence of orography on shaping the northern Hadley circulation (HC) and its poleward expansion during boreal summer. Our analysis shows that after considering the orography (i.e., surface pressure), the Northern Hemisphere (NH) HC is dramatically different to that without considering the orography. In addition, the significance and degree of the poleward expansion of the HC are obviously reduced during boreal summer after considering the orography. While the evolution characteristics of the HC and its response to different forcings have been investigated by many previous studies, our study emphasizes that some of these studies should be corrected for the orography. Because regional biases in the meridional wind can cause considerable biases throughout the entire NH HC via the vertical integration process, we also claim that the orography should be considered when conducting the vertical integration of any variable. The results from our study have substantial implications for the study related to the HC. Key Points The concept of minor circulation is introduced by not considering orography (i.e., surface pressure) Regional biases in the meridional wind can cause considerable biases throughout the entire NH HC via the vertical integration process The significance and degree of the poleward expansion of the NH HC duri