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

Abstract

AbstractThe 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.