Competing Tropical and Extra-tropical Processes drive Zonal Asymmetries in Hadley Circulation Expansion

Dynamics associated with the expanding Hadley Circulation (HC) are gaining attention owing to their far-reaching consequences on tropical and subtropical weather and climate. In the present communication, the co-variability of ascending and descending regions of the HC at regional scales is investigated. The results from four decades of ERA-5 reanalysis emphatically show that the ascending and descending regions of the HC move in tandem with profound zonal asymmetry in their degree of co-variability, and their long-term trends show similar signs with notable differences in magnitude. In more than half of the longitudinal sectors, the relative movement between the ascending and descending boundaries of each hemisphere is in unison. At this juncture, where there are debates on the causative mechanism for the observed expansion of HC as well as their zonal asymmetries, the present results provide new insights into the regional HC dynamics by emphasizing the coherent variations of ascending and descending regions of the HC at regional scales for the first time. The results discussed in the study will be useful for identifying differences in the regional forcing of HC expansion and for quantifying the regional expansion rates of both ascending and descending regions of the HC.