Ocean Heat Transport and the Width of the Hadley Circulation in an Aquaplanet GCM

Abstract

Despite decades of research, the fundamental dynamics that govern the width of the Hadley circulation (HC) remain an active area of investigation. This study seeks to further theoretical understanding of HC width and of the mechanisms leading to HC expansion by exploring the response of a zonally-symmetric slab-ocean aquaplanet general circulation model (GCM) to imposed poleward ocean heat transport (OHT). It is found that the presence of OHT causes the HC to extend up to 3° farther into the subtropics than it would in the absence of OHT. This widening of the HC by OHT is interpreted as being driven by a decrease in baroclinicity near the poleward edge of the HC and is decomposed into three components: a decrease in baroclinicity due to (1) a systematic poleward shift of the Intertropical Convergence Zone (ITCZ) throughout the seasonal cycle that drives a decrease in the angular momentum of the HC and, consequently, a weakening of the vertical shear of the zonal wind; (2) an increase in subtropical static stability due to the quasi-moist adiabatic adjustment to warmed subtropical sea surface temperatures (SSTs); and (3) a diabatic relaxation of the meridional SST gradient in the outer tropics and subtropics by OHT. Although mechanism (3) appears contribute the most to the response of HC width to OHT, the contributions from (1) and (2) are by no means negligible (each accounting for up to 20–30% of the HC response). By highlighting the role of ITCZ position in setting the mean width of the HC and in producing HC expansion, a role which has previously been under-appreciated, this study represents an important addition to the existing understanding of theoretical constraints on HC width. Overall, this study indicates a fundamental role for baroclinicity in limiting the poleward extent of the HC.