The Response of Hadley Circulation Extent to an Idealized Representation of Poleward Ocean Heat Transport in an Aquaplanet GCM

Abstract

Deeper theoretical understanding of Hadley circulation (HC) width and the mechanisms leading to HC expansion is gained by exploring the response of a zonally symmetric slab ocean aquaplanet general circulation model (GCM) to imposed poleward ocean heat transport (OHT). Poleward OHT causes the subtropical edge of the HC to shift poleward by up to 3° compared to its position in simulations without OHT. This HC widening is interpreted as being driven by a decrease in baroclinicity near the poleward edge of the HC and is divided into three components: a decrease in baroclinicity due to 1) a systematic poleward shift of the intertropical convergence zone (ITCZ) during 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 and the vertical extent of the HC, both of which result from OHT’s effect on global-mean temperature; and 3) a relaxation of the meridional sea surface temperature (SST) gradient in the outer tropics and subtropics by OHT. Although the third mechanism contributes the most to the response of HC width to OHT, the contributions from the first two mechanisms each account for up to 20%–30% of the HC response. This work highlights the role of ITCZ position in producing HC expansion and in setting the climatological width of the HC, a role which has been underappreciated. This study indicates a fundamental role for baroclinicity in limiting the poleward extent of the HC.

Modulation of Monsoon Circulations by Cross-Equatorial Ocean Heat Transport

“…Separate from the question of monsoons, the relationship between the zonal-mean atmospheric circulation and OHT has been investigated in a number of recent studies. It has been shown that including interactive OHT in idealized models substantially damps the Hadley circulation (Clement 2006; Levine and Schneider 2011;Singh et al 2017;Hilgenbrink and Hartmann 2018), as well as meridional shifts of the intertropical convergence zone [ITCZ; Green and Marshall 2017;Schneider 2017; however, note that Clement (2006) found that OHT increases the amplitude of the seasonal migration of the ITCZ]. The reason for this is that, because of its small gross moist stability, the tropical atmosphere is an inefficient transporter of energy (Held 2001).…”

ITCZ Width Controls on Hadley Cell Extent and Eddy-Driven Jet Position and Their Response to Warming

“…Thus, when examining models that include a seasonal cycle, or observations, it would be necessary to examine the dynamics of individual seasons in addition to the annual mean. Furthermore, in individual seasons, shifts in the position of the ITCZ can cause changes in the angular momentum budget of the tropical atmosphere, and hence Hadley cell extent, in a similar fashion to the width changes discussed in this work (Kang and Lu 2012;Hilgenbrink and Hartmann 2018).…”

“…Based on this idea, it is reasonable to suppose that changes in the position, width, and strength of ascent in the deep tropics under global warming may have an impact on the higher-latitude response. Previous work has found that ITCZ position changes play a role in setting the Hadley cell edge response to warming in comprehensive climate models (Kang and Lu 2012) and in response to imposed ocean heat transport in a slab ocean aquaplanet model (Hilgenbrink and Hartmann 2018).…”

The Dynamics of the Global Monsoon: Connecting Theory and Observations

“…Held & Coauthors, 2000). Hilgenbrink and Hartmann (2018) found that the reduced upper-level zonal velocity associated with an ITCZ located further from the Equator tends to reduce vertical wind shear, and consequently baroclinicity, over the winter hemisphere cell edge, so that the cell extends deeper into the winter hemisphere.…”