The Intertropical Convergence Zone (ITCZ) — responsible for most of the precipitation on Earth — is defined by a pronounced maximum rainfall occurring 5◦ north of the equator over most ocean basins. The existence of an ITCZ directly derives from large meridional-overturning atmospheric cells in Earth’s tropical regions, known as Hadley cells. Near the surface, Hadley cells carry moist, warm air toward the ITCZ , while in the upper troposphere, drier, cooler air is carried away. This circulation transports energy away from the ITCZ.
The ITCZ represents an “energy-flux equator” for the atmosphere: Hadley circulation transports energy, northward and southward, with the mean ITCZ location in the NH. Earth’s atmosphere transports energy from the NH to the SH across the geographic equator. This well-know behavior, which induces cooling in the NH and warming in the SH, has been extensively documented in observations (Trenberth and Caron, 2001) and global climate model simulations.
Presently, the mean position of the ITCZ is north of the equator. Yet, what controls the location of the ITCZ and how its position relates to the cross-equatorial energy transport is not completely understood. Local feedbacks, such as coastlines and continental distribution, have been invoked as the primary cause of the northern ITCZ location (e.g., Xie and Philander, 1997; Takahashi and Battisti, 2007). However, nonlocal processes, such as hemispheric differences in extratropical and polar cloud cover (e.g., Hwang and Frierson, 2013), differences in ice and snow cover in polar regions (e.g., Chiang and Bitz, 2005), and cross-equatorial heat transport by the ocean circulation (e.g., Marshall et al., 2013), may also be important.
In Frierson et al. (2013), the northern ITCZ position is attributed to cross-equatorial heat transport by the ocean circulation in the Atlantic basin, which is part of the global thermohaline circulation. Global oceanic circulation leads to a net heat transport from the SH to the NH that nearly equals that of the atmosphere (Trenberth and Caron, 2001).
Frierson et al. (2013) suggests that radiative imbalances from clouds, ice cover, or land albedo have a negligible effect on the mean ITCZ location. If radiative imbalances were to remain insignificant with global warming, changes in the location of the ITCZ and cross-equatorial atmospheric heat transport could be constrained solely from changes in the ocean transport. However, a number of processes (e.g., cloud cover), can increase radiative imbalance between hemispheres as climate warms in the deep tropics in which case changes in the mean ITCZ location cannot be constrained solely from changes in the cross-equatorial ocean heat transport.
Identifying the sensitivity of the ITCZ location and cross-equatorial atmospheric heat transport to climate change requires designing a quantitative model that accounts for both ocean heat transport and top-of-atmosphere radiative imbalances. Frierson et al. (2013) lacks this. Further work is needed to better comprehend the sensitivity of the ITCZ location to past and future climate changes.
1. Frierson, D.M.W. et al., 2013: Contribution of ocean overturning circulation to tropical rainfall peak in the Northern Hemisphere, Nature Geoscience, 6, 940–944.
Chiang, J.C. and C.M. Bitz, 2005: Influence of high latitude ice cover on the marine Intertropical Convergence Zone. Climate Dynamics, 25, 477-496.
Frierson, D.M.W. et al., 2013: Contribution of ocean overturning circulation to tropical rainfall peak in the Northern Hemisphere, Nature Geoscience, 6, 940–944.
Hwang, Y-T., D.M.W. Frierson, and S.M. Kang, 2013: Anthropogenic sulfate aerosol and the southward shift of tropical precipitation in the 20th century. Geophysical Research Letter, 40, 1-6.
Marshall, J. et al., 2013: The role of the ocean circulation in setting the mean position of the ITCZ. Climate Dynamics.
Takahashi, K. and D.S. Battisti, 2007: Processes controlling the mean tropical Pacific precipitation pattern. Part I: The Andes and the eastern Pacific ITCZ. Journal of Climate, 20, 3434-3451.
Trenberth, K.E. and J.M. Caron , 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports. Journal of Climate, 14, 3433-3443.
Xie, S-P. and S.G.H. Philander, 1997: A couple ocean-atmosphere model of relevance to the ITCZ in the eastern Pacific. Tellus, 46A, 340-350.