NATURE GEOSCIENCE | VOL 6 | NOVEMBER 2013 | www.nature.com/naturegeoscience 901 news & views N arrow bands of intense rainfall in the deep tropics stretch around the globe in close proximity to arid regions where rainfall is scarce. his rainfall-rich zone, termed the intertropical convergence zone (ITCZ; Fig. 1), is neither centred on the equator, nor symmetric about it. Instead, much of the rainfall is located over the northern tropical oceans. Writing in Nature Geoscience, Frierson and colleagues 1 use model simulations and observational data to show that the ocean’s overturning circulation and its associated transports of energy are key determinants of rainfall’s boreal disposition. According to conventional understanding, the hemispheric asymmetry of the ITCZ is rooted in geography, resulting from the inluence of tropical continental landmasses on ocean currents and temperatures, and associated atmospheric circulation and cloud ields 2 . However, the Southern Hemisphere absorbs, on average, slightly more energy at the top of the atmosphere than the Northern Hemisphere. he ocean’s meridional overturning circulation redresses the resultant energetic imbalance by transporting heat northwards across the equator, and into the boreal extratropics, where it escapes into the atmosphere. Frierson and colleagues 1 demonstrate the sensitivity of the ITCZ to the redistribution of heat by this meridional overturning circulation in various model simulations. In general, the oceans absorb energy at the surface in the tropics and release it in the extratropics 3 . Using an aquaplanet model (that is, a model set- up devoid of continents), Frierson and colleagues explore perturbations to this mean state by imposing a small inter-hemispheric gradient in extratropical surface heat lux, such that slightly more heat lows out of the Northern Hemisphere oceans into the atmosphere than usual. his asymmetric lux is intended to mimic the ocean’s meridional overturning circulation. In this coniguration, they are able to induce a hemispheric asymmetry in the ITCZ. To further test their hypothesis that the meridional overturning circulation, as opposed to geography, determines the ITCZ asymmetry, Frierson et al. impose a symmetrical surface heat lux between the tropics and extratropics on both hemispheres, in a model that includes realistic landmasses. hey ind that the asymmetry in the ITCZ, which is captured well by the model when forced with the observed, lopsided surface heat lux, disappears when the symmetric heat lux is imposed. he indings suggest that hemispheric diferences in ocean– atmosphere heat lux in the extratropics associated with the meridional overturning circulation may be a necessary condition for ITCZ asymmetry. It is worth pointing out that the hemispheric imbalance in the surface heat lux is itself a consequence of continental geometry, which inluences the circulations of both the atmosphere and ocean 4 . However, by emphasizing the role of energetics, rather than the location of landmasses, the indings provide new insight into the processes that might dictate changes in the distribution of tropical rainfall. For instance, inter- hemispheric contrasts in aerosol abundance may modulate rainfall not just in their immediate vicinity (for instance by seeding clouds), but also throughout the tropics by modulating the surface radiative budget. As the boreal economies, especially those of India and China, transition to irst-world environmental standards, the implications for aerosols, and by extension patterns of tropical rainfall, are substantial. Speciically, a reduction in aerosol levels could increase the amount of sunlight reaching the surface in the Northern Hemisphere, thereby reducing the hemispheric heat contrast and ITCZ asymmetry. Frierson et al.’s conclusion that the ocean overturning circulation inluences the location of rainfall in the tropics needs to be explored further, both to test whether it is robust across a range of models, and to determine the relative signiicance of the meridional overturning circulation compared to other inluences — such as land-surface feedbacks and greenhouse gas concentrations — on rainfall distribution 5 . Furthermore, even if veriied, the natural variability in tropical rainfall, which spans ATMOSPHERIC SCIENCE Rainfall’s oceanic underpinnings Understanding the processes that govern the complex spatial structure of rainfall is crucial. Idealized numerical simulations reveal the strong inluence that ocean heat transport exerts on this structure. John Fasullo ITCZ Equator ITCZ Figure 1 | Tropical rain band. A narrow band of intense rainfall, depicted here by the bright white clouds that cut across the centre of the image, and termed the intertropical convergence zone (ITCZ), stretches from east to west just north of the equator. As a result, more rain falls to the north of the equator than the south. Frierson and colleagues 1 show that the redistribution of ocean heat from the Northern to the Southern Hemisphere by the meridional ocean overturning circulation can account for this hemispheric asymmetry in tropical rainfall. Cloud retrievals from NOAA’s Geostationary Operational Environmental Satellite (GOES-11); image courtesy of the NASA Earth Observatory. © 2013 Macmillan Publishers Limited. All rights reserved