Contribution of ocean overturning circulation to tropical rainfall peak in the Northern Hemisphere

Abstract

Rainfall in the tropics is largely focused in a narrow zonal band near the Equator, known as the intertropical convergence zone. On average, substantially more rain falls just north of the Equator1. This hemispheric asymmetry in tropical rainfall has been attributed to hemispheric asymmetries in ocean temperature induced by tropical landmasses. However, the ocean meridional overturning circulation also redistributes energy, by carrying heat northwards across the Equator. Here, we use satellite observations of the Earth’s energy budget2, atmospheric reanalyses3 and global climate model simulations to study tropical rainfall using a global energetic framework. We show that the meridional overturning circulation contributes significantly to the hemispheric asymmetry in tropical rainfall by transporting heat from the Southern Hemisphere to the Northern Hemisphere, and thereby pushing the tropical rain band north. This northward shift in tropical precipitation is seen in global climate model simulations when ocean heat transport is included, regardless of whether continents are present or not. If the strength of the meridional overturning circulation is reduced in the future as a result of global warming, as has been suggested4, precipitation patterns in the tropics could change, with potential societal consequences.

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Figure 1: Observed and modelled annual mean precipitation.
Figure 2: Observed net TOA radiation and surface energy fluxes.
Figure 3: Schematic of the role of the oceanic MOC in forcing the Northern Hemisphere maximum of tropical precipitation.

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Acknowledgements

We acknowledge helpful conversations with L. Thompson, C. Bitz, K. Armour, B. Rose, I. Held, R. Pierrehumbert, D. Hartmann, J. Scheff and M. Wallace. J. Fasullo provided the ERA-Interim energy transports. We acknowledge the Program for Climate Model Diagnosis and Intercomparison and the WCRP’s Working Group on Coupled Modelling for their roles in making available the CMIP3 and CMIP5 data sets. D.M.W.F. and Y-T.H. are supported by NSF Grants AGS-0846641 and AGS-0936069, and a University of Washington Royalty Research Fund grant. N.S.F. is supported by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), by NASA through grant No. NNX07AG53G, and by NOAA through grant No. NA11NMF4320128, which sponsor research at the International Pacific Research Center. R.S. is supported by NSF award AGS-0804107. S.M.K. is supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013R1A1A3004589). A.D. is supported by the NOAA Climate and Global Change Fellowship, administered by the University Corporation for Atmospheric Research. E.A.M. is supported by the National Defense Science and Engineering Graduate Fellowship Program.

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D.M.W.F., N.S.F. and Y-T.H. designed the original diagnostics and experiments, with frequent subsequent input on diagnostic techniques and experimental design from all co-authors. Y-T.H. and D.M.W.F. analysed the observations. Y-T.H. performed experiments with the GFDL model. X.L. and D.S.B. designed experiments with the ECHAM model, and X.L. ran the ECHAM model experiments. D.M.W.F. led the writing of the paper, with substantial input from all co-authors.

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Correspondence to Dargan M. W. Frierson.

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The authors declare no competing financial interests.

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Frierson, D., Hwang, Y., Fučkar, N. et al. Contribution of ocean overturning circulation to tropical rainfall peak in the Northern Hemisphere. Nature Geosci 6, 940–944 (2013). https://doi.org/10.1038/ngeo1987

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