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Changes in tropical Atlantic interannual variability from a substantial weakening of the meridional overturning circulation

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Abstract

In response to a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC)—from a coupled ocean–atmosphere general circulation model experiment—significant changes in the interannual variability are found over the tropical Atlantic, characterized by an increase of variance (by ~150 %) in boreal late spring-early summer and a decrease of variance (by ~60 %) in boreal autumn. This study focuses on understanding physical mechanisms responsible for these changes in interannual variability in the tropical Atlantic. It demonstrates that the increase of variability in spring is a consequence of an increase in the variance of the El Niño-Southern Oscillation, which has a large impact on the tropical Atlantic via anomalous surface heat fluxes. Winter El Niño (La Niña) affects the eastern equatorial Atlantic by decreasing (increasing) cloud cover and surface wind speed which is associated with anomalous downward (upward) short wave radiation and reduced (enhanced) upward latent heat fluxes, creating anomalous positive (negative) sea surface temperature (SST) anomalies over the region from winter to spring. On the other hand, the decrease of SST variance in autumn is due to a deeper mean thermocline which weakens the impact of the thermocline movement on SST variation. The comparison between the model results and observations is not straightforward owing to the influence of model biases and the lack of a major MOC weakening event in the instrumental record. However, it is argued that the basic physical mechanisms found in the model simulations are likely to be robust and therefore have relevance to understanding tropical Atlantic variability in the real world, perhaps with modified seasonality.

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Acknowledgments

Irene Polo has been supported by a postdoctoral fellowship funded by the Spanish Government. This work has been also possible thanks to the Spanish projects: Tropical Atlantic Variability and the Climate Shift (TRACS-CGL2009-10285) and MOVAC. BD and RTS are supported by the U.K. National Centre for Atmospheric Science-Climate (NCAS-Climate) at the University of Reading. We also wish to thank two anonymous reviewers for their constructive comments and suggestions.

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  1. NCAS-Climate, Department of Meteorology, University of Reading, PO Box 243, Earley Gate, Reading, RG6 6BB, UK

    Irene Polo, Buwen W. Dong & Rowan T. Sutton

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  1. Irene Polo
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Correspondence to Irene Polo.

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Polo, I., Dong, B.W. & Sutton, R.T. Changes in tropical Atlantic interannual variability from a substantial weakening of the meridional overturning circulation. Clim Dyn 41, 2765–2784 (2013). https://doi.org/10.1007/s00382-013-1716-x

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