Energetics of the Tropical Atlantic Zonal Mode

N. J. Burls University of Cape Town, Cape Town, South Africa

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C. J. C Reason University of Cape Town, Cape Town, South Africa

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P. Penven Laboratoire de Physique des Oceans (UMR 6523 CNRS, IFREMER, IRD, UBO), LMI ICEMASA, Plouzane, France, and University of Cape Town, Cape Town, South Africa

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S. G. Philander Princeton University, Princeton, New Jersey

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Abstract

Sea surface temperature in the central-eastern equatorial Atlantic has a seasonal cycle far bigger than that of the Pacific, but interannual anomalies smaller than those of the Pacific. Given the amplitude of seasonal SST variability, one wonders whether the seasonal cycle in the Atlantic is so dominant that it is able to strongly influence the evolution of its interannual variability. In this study, interannual upper-ocean variability within the tropical Atlantic is viewed from an energetics perspective, and the role of ocean dynamics, in particular the role of ocean memory, within zonal mode events is investigated. Unlike in the Pacific where seasonal and interannual variability involve distinctly different processes, the results suggest that the latter is a modulation of the former in the Atlantic, whose seasonal cycle has similarities with El Niño and La Niña in the Pacific. The ocean memory mechanism associated with the zonal mode appears to operate on much shorter time scales than that associated with the El Niño–Southern Oscillation, largely being associated with interannual modulations of a seasonally active delayed negative feedback response. Differences between the El Niño–Southern Oscillation and the zonal mode can then be accounted for in terms of these distinctions. Anomalous wind power over the tropical Atlantic is shown to be a potential predictor for zonal mode events. However, because zonal mode events are due to a modulation of seasonally active coupled processes, and not independent processes operating on interannual time scales as seen in the Pacific, the lead time of this potential predictability is limited.

Corresponding author address: N. J. Burls, Department of Oceanography, University of Cape Town, Private Bag X3, Rondebosch, Cape Town 7701, South Africa. E-mail: nats.burls@gmail.com

Abstract

Sea surface temperature in the central-eastern equatorial Atlantic has a seasonal cycle far bigger than that of the Pacific, but interannual anomalies smaller than those of the Pacific. Given the amplitude of seasonal SST variability, one wonders whether the seasonal cycle in the Atlantic is so dominant that it is able to strongly influence the evolution of its interannual variability. In this study, interannual upper-ocean variability within the tropical Atlantic is viewed from an energetics perspective, and the role of ocean dynamics, in particular the role of ocean memory, within zonal mode events is investigated. Unlike in the Pacific where seasonal and interannual variability involve distinctly different processes, the results suggest that the latter is a modulation of the former in the Atlantic, whose seasonal cycle has similarities with El Niño and La Niña in the Pacific. The ocean memory mechanism associated with the zonal mode appears to operate on much shorter time scales than that associated with the El Niño–Southern Oscillation, largely being associated with interannual modulations of a seasonally active delayed negative feedback response. Differences between the El Niño–Southern Oscillation and the zonal mode can then be accounted for in terms of these distinctions. Anomalous wind power over the tropical Atlantic is shown to be a potential predictor for zonal mode events. However, because zonal mode events are due to a modulation of seasonally active coupled processes, and not independent processes operating on interannual time scales as seen in the Pacific, the lead time of this potential predictability is limited.

Corresponding author address: N. J. Burls, Department of Oceanography, University of Cape Town, Private Bag X3, Rondebosch, Cape Town 7701, South Africa. E-mail: nats.burls@gmail.com
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