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Interannual atmospheric variability forced by the deep equatorial Atlantic Ocean


Climate variability in the tropical Atlantic Ocean is determined by large-scale ocean–atmosphere interactions, which particularly affect deep atmospheric convection over the ocean and surrounding continents1. Apart from influences from the Pacific El Niño/Southern Oscillation2 and the North Atlantic Oscillation3, the tropical Atlantic variability is thought to be dominated by two distinct ocean–atmosphere coupled modes of variability that are characterized by meridional4,5 and zonal6,7 sea-surface-temperature gradients and are mainly active on decadal and interannual timescales, respectively8,9. Here we report evidence that the intrinsic ocean dynamics of the deep equatorial Atlantic can also affect sea surface temperature, wind and rainfall in the tropical Atlantic region and constitutes a 4.5-yr climate cycle. Specifically, vertically alternating deep zonal jets of short vertical wavelength with a period of about 4.5 yr and amplitudes of more than 10 cm s−1 are observed, in the deep Atlantic, to propagate their energy upwards, towards the surface10,11. They are linked, at the sea surface, to equatorial zonal current anomalies and eastern Atlantic temperature anomalies that have amplitudes of about 6 cm s−1 and 0.4 °C, respectively, and are associated with distinct wind and rainfall patterns. Although deep jets are also observed in the Pacific12 and Indian13 oceans, only the Atlantic deep jets seem to oscillate on interannual timescales. Our knowledge of the persistence and regularity of these jets is limited by the availability of high-quality data. Despite this caveat, the oscillatory behaviour can still be used to improve predictions of sea surface temperature in the tropical Atlantic. Deep-jet generation and upward energy transmission through the Equatorial Undercurrent warrant further theoretical study.

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Figure 1: Interannual variability in the tropical Atlantic associated with a 1,670-d cycle.
Figure 2: Zonal velocities at the Equator, 23° W.
Figure 3: Mean zonal velocity, zonal temperature gradient and harmonic analysis of 1,670-d oscillation.
Figure 4: Equatorial zonal velocities from 1,000-m Argo float drift data.

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This study was supported by the German Federal Ministry of Education and Research as part of the co-operative project ‘North Atlantic’ and by the German Science Foundation as part of the Sonderforschungsbereich 754 ‘Climate-Biogeochemistry Interactions in the Tropical Ocean’. The contribution of J.M.T. was facilitated by support from the Woods Hole Oceanographic Institution’s Columbus O’Donnell Iselin Chair for Excellence in Oceanography. We thank J. Fischer for mooring planning and field-work participation, F. Ascani for discussion and S.-H. Didwischus for data processing. This study uses PIRATA velocity and temperature data provided through the TAO project office, Argo float drift data provided by APDRC/IPRC19, rainfall data from the Global Precipitation Climatology Project, Met Office Hadley Centre and microwave optimally interpolated SST data, NCEP/NCAR reanalysis wind data, and AVISO sea level anomaly data (Supplementary Table 1).

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P.B. led the project and designed the study including sea-going work and data analysis. A.F. and V.H. processed and analysed moored velocity, Argo float and satellite data. J.M.T. performed moored profiler measurements, its data processing and its analysis. P.B., M.D. and R.J.G. led the drafting of the manuscript. All authors contributed to the interpretation of the results and provided substantial input to the manuscript.

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Correspondence to Peter Brandt.

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Supplementary Information

This file contains Supplementary Table 1-2 and Supplementary Figures 1-8 with legends. (PDF 1746 kb)

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Brandt, P., Funk, A., Hormann, V. et al. Interannual atmospheric variability forced by the deep equatorial Atlantic Ocean. Nature 473, 497–500 (2011).

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