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Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing


Since the mid-nineteenth century the Earth's surface has warmed1,2,3, and models indicate that human activities have caused part of the warming by altering the radiative balance of the atmosphere1,3. Simple theories suggest that global warming will reduce the strength of the mean tropical atmospheric circulation4,5. An important aspect of this tropical circulation is a large-scale zonal (east–west) overturning of air across the equatorial Pacific Ocean—driven by convection to the west and subsidence to the east—known as the Walker circulation6. Here we explore changes in tropical Pacific circulation since the mid-nineteenth century using observations and a suite of global climate model experiments. Observed Indo-Pacific sea level pressure reveals a weakening of the Walker circulation. The size of this trend is consistent with theoretical predictions, is accurately reproduced by climate model simulations and, within the climate models, is largely due to anthropogenic forcing. The climate model indicates that the weakened surface winds have altered the thermal structure and circulation of the tropical Pacific Ocean. These results support model projections of further weakening of tropical atmospheric circulation during the twenty-first century4,5,7.

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Figure 1: Spatial pattern of observed and modelled sea level pressure linear trends.
Figure 2: Summary of the linear trends in SLP gradient across the Indo-Pacific (ΔSLP) from observations and the various GCM historical radiative forcing experiments.
Figure 3: Observed and modelled evolution of ΔSLP since the nineteenth century.
Figure 4: Observed and modelled equatorial Pacific zonal-mean zonal wind-stress anomaly, < τx > , and equatorial thermocline depth anomaly, Ztc.

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G.A.V. was supported by the Visiting Scientist Program at the NOAA/GFDL administered by UCAR. We are grateful to the model development teams at GFDL, and thank A. E. Johansson, M. P. Vecchi, T. Knutson, T. Delworth and J. Russell for comments and suggestions.

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Correspondence to Gabriel A. Vecchi.

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Supplementary information

Supplementary Methods

This file contains additional details of the methods used in this study. (DOC 44 kb)

Supplementary Figure 1

Evolution of ΔSLP from GFDL-CM2.1 historical integrations. (PDF 428 kb)

Supplementary Figure 2

Statistical significance limits of single-member Δ SLP trends of different lengths, estimated from the 2,000-year control GFDL-CM2.1 integration. (PDF 595 kb)

Supplementary Figure 3

Two-sided confidence intervals on zero Δ SLP trend from pre-Industrial IPCC-AR4 GCM control experiments. (PDF 359 kb)

Supplementary Figure 4

Linear trends of Δ SLP from IPCC-AR4 models. (PDF 276 kb)

Supplementary Figure 5

Modelled changes in equatorial Pacific oceanic currents. (PDF 688 kb)

Supplementary Figure 6

Evolution of equatorial Pacific thermocline depth and slope. (PDF 291 kb)

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Vecchi, G., Soden, B., Wittenberg, A. et al. Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing. Nature 441, 73–76 (2006).

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