Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus

Journal name:
Nature Climate Change
Volume:
4,
Pages:
222–227
Year published:
DOI:
doi:10.1038/nclimate2106
Received
Accepted
Published online
Corrected online

Abstract

Despite ongoing increases in atmospheric greenhouse gases, the Earths global average surface air temperature has remained more or less steady since 2001. A variety of mechanisms have been proposed to account for this slowdown in surface warming. A key component of the global hiatus that has been identified is cool eastern Pacific sea surface temperature, but it is unclear how the ocean has remained relatively cool there in spite of ongoing increases in radiative forcing. Here we show that a pronounced strengthening in Pacific trade winds over the past two decades—unprecedented in observations/reanalysis data and not captured by climate models—is sufficient to account for the cooling of the tropical Pacific and a substantial slowdown in surface warming through increased subsurface ocean heat uptake. The extra uptake has come about through increased subduction in the Pacific shallow overturning cells, enhancing heat convergence in the equatorial thermocline. At the same time, the accelerated trade winds have increased equatorial upwelling in the central and eastern Pacific, lowering sea surface temperature there, which drives further cooling in other regions. The net effect of these anomalous winds is a cooling in the 2012 global average surface air temperature of 0.1–0.2 °C, which can account for much of the hiatus in surface warming observed since 2001. This hiatus could persist for much of the present decade if the trade wind trends continue, however rapid warming is expected to resume once the anomalous wind trends abate.

At a glance

Figures

  1. Global average SAT and Pacific trade wind anomalies
    over the past century.
    Figure 1: Global average SAT and Pacific trade wind anomalies over the past century.

    a, Temperature anomalies are shown as the annual mean relative to 1951–1980, with individual years shown as grey bars and a five-year running mean overlaid in bold. b, Pacific wind stress anomalies are computed over the region 6°N–6°S and 180°–150°W (green rectangle in Fig. 2a), corresponding to where the IPO exhibits maximum regression onto Pacific Ocean winds. Anomalies are shown relative to the historical record for two climatologies (Methods), with a bold line indicating the strength of the 20-year trends leading up to each year shown. In both reanalysis products shown, the recent multidecade acceleration in Pacific trade winds is the highest on record, although estimates of observed winds are not well constrained by measurements previous to the satellite era. The sign of the low-pass filtered IPO index18, 19 is indicated in both panels, with negative phases of the IPO shaded in blue.

  2. Observed trends in winds, SLP, sea surface height, SST and SAT during 1992-2011.
    Figure 2: Observed trends in winds, SLP, sea surface height, SST and SAT during 1992–2011.

    a, Observed trends in surface wind stress (Nm−2yr−1) shown as vectors with observed trends in atmospheric SLP overlaid in colour shading (Payr−1). The maximum vector is 0.003Nm−2yr−1 and only vector trends that are significant at the 95% confidence level are shown. The green rectangle is the region computed in Fig. 1b. b, Observed trends in sea surface height (cmyr−1) from satellite altimetry. c,d, Observed trends in SST (c) and surface layer air temperature (d), respectively (°Cyr−1). In all panels, stippling indicates where the trends are significant at the 95% confidence level given the linear regression standard error over the entire period of 1992–2011.

  3. Schematic of the trends in temperature and
        ocean-atmosphere circulation in the Pacific over the past two
        decades.
    Figure 3: Schematic of the trends in temperature and ocean–atmosphere circulation in the Pacific over the past two decades.

    Colour shading shows observed temperature trends (°C per decade) during 1992–2011 at the sea surface (Northern Hemisphere only), zonally averaged in the latitude-depth sense (as per Supplementary Fig. 6) and along the equatorial Pacific in the longitude-depth plane (averaged between 5°N–5°S). Peak warming in the western Pacific thermocline is 2.0°C per decade in the reanalysis data and 2.2°C per decade in the model. The mean and anomalous circulation in the Pacific Ocean is shown by bold and thin arrows, respectively, indicating an overall acceleration of the Pacific Ocean shallow overturning cells, the equatorial surface currents and the Equatorial Undercurrent (EUC). The accelerated atmospheric circulation in the Pacific is indicated by the dashed arrows; including the Walker cell (black dashed) and the Hadley cell (red dashed; Northern Hemisphere only). Anomalously high SLP in the North Pacific is indicated by the symbol ‘H. An equivalent accelerated Hadley cell in the Southern Hemisphere is omitted for clarity.

  4. Model temperature and ocean circulation anomalies due
    to observed 1992-2011 wind trends over the Pacific Ocean, and projections to
    2050.
    Figure 4: Model temperature and ocean circulation anomalies due to observed 1992–2011 wind trends over the Pacific Ocean, and projections to 2050.

    a, SST and ocean circulation trends in the model experiment including both historical CO2 forcing and observed 1992–2011 Pacific Ocean wind trends. Note the colour scale used is different from the observed trends (Fig. 2c). b, As in a but showing the zonal average temperature and meridional overturning circulation trends in the Pacific Ocean. Units are °Cyr−1 and Svyr−1, respectively. Maximum vector in a is 1cms−1 per 20 yr (trends larger than this are shown on this maximum scale, for clarity). Contour interval is 0.2Svyr−1 in b with contours drawn at ±0.1,±0.3,±0.5,±0.7,…Svyr−1. Solid (dashed) contours indicate a clockwise (anticlockwise) circulation. c, Wind-induced annual mean global SAT anomalies relative to the increasing CO2 experiment during 1980–2012 and projected for 2013–2050 assuming (i) a return to climatological winds by 2030, (ii) a fixed wind anomaly from 2012 onwards and (iii) a continuation of the recent trend until 2020 and stabilization thereafter. Future SAT anomalies essentially track the wind trend after 2013: either returning to zero by 2030, persisting at around 0.1°C cooling or increasing up to 0.18°C cooling by 2025.

  5. Recent annual average global air temperature
    anomalies and Pacific wind trends compared with model projections.
    Figure 5: Recent annual average global air temperature anomalies and Pacific wind trends compared with model projections.

    a, Observations are shown as annual anomalies relative to the 1980–2012 mean (grey bars) and a five-year running mean (black solid line). Model projections are shown relative to the year 2000 and combine the CMIP3 and CMIP5 multi-model mean (red dashed line) and range (red shaded envelope). The projections branch off the five-year running mean of observed anomalies and include all simulations as evaluated by the IPCC AR4 and AR5. The cyan, blue and purple dashed lines and the blue shading indicate projections adjusted by the trade-wind-induced SAT cooling estimated by the ocean model (OGCM), under three scenarios: the recent trend extends until 2020 before stabilizing (purple dashed line); the trend stabilizes in year 2012 (blue dashed line); and the wind trend reverses in 2012 and returns to climatological mean values by 2030 (cyan dashed line). The black, dark green and light green dashed lines are as per the above three scenarios, respectively, only using the trade-wind-induced SAT cooling derived from the full coupled model (CGCM). Shading denotes the multi-model range throughout. b, Normalized histograms of Pacific trade wind trends (computed over 6°N–6°S and 180°–150°W) for all 20-year periods using monthly data in observations (1980–2011) versus available CMIP5 models (1980–2013). The observed trend strength during 1992–2011 is indicated.

Change history

Corrected online 14 February 2014
In the version of this Article originally published online, the y axis label 'Zonal wind stress anomalies' of Fig. 1b should have had units of ×10–1 N m–2. This error has now been corrected in all versions of the Article.

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Author information

Affiliations

  1. ARC Centre of Excellence for Climate System Science, University of New South Wales New South Wales 2052 Australia

    • Matthew H. England,
    • Shayne McGregor,
    • Paul Spence,
    • Alex Sen Gupta &
    • Agus Santoso
  2. Climate Change Research Centre, University of New South Wales New South Wales 2052 Australia

    • Matthew H. England,
    • Shayne McGregor,
    • Paul Spence,
    • Alex Sen Gupta &
    • Agus Santoso
  3. National Center for Atmospheric Research Boulder Colorado 80307 USA

    • Gerald A. Meehl
  4. International Pacific Research Centre, University of Hawaii Hawaii 96822 USA

    • Axel Timmermann
  5. CSIRO Marine and Atmospheric Research Aspendale Victoria 3195 Australia

    • Wenju Cai &
    • Ariaan Purich
  6. NOAA/Pacific Marine Environmental Laboratory Seattle Washington 98115 USA

    • Michael J. McPhaden

Contributions

M.H.E. conceived the study and wrote the initial draft of the paper. M.H.E., S.M. and P.S. formulated the experimental design and observational data analyses. P.S. conducted and analysed the ocean model experiments, S.M. analysed the observational data. M.H.E. designed and A.P. computed the analysis of the CMIP5 experiments, A.S.G. analysed the ocean reanalysis trends and A.S. ran the coupled model experiments. All authors contributed to interpreting the results, discussion of the associated dynamics and refinement of the paper.

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The authors declare no competing financial interests.

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