Research ArticleCLIMATOLOGY

Human-caused Indo-Pacific warm pool expansion

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Science Advances  01 Jul 2016:
Vol. 2, no. 7, e1501719
DOI: 10.1126/sciadv.1501719
  • Fig. 1 Geographical distribution of SST trends and warm pool expansion over the Indo-Pacific during 1953–2012.

    (A to D) HadISST (39) observations (A) and CMIP5 (18) multimodel means for each type of forcing: (B) anthropogenic plus natural external (29 models), (C) greenhouse gas only (6 models), and (D) natural external only (6 models). In each panel, trends are °C per 60 years, and the mean IPWP area is shown for two periods: 1953–1959 (dashed line) and 2000–2012 (solid lines). Gray boxes in (A) depict the area over which warm pool regions are calculated for the IPWP, divided into Indian and Pacific sectors by the 120°E meridian.

  • Fig. 2 Time series of 5-year mean anomalies of warm pool intensity and area.

    (A) Observed (39) (black) SST anomalies (°C) with (solid) and without (dashed) PDO influence are compared with multimodel mean–simulated responses to anthropogenic plus natural external forcings (ALL; green), anthropogenic forcing (ANT; calculated as ALL minus NAT; orange), greenhouse gas only forcing (GHG; red), and natural external only forcings (NAT; blue) for the Indo-Pacific (top), Indian Ocean (center), and Pacific Ocean (bottom) warm pools. Gray dashed lines represent the 5 to 95% range of internal variability taken from control (CTL) simulations. Linear trends [error bars representing 5 to 95% confidence intervals (CIs)] for observations with (black) and without (white) PDO and multimodel mean over the period 1953–2012 are displayed to the right. (B) The same as (A) but for warm pool area anomalies (as a percentage of each 1971–2000 mean) in the three warm pool regions.

  • Fig. 3 Results from optimal detection analysis of warm pool intensity and area with corresponding attributable trends.

    (A to C) Scaling factors of warm pool SST (A), area (B), and combined SST and area anomalies (C) for single-signals of anthropogenic plus natural external (ALL; green), greenhouse gas only (GHG; red), anthropogenic (ANT; orange), and natural external only (NAT; blue) forcings (left), and for two signals of ANT (x axis) and NAT (y axis) (center). In two-signal panels, dark, medium, and light colors indicate the Indo-Pacific, Indian, and Pacific warm pools, respectively. Best estimates (data points) and 5 to 95% Cls (error bars) of scaling factors are displayed, and the 5 to 95% joint confidence for two signals are represented by ellipses. Detectable response to an individual forcing occurs when scaling factors are significantly greater than zero. Consistency between observed and simulated responses is determined when scaling factors are not significantly different from unity. Also shown in corresponding bar graphs (right) of (A) and (B) are ANT- and NAT-attributable intensity and area trends (bar and 5 to 95% CI) from two-signal analysis (center) compared to observed trends (horizontal solid lines). The right panel of (C) compares the detection strength (mean and 5 to 95% CI) of the multimodel fingerprint of ANT in model runs (orange) and observations (black).

  • Table 1 Comparison of trends in warm pool intensity and area between observations and climate model simulations.

    Multimodel means of linear trend slopes are defined as the signal (SALL, SANT, SGHG, or SNAT), and the SD of trends across nonoverlapping CTL chunks is defined as the noise (N). The 5 to 95% CIs are shown in Fig. 2. Signal-to-noise ratios (SNRs) are then calculated from slopes of SST and area series averaged over the three warm pool regions during 1953–2012 for ALL and ANT simulations. Observational (39) trend slopes with (SOBS) and without the influence of the PDO (SOBS*) are given for comparison. Units for S and N are °C and % per 60 years for warm pool intensity and area, respectively.

    SALL
    (SNRALL)
    SANT
    (SNRANT)
    SGHGSNATNCTLSOBS
    SOBS*
    Intensity (°C per 60 years)
    Indo-Pacific0.25 (7.24)0.24 (7.01)0.390.010.030.30 (0.26*)
    Indian0.26 (9.16)0.25 (8.97)0.430.010.030.34 (0.28*)
    Pacific0.28 (6.14)0.27 (6.02)0.430.010.050.33 (0.29*)
    Area (percentage per 60 years)
    Indo-Pacific37.3 (2.99)35.3 (2.83)67.12.012.532.2 (27.4*)
    Indian38.4 (2.1)36.4 (1.99)78.772.118.351.2 (44.3*)
    Pacific36.7 (2.36)34.7 (2.23)61.02.015.621.6 (18.0*)

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/7/e1501719/DC1

    fig. S1. Relationship of observed and simulated SST trends in the Indo-Pacific region with global mean temperatures during 1953–2012.

    fig. S2. Simulated IPWP area and intensity.

    fig. S3. Relationships between warm pool intensity and area.

    fig. S4. Time series of tropical ocean and warm pool intensity and zonal gradient during 1953–2012.

    fig. S5. Sensitivity of observed SST trends and warm pool expansion over the Indo-Pacific during 1953–2012 to data set.

    fig. S6. Observed rainfall anomalies associated with warm pool intensity and area variations.

    fig. S7. Observed rainfall trend during the satellite era and intermodel relationship between zonal contrast of warm pool expansion and Pacific decadal variability.

    fig. S8. Simulated rainfall trends from two groups of CMIP5 models and their difference.

    fig. S9. Intermodel relationships between trends in convection and the IPWP.

    fig. S10. Sensitivity of simulated warm pool intensity and area anomalies to model samples.

    fig. S11. Sensitivity of optimal detection results to model samples.

    table S1. List of CMIP5 model simulations used in this study.

    table S2. Performance of 42 CMIP5 models integrated with observed time-evolving changes in anthropogenic and natural forcings.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Relationship of observed and simulated SST trends in the Indo-Pacific region with global mean temperatures during 1953–2012.
    • fig. S2. Simulated IPWP area and intensity.
    • fig. S3. Relationships between warm pool intensity and area.
    • fig. S4. Time series of tropical ocean and warm pool intensity and zonal gradient during 1953–2012.
    • fig. S5. Sensitivity of observed SST trends and warm pool expansion over the Indo-Pacific during 1953–2012 to data set.
    • fig. S6. Observed rainfall anomalies associated with warm pool intensity and area variations.
    • fig. S7. Observed rainfall trend during the satellite era and intermodel relationship between zonal contrast of warm pool expansion and Pacific decadal variability.
    • fig. S8. Simulated rainfall trends from two groups of CMIP5 models and their difference.
    • fig. S9. Intermodel relationships between trends in convection and the IPWP.
    • fig. S10. Sensitivity of simulated warm pool intensity and area anomalies to model samples.
    • fig. S11. Sensitivity of optimal detection results to model samples.
    • table S1. List of CMIP5 model simulations used in this study.
    • table S2. Performance of 42 CMIP5 models integrated with observed time-evolving changes in anthropogenic and natural forcings.

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