Research ArticleCLIMATOLOGY

Weakening Atlantic Niño–Pacific connection under greenhouse warming

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Science Advances  21 Aug 2019:
Vol. 5, no. 8, eaax4111
DOI: 10.1126/sciadv.aax4111
  • Fig. 1 Atlantic Niño–Pacific connection in observation and CMIP5 models.

    Correlation coefficients of the Atl-EOF index (JJA0) and the grid-point D0JF1 equatorial Pacific (5°S to 5°N and 160°E to 90°W) SST anomalies over a 20-year running window from 1900 to 2017 (recorded at end year of the window) in Hadley Centre Sea Ice and Sea Surface Temperature dataset (HadISST) (A) and over the 1900–1999 period in HadISST (black bar) and 33 CMIP5 models (B). Black solid line in (A) indicates area-averaged values of all the correlation coefficients in the equatorial Pacific. Red dots in (A) and bars in (B) indicate the average of only significant (more than 95% confidence level) correlation coefficients in the respective period (see “Sign-dependent average” section in Materials and Methods). Values of models that fail to produce any significant impact of Atlantic Niño on the equatorial Pacific are set to be zero in (B). Black dashed lines in (A) denote the value (±0.466) of the 95% confidence level based on Student’s t test. (C to E) Spatial pattern of the correlation between the Atl-EOF index (JJA0) and the grid-point D0JF1 tropical Pacific SST anomalies over the 1900–1999 period based on HadISST (C), multimodel average of the 17 selected models (D), and multimodel average of the six models [orange bars in (B)] with opposite Atlantic Niño–Pacific connection (E). The values more than the 95% confidence level in (C) and the most robust features of ensemble where the mean exceeds 1 SD in (D) and (E) are hatched.

  • Fig. 2 Projected decrease in the Atlantic Niño–Pacific connection.

    (A) Comparison of the Pacific SST response (°C per SD) over the present-day (1900–1999; blue bars) and future (2000–2099; orange bars) 100-year periods in the 17 selected models. The four models that simulate an increase in response are grayed out. Error bars in the multimodel mean are calculated as 2 SDs (a 95% confidence interval based on normal distribution) of the 10,000 inter-realizations of a bootstrap method (see “Bootstrap test” section in Materials and Methods). The Pacific SST response is measured by regression coefficients of grid-point D0JF1 equatorial Pacific (5°S to 5°N and 160°E to 90°W) SST anomalies onto the Atl-EOF index (see “Sign-dependent average” section in Materials and Methods). (B and C) Comparison of the occurrence ratio of Atlantic Niño (Atl-EOF index > 0.5 SD) followed by La Niña (Niño3.4 index < −1 SD) over the total Atlantic Niño events (B) and that of Atlantic Niña (Atl-EOF index < −0.5 SD) followed by El Niño (Niño3.4 index >1 SD) over the total Atlantic Niña events (C) over the present-day (1900–1999; x axis) and future (2000–2099; y axis) 100-year periods in the 17 selected models. Numbers in the top left (bottom right) indicate the number of models that produce stronger (weaker) impact of Atlantic Niño/Niña on ENSO under future climate.

  • Fig. 3 Projection of decreased response to a diabatic equatorial Atlantic heating.

    Multimodel (the 17 selected models) average of boreal summer (JJA0) regression of equatorial (10°S to 10°N) vertical atmospheric velocity (pascals per second; color) and equatorial atmospheric flow vectors (zonal wind and vertical velocity scaled by a factor of 300) onto the Atl-EOF index of the present-day (1900–1999) (A) and future (2000–2099) (B) 100-year periods. Values exceeding 1 SD are shown in shaded contour and black vectors. The color map and reference vector are labeled in the middle of (A) and (B). (C) Comparison of vertical velocity response to the Atl-EOF index over the present-day (blue bars) and future (orange bars) 100-year periods in the 17 selected models. The three models that simulate an increase in response are marked in gray. Error bars in the multimodel mean are calculated as 2 SDs (the 95% confidence interval based on normal distribution) of the 10,000 inter-realizations of a bootstrap method (see “Bootstrap test” section in Materials and Methods). The vertical velocity response is measured by the regression coefficients (pascals per second per SD) of the JJA0 vertical velocity anomalies at 600 hPa averaged over the equatorial Atlantic (5°S to 5°N and 45°W to 20°E) and the respective Atl-EOF index. (D) Responses of JJA0 vertical velocity anomalies at 600 hPa averaged over the equatorial Atlantic (5°S to 5°N and 45°W to 20°E) (pascals per second) to JJA0 Atl-EOF index using all samples of 17 selected models. The vertical velocity anomalies are binned in 0.1 SD. Atl-EOF index intervals and the median vertical velocity anomaly and index are identified for each bin (dots). Blue and red dots indicate values in the present-day and future 100-year periods, respectively. The corresponding linear fitting lines (with the values of the slopes plus and minus the 95% confidence values from the Student’s t test) are also shown.

  • Fig. 4 Mechanism for the projected decrease in the Atlantic Niño–Pacific connection.

    (A) Warming trend of the boreal summer mean (JJA0) air temperature (AirT) over the equatorial Atlantic (5°S to 5°N and 45°W to 20°E) at different levels during 2000–2099. (B) Intermodel relationship between the change (between future and present-day climates) of boreal summer mean (JJA0) atmospheric stratification and the Pacific SST response. The atmospheric stratification is calculated as the difference between the boreal summer mean (JJA0) temperature at 600 hPa and the temperature at 925 hPa, both averaged over the equatorial Atlantic. The linear fit (solid line) is displayed together with the correlation coefficient r, slope, and P value from the regression. To enhance the intermodel comparability, we scale both the warming trend in (A) and the changes in (B) by the increase in global mean temperature over the present-day and future periods.

  • Fig. 5 Weakened Atlantic Niño–Pacific teleconnection in the CAM3.1-RGO experiments.

    Histograms of 10,000 realizations of the bootstrap method for the Pacific SST response (A) and vertical velocity anomalies at 600 hPa (B) in the initial experiments under control (blue) and 4× CO2 (red) (see “Model experiments” section in Materials and Methods). The Pacific SST response is measured by the area-averaged D0JF1 equatorial Pacific (5°S to 5°N and 160°E to 90°W) SST anomalies, and the vertical velocity anomalies at 600 hPa are averaged over the equatorial Atlantic (5°S to 5°N and 45°W to 20°E) during boreal summer (pascals per second; JJA0). The blue and red vertical lines indicate the mean values of 10,000 inter-realizations for the present-day and future periods, respectively. The gray shaded regions indicate the respective doubled SDs (the 95% confidence interval based on normal distribution) of the 10,000 inter-realization (see “Bootstrap test” section in Materials and Methods).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/8/eaax4111/DC1

    Fig. S1. Observed development of the Atlantic Niño–Pacific connection.

    Fig. S2. Comparison of Atlantic Niño patterns in observation and 17 selected models.

    Fig. S3. Intermodel relationship of Atlantic Niño–Pacific teleconnection over 100 years and its multidecadal fluctuations.

    Fig. S4. Modeled development of the Atlantic Niño–Pacific teleconnection.

    Fig. S5. Impact of the Atlantic Niño amplitude change on the Pacific SST response.

    Fig. S6. Projected decrease in the Atlantic Niño–Pacific connection in terms of bootstrap test and extreme ENSO.

    Fig. S7. Projection of decreased precipitation response to a diabatic equatorial Atlantic heating.

    Fig. S8. Projected warming pattern of equatorial Pacific and Atlantic Ocean.

    Fig. S9. Impact of model biases on the Pacific SST response.

    Table S1. Observed relationship between ENSO events and the Atlantic Niño.

    Table S2. CMIP5 models and their EOF modes of tropical Atlantic used in this study.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Observed development of the Atlantic Niño–Pacific connection.
    • Fig. S2. Comparison of Atlantic Niño patterns in observation and 17 selected models.
    • Fig. S3. Intermodel relationship of Atlantic Niño–Pacific teleconnection over 100 years and its multidecadal fluctuations.
    • Fig. S4. Modeled development of the Atlantic Niño–Pacific teleconnection.
    • Fig. S5. Impact of the Atlantic Niño amplitude change on the Pacific SST response.
    • Fig. S6. Projected decrease in the Atlantic Niño–Pacific connection in terms of bootstrap test and extreme ENSO.
    • Fig. S7. Projection of decreased precipitation response to a diabatic equatorial Atlantic heating.
    • Fig. S8. Projected warming pattern of equatorial Pacific and Atlantic Ocean.
    • Fig. S9. Impact of model biases on the Pacific SST response.
    • Table S1. Observed relationship between ENSO events and the Atlantic Niño.
    • Table S2. CMIP5 models and their EOF modes of tropical Atlantic used in this study.

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