Research ArticleEARTH SCIENCE

Recent intensification of Amazon flooding extremes driven by strengthened Walker circulation

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Science Advances  19 Sep 2018:
Vol. 4, no. 9, eaat8785
DOI: 10.1126/sciadv.aat8785
  • Fig. 1 Frequency, severity, and duration of historical extreme floods and droughts in Amazonia.

    (A) Daily water level of Negro River at Manaus (black; 1903–2015) and Amazon River at Óbidos (red; 1968–2015). The horizontal dotted lines denote the thresholds used to define drought (15.8 m) and flood (29.0 m) events from the Negro River daily water level data. The resulting events are indicated by orange (droughts) and blue (floods) vertical ticks at the bottom of the panel. (B) Time-varying frequency of the identified droughts and floods between 1903 and 2015. (C and D) Duration, severity, and frequency of flood and drought events. The dotted lines denote the bivariate return period of a drought or flood event with a given duration or intensity. The dates of the events are indicated by the last two digits of the year of occurrence (for example, 12 for 2012). Events during the 21st century are indicated by red triangles.

  • Fig. 2 Intensification of the equatorial Walker circulation and increased flooding in Amazonia.

    (A) Trends in local Walker circulation (shading) (25) during the wet season (December to February) based on meridionally averaged (10°S–10°N) zonal vertical mass flux in the ERA-Interim (ERA-I) reanalysis. Overlying vectors represent the wet season climatological zonal wind and the vertical velocity scaled by a factor of −50. (B) Observed wet season SST trends from the ERSST (extended reconstructed SST) data set (39) and schematic representation of the overturning Walker cells in blue. (C) Trends in wet season SLP and wind stress from ERA-Interim reanalysis (26). (D) Tropical trans-basin variability indices [Pacific minus Atlantic SST and SLP anomalies averaged over the boxes in (C)] (24). (E) Comparison between September–February tropical Pacific trade winds over the region 5°S–5°N/160°E–180°E and wet season precipitation anomalies averaged over Amazonia. (F) Observed change in flood recurrence and strength of the ascending branch of the Walker circulation over Amazonia during the wet season, as represented by the mean zonal vertical mass flux averaged over the box in (A). All linear trends are given as the cumulative change over 26 years (1990–2015), and stippling indicates significance at the 90% confidence level.

  • Fig. 3 Atlantic warming and recent increase in Agulhas leakage with the progressive poleward displacement of the Southern Hemisphere westerlies.

    (A) Annual mean (January to December) SSTs in the tropical Atlantic (5°S–20°N) compared with annual Agulhas leakage (34) lagged by 16 years. (B) Annual mean Southern Annular Mode (SAM) from observations (black) (40) and NOAA 20th century reanalysis (gray) (28). wrt, with respect to.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/9/eaat8785/DC1

    Fig. S1. Location of Manaus and Óbidos gauges in the Amazon system.

    Fig. S2. Increasing seasonal hydrological variability in Amazonia during 1970–2015.

    Fig. S3. Time-varying frequency of floods (left) and droughts (right) between 1903 and 2015 using different thresholds to define extreme events.

    Fig. S4. Correlations of monthly Pacific and Atlantic climate indices with seasonal water levels of the Negro River at Manaus and the Amazon River at Óbidos.

    Fig. S5. Decadal fluctuations in drought frequency in Amazonia and Atlantic climate modes.

    Fig. S6. Comparison of tropical Atlantic and Pacific SST averages along with the IPO and AMO indices during the Amazon wet season.

    Fig. S7. Trends and average time series of local Walker circulation based on meridionally averaged (10°S–10°N) zonal vertical mass flux in the ERA-Interim (ERA-I), NOAA 20th century (N20CR), and NCEP-2 (NCEP) reanalyses.

    References (41, 42)

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Location of Manaus and Óbidos gauges in the Amazon system.
    • Fig. S2. Increasing seasonal hydrological variability in Amazonia during 1970–2015.
    • Fig. S3. Time-varying frequency of floods (left) and droughts (right) between 1903 and 2015 using different thresholds to define extreme events.
    • Fig. S4. Correlations of monthly Pacific and Atlantic climate indices with seasonal water levels of the Negro River at Manaus and the Amazon River at Óbidos.
    • Fig. S5. Decadal fluctuations in drought frequency in Amazonia and Atlantic climate modes.
    • Fig. S6. Comparison of tropical Atlantic and Pacific SST averages along with the IPO and AMO indices during the Amazon wet season.
    • Fig. S7. Trends and average time series of local Walker circulation based on meridionally averaged (10°S–10°N) zonal vertical mass flux in the ERA-Interim (ERA-I), NOAA 20th century (N20CR), and NCEP-2 (NCEP) reanalyses.
    • References (41, 42)

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