Observed changes in extreme wet and dry spells during the South Asian summer monsoon season

Journal name:
Nature Climate Change
Volume:
4,
Pages:
456–461
Year published:
DOI:
doi:10.1038/nclimate2208
Received
Accepted
Published online

The South Asian summer monsoon directly affects the lives of more than 1/6th of the world’s population. There is substantial variability within the monsoon season, including fluctuations between periods of heavy rainfall (wet spells) and low rainfall (dry spells)1. These fluctuations can cause extreme wet and dry regional conditions that adversely impact agricultural yields, water resources, infrastructure and human systems2, 3. Through a comprehensive statistical analysis of precipitation observations (1951–2011), we show that statistically significant decreases in peak-season precipitation over the core-monsoon region have co-occurred with statistically significant increases in daily-scale precipitation variability. Further, we find statistically significant increases in the frequency of dry spells and intensity of wet spells, and statistically significant decreases in the intensity of dry spells. These changes in extreme wet and dry spell characteristics are supported by increases in convective available potential energy and low-level moisture convergence, along with changes to the large-scale circulation aloft in the atmosphere. The observed changes in wet and dry extremes during the monsoon season are relevant for managing climate-related risks, with particular relevance for water resources, agriculture, disaster preparedness and infrastructure planning.

At a glance

Figures

  1. July-August precipitation characteristics.
    Figure 1: July–August precipitation characteristics.

    a, Time series of mean precipitation, daily-variability and probability of daily precipitation (fraction of total days with precipitation >1 mm d−1) over the monsoon ‘core’ (red rectangle in the inset map in b, 18°–28° N, 73°–82° E; ref. 6). Numbers indicate linear trend magnitudes of the time series. b, Daily precipitation distributions over the ‘core’ in 1951–1980 and 1981–2011 and the p value obtained from testing the difference in means of the distributions. Colours indicate the significance of trends (a) and p values (b). cf, Composite precipitation anomalies from the July–August 1951–2011 mean for all extreme wet/dry spells in 1951–1980 (c,e) and 1981–2011 (d,f).

  2. Extreme wet and dry spell characteristics.
    Figure 2: Extreme wet and dry spell characteristics.

    ah, Time series of wet (blue) and dry (red) spell frequency, duration, intensity and cumulative days over the core monsoon domain (see Methods for definitions). Missing links in the time series are years with no wet or dry spells. Trend lines are estimated using the non-parametric LOESS regression technique; shading represents the 90% confidence intervals of the estimated trends. p values are obtained from testing the difference in means of the distributions of each variable between 1951–1980 and 1981–2011 using the non-parametric moving block bootstrap test. Colours indicate the significance level of the p values.

  3. Dynamics of extreme wet and dry spells.
    Figure 3: Dynamics of extreme wet and dry spells.

    ae, Composite values of 850 mb moisture convergence (conv.) or divergence (div.), and composite anomalies of 200 mb and 850 mb winds, 200 mb geopotential heights and CAPE, for 1951–1980. Composite values are calculated by averaging the variable on all extreme wet/dry days. Composite anomalies are differences between composite values and the average of all July–August days in the analysis period. fj, Differences in composite values/anomalies in 1981–2011 relative to 1951–1980. (See Supplementary Fig. 3 for significance of differences.) Grey rectangle in b defines the primary analysis domain.

  4. Sensitivity of statistical results to spatial domain.
    Figure 4: Sensitivity of statistical results to spatial domain.

    a,b, Trends in wet (a) and dry (b) spell characteristics (frequency (fr) events in a year per year), intensity (int) mm d−1 per year, duration (dur) days per year and cumulative days (c.d.) days per year derived from precipitation over three domains (see inset map—ref. 6 (red), ref. 1 (green), and ref. 7 (blue)). Colours indicate the significance level of p values from the significance test comparing the mean of the distributions of wet and dry spell characteristics in 1951–1980 and 1981–2011. (Supplementary Fig. 7 for the time series of these characteristics over the three domains.)

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Author information

Affiliations

  1. Department of Environmental Earth System Science, Stanford University, California 94305, USA

    • Deepti Singh,
    • Michael Tsiang,
    • Bala Rajaratnam &
    • Noah S. Diffenbaugh
  2. Woods Institute for the Environment, Stanford University, California 94305, USA

    • Bala Rajaratnam &
    • Noah S. Diffenbaugh
  3. Department of Statistics, Stanford University, California 94305, USA

    • Bala Rajaratnam

Contributions

D.S., B.R. and N.S.D. conceived and designed the study; D.S. and M.T. implemented the analytical tools; B.R. developed and supervised the statistical analyses; and D.S., M.T. and N.S.D. analysed the data and co-wrote the manuscript. All authors discussed the results and commented on the draft.

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The authors declare no competing financial interests.

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